diff --git a/changelog.d/2014.fixed.md b/changelog.d/2014.fixed.md
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+- Charge Universal Credit, Housing Benefit and national Council Tax Reduction non-dependant deductions for members of the claimant's own benefit unit who are not the claimant, partner or a child or young person, such as an adult son (UC Regs 2013 Sch 4 para 9; HB Regs 2006 and HB (SPC) Regs 2006 reg 3; SI 2012/2885 reg 9 and the Welsh and Scottish equivalents), and apply the Universal Credit renter exemption from housing cost contributions (Sch 4 para 15).
diff --git a/policyengine_uk/tests/policy/baseline/finance/benefit/family/benefit_unit_non_dependants.yaml b/policyengine_uk/tests/policy/baseline/finance/benefit/family/benefit_unit_non_dependants.yaml
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+# Non-dependants within the claimant's own benefit unit.
+#
+# A benefit unit can include someone aged 16 or over who is neither the
+# claimant or partner nor a child or young person the claimant is responsible
+# for, such as an adult son or a 19-year-old who has left education. In law
+# that person is a non-dependant of the renter (UC Regs 2013 Sch 4 para
+# 9(1)(c) and (2); HB Regs 2006 and HB (SPC) Regs 2006 reg 3; CTR SI
+# 2012/2885 reg 9, WSI 2013/3029 reg 9, SSI 2021/249 reg 8), so their family's
+# claim bears a deduction for them, subject to each scheme's exemptions.
+#
+# Amounts for 2026-27:
+# - UC housing cost contribution £96.55 a month (Sch 4 para 14(1)), aged 21
+#   or over (para 16(2)(a)): £1,158.60 a year.
+# - HB non-dependant aged 18 or over not in remunerative work £20.40 a week
+#   (reg 74(1)(b)): £1,060.80 a year. In remunerative work with gross income
+#   of £300 a week: £64.35 (reg 74(2)(c)), £3,346.20 a year. The HB cases
+#   give part-time earners £50 a week for 10 hours, below remunerative work
+#   (16 hours, reg 6) and inside the lowest band.
+
+- name: UC, an adult son in the renter's benefit unit bears a housing cost contribution
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+      son:
+        age: 25
+        is_claimant_or_partner: false
+    benunits:
+      benunit:
+        members: [parent, son]
+    households:
+      household:
+        members: [parent, son]
+  output:
+    is_benefit_unit_non_dependant_for_universal_credit: [false, true]
+    uc_individual_non_dep_deduction: [0, 1_158.60]
+    uc_non_dep_deductions: 1_158.60
+
+- name: UC, no contribution for a non-dependant in the benefit unit under 21
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+      son:
+        # Left education: not a qualifying young person.
+        age: 19
+        is_claimant_or_partner: false
+        is_qualifying_young_person_for_universal_credit: false
+    benunits:
+      benunit:
+        members: [parent, son]
+    households:
+      household:
+        members: [parent, son]
+  output:
+    is_benefit_unit_non_dependant_for_universal_credit: [false, true]
+    uc_non_dep_deductions: 0
+
+- name: UC, a qualifying young person the renter is responsible for is not a non-dependant
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+      daughter:
+        age: 19
+        is_claimant_or_partner: false
+        is_qualifying_young_person_for_universal_credit: true
+    benunits:
+      benunit:
+        members: [parent, daughter]
+    households:
+      household:
+        members: [parent, daughter]
+  output:
+    is_benefit_unit_non_dependant_for_universal_credit: [false, false]
+    uc_non_dep_deductions: 0
+
+# Sch 4 para 9(2)(g): a qualifying young person for whom no one in the
+# renter's extended benefit unit is responsible is not a non-dependant.
+- name: UC, a qualifying young person looked after by a local authority is not a non-dependant
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      carer:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+      young_person:
+        age: 17
+        is_claimant_or_partner: false
+        is_qualifying_young_person_for_universal_credit: true
+        is_looked_after_by_local_authority: true
+    benunits:
+      benunit:
+        members: [carer, young_person]
+    households:
+      household:
+        members: [carer, young_person]
+  output:
+    is_child_or_qualifying_young_person_for_universal_credit: [false, false]
+    is_benefit_unit_non_dependant_for_universal_credit: [false, false]
+    uc_non_dep_deductions: 0
+
+# Sch 4 para 15: no contribution for any non-dependant if the renter, or
+# either joint renter of a couple, receives the PIP daily living component.
+- name: UC, no contribution where the renter receives the PIP daily living component
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+        pip_dl: 3_000
+      son:
+        age: 25
+        is_claimant_or_partner: false
+      aunt:
+        # A non-dependant in another family.
+        age: 30
+    benunits:
+      benunit_1:
+        members: [parent, son]
+      benunit_2:
+        members: [aunt]
+    households:
+      household:
+        members: [parent, son, aunt]
+  output:
+    uc_non_dep_deductions_renter_exempt: [true, false]
+    uc_non_dep_deductions: [0, 0]
+
+- name: UC, no contribution where the renter's partner is blind
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent_1:
+        age: 52
+        is_household_head: true
+        is_claimant_or_partner: true
+      parent_2:
+        age: 50
+        is_claimant_or_partner: true
+        is_blind: true
+      son:
+        age: 25
+        is_claimant_or_partner: false
+    benunits:
+      benunit:
+        members: [parent_1, parent_2, son]
+    households:
+      household:
+        members: [parent_1, parent_2, son]
+  output:
+    uc_non_dep_deductions: 0
+
+# Para 15(1)(a) exempts "any renter who is a single person to whom
+# sub-paragraph (2) applies": the renter's own disability, not a child's.
+- name: UC, a disabled child in the renter's family does not exempt the renter
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+      son:
+        age: 25
+        is_claimant_or_partner: false
+      child:
+        age: 10
+        is_claimant_or_partner: false
+        dla_sc_category: MIDDLE
+    benunits:
+      benunit:
+        members: [parent, son, child]
+    households:
+      household:
+        members: [parent, son, child]
+  output:
+    # A child under 16 is never a non-dependant (WRA 2012 s.40).
+    is_benefit_unit_non_dependant_for_universal_credit: [false, true, false]
+    is_benefit_unit_non_dependant_for_legacy_benefits: [false, true, false]
+    uc_non_dep_deductions_renter_exempt: false
+    uc_non_dep_deductions: 1_158.60
+
+# Sch 4 para 16(2)(d): no contribution for a non-dependant receiving
+# Attendance Allowance.
+- name: UC, no contribution for a non-dependant in the benefit unit receiving Attendance Allowance
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+      son:
+        age: 25
+        is_claimant_or_partner: false
+        attendance_allowance: 3_000
+    benunits:
+      benunit:
+        members: [parent, son]
+    households:
+      household:
+        members: [parent, son]
+  output:
+    uc_non_dep_deduction_exempt: [false, true]
+    uc_non_dep_deductions: 0
+
+# Pension Credit is the claimant's award: an adult son in a family on Pension
+# Credit is not receiving it, so para 16(2)(b) does not exempt him.
+- name: UC, a family's Pension Credit does not exempt the non-dependant in its benefit unit
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 70
+        is_claimant_or_partner: true
+      son:
+        age: 40
+        is_claimant_or_partner: false
+    benunits:
+      benunit:
+        members: [parent, son]
+        pension_credit: 2_000
+  output:
+    uc_non_dep_deduction_exempt: [true, false]
+
+- name: UC, the head's family bears non-dependants in its own benefit unit and in other families
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+      son:
+        age: 25
+        is_claimant_or_partner: false
+      aunt:
+        age: 60
+        is_claimant_or_partner: true
+    benunits:
+      benunit_1:
+        members: [parent, son]
+      benunit_2:
+        members: [aunt]
+    households:
+      household:
+        members: [parent, son, aunt]
+  output:
+    uc_non_dep_deductions: [2_317.20, 0]
+
+# A non-dependant family's own adult member is a non-dependant of the
+# household head, counted once, in the head's claim (Sch 4 para 9(2)(f)).
+- name: UC, an adult in a non-dependant family is the household head's non-dependant
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      head:
+        age: 70
+        is_household_head: true
+        is_claimant_or_partner: true
+      daughter:
+        age: 45
+        is_claimant_or_partner: true
+      grandson:
+        age: 23
+        is_claimant_or_partner: false
+    benunits:
+      benunit_1:
+        members: [head]
+      benunit_2:
+        members: [daughter, grandson]
+    households:
+      household:
+        members: [head, daughter, grandson]
+  output:
+    uc_non_dep_deductions: [2_317.20, 0]
+
+# A sharer of the rent bears the non-dependant in its own benefit unit; the
+# household head's family does not (para 9(2)(f)).
+- name: UC, a sharer of the rent bears the non-dependant in its own benefit unit
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      head:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+      sharer:
+        age: 48
+        is_claimant_or_partner: true
+      sharer_son:
+        age: 26
+        is_claimant_or_partner: false
+    benunits:
+      benunit_1:
+        members: [head]
+      benunit_2:
+        members: [sharer, sharer_son]
+        liable_for_share_of_household_rent: true
+    households:
+      household:
+        members: [head, sharer, sharer_son]
+        rent: 12_000
+  output:
+    uc_non_dep_deductions: [0, 1_158.60]
+
+- name: UC, the housing costs element nets off the contribution for an adult son
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+      son:
+        age: 25
+        is_claimant_or_partner: false
+    benunits:
+      benunit:
+        members: [parent, son]
+        benunit_tenure_type: RENT_FROM_COUNCIL
+        benunit_rent: 6_000
+    households:
+      household:
+        members: [parent, son]
+  output:
+    # £6,000 - £1,158.60.
+    uc_housing_costs_element: 4_841.40
+
+- name: HB, an adult son in the claimant's benefit unit working part time
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+      son:
+        age: 25
+        is_claimant_or_partner: false
+        employment_income: 2_600
+        weekly_hours: 10
+    benunits:
+      benunit:
+        members: [parent, son]
+    households:
+      household:
+        members: [parent, son]
+  output:
+    is_benefit_unit_non_dependant_for_legacy_benefits: [false, true]
+    housing_benefit_non_dep_deductions: 1_060.80
+
+- name: HB, an adult daughter in the claimant's benefit unit working full time on £300 a week
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+      daughter:
+        age: 22
+        is_claimant_or_partner: false
+        employment_income: 15_600
+        weekly_hours: 37.5
+    benunits:
+      benunit:
+        members: [parent, daughter]
+    households:
+      household:
+        members: [parent, daughter]
+  output:
+    housing_benefit_non_dep_deductions: 3_346.20
+
+- name: HB, no deduction for a non-dependant in the benefit unit under 18
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+      son:
+        age: 17
+        is_claimant_or_partner: false
+        is_child_or_qualifying_young_person_for_child_benefit: false
+    benunits:
+      benunit:
+        members: [parent, son]
+    households:
+      household:
+        members: [parent, son]
+  output:
+    is_benefit_unit_non_dependant_for_legacy_benefits: [false, true]
+    housing_benefit_non_dep_deductions: 0
+
+# HB reg 19: a Child Benefit qualifying young person is in the claimant's
+# family, not a non-dependant.
+- name: HB, a young person in the claimant's family is not a non-dependant
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+      daughter:
+        age: 19
+        is_claimant_or_partner: false
+        is_child_or_qualifying_young_person_for_child_benefit: true
+    benunits:
+      benunit:
+        members: [parent, daughter]
+    households:
+      household:
+        members: [parent, daughter]
+  output:
+    is_benefit_unit_non_dependant_for_legacy_benefits: [false, false]
+    housing_benefit_non_dep_deductions: 0
+
+# A joint tenant's adult son normally resides with both joint tenants (reg
+# 3(1) and (4)) and is in neither's family under reg 3(2)(a), so he is a
+# non-dependant of each, apportioned by their shares of the rent (reg 74(5)).
+- name: HB, joint tenants share the deduction for one tenant's adult son
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      head:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+      son:
+        age: 25
+        is_claimant_or_partner: false
+        employment_income: 2_600
+        weekly_hours: 10
+      sharer:
+        age: 48
+        is_claimant_or_partner: true
+    benunits:
+      benunit_1:
+        members: [head, son]
+      benunit_2:
+        members: [sharer]
+        liable_for_share_of_household_rent: true
+    households:
+      household:
+        members: [head, son, sharer]
+        rent: 12_000
+  output:
+    share_of_household_rent: [0.5, 0.5]
+    housing_benefit_non_dep_deductions: [530.40, 530.40]
+
+# The same son coded as his own benefit unit gives the same deductions: the
+# result does not depend on how the survey groups him.
+- name: HB, joint tenants share the deduction for an adult son in his own benefit unit
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      head:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+      son:
+        age: 25
+        is_claimant_or_partner: true
+        employment_income: 2_600
+        weekly_hours: 10
+      sharer:
+        age: 48
+        is_claimant_or_partner: true
+    benunits:
+      benunit_1:
+        members: [head]
+      benunit_2:
+        members: [sharer]
+        liable_for_share_of_household_rent: true
+      benunit_3:
+        members: [son]
+    households:
+      household:
+        members: [head, son, sharer]
+        rent: 12_000
+  output:
+    housing_benefit_non_dep_deductions: [530.40, 530.40, 0]
+
+# A lodger's adult son is the lodger's non-dependant, and the householder's
+# too: reg 3(2)(e)(i) excludes only the person liable to pay the householder.
+# The lodger is not a joint occupier with the householder, so reg 74(5) does
+# not apportion him and each bears the whole deduction.
+- name: HB, a lodger's adult son is a non-dependant of the lodger and of the householder
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      head:
+        age: 60
+        is_household_head: true
+        is_claimant_or_partner: true
+      lodger:
+        age: 45
+        is_claimant_or_partner: true
+        rent_paid_as_lodger: 5_200
+      lodger_son:
+        age: 21
+        is_claimant_or_partner: false
+        employment_income: 2_600
+        weekly_hours: 10
+    benunits:
+      benunit_1:
+        members: [head]
+      benunit_2:
+        members: [lodger, lodger_son]
+    households:
+      household:
+        members: [head, lodger, lodger_son]
+        rent: 9_000
+  output:
+    housing_benefit_non_dep_deductions: [1_060.80, 1_060.80]
+
+- name: CTR, the claimant's family bears the non-dependant in its benefit unit
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+      son:
+        age: 25
+        is_claimant_or_partner: false
+        council_tax_reduction_individual_non_dep_deduction: 300
+    benunits:
+      benunit:
+        members: [parent, son]
+    households:
+      household:
+        members: [parent, son]
+  output:
+    council_tax_reduction_individual_non_dep_deduction_eligible: [false, true]
+    council_tax_reduction_non_dep_deductions: 300
+
+- name: CTR, no deduction for a non-dependant in the benefit unit under 18
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+      son:
+        age: 17
+        is_claimant_or_partner: false
+        is_child_or_qualifying_young_person_for_child_benefit: false
+    benunits:
+      benunit:
+        members: [parent, son]
+    households:
+      household:
+        members: [parent, son]
+  output:
+    council_tax_reduction_individual_non_dep_deduction_eligible: [false, false]
+    council_tax_reduction_non_dep_deductions: 0
+
+# An applicant's adult son normally resides with both jointly liable people
+# and is in neither's family (SI 2012/2885 reg 9(1), (2)(a)), so the deduction
+# is apportioned equally between them (Sch 1 para 8(5)).
+- name: CTR, jointly liable applicants share the deduction for one applicant's adult son
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      head:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+      son:
+        age: 25
+        is_claimant_or_partner: false
+        council_tax_reduction_individual_non_dep_deduction: 300
+      sharer:
+        age: 48
+        is_claimant_or_partner: true
+    benunits:
+      benunit_1:
+        members: [head, son]
+      benunit_2:
+        members: [sharer]
+        liable_for_share_of_household_rent: true
+    households:
+      household:
+        members: [head, son, sharer]
+        rent: 12_000
+  output:
+    council_tax_reduction_joint_liability_share: [0.5, 0.5]
+    council_tax_reduction_non_dep_deductions: [150, 150]
+
+# Para 8(5) still apportions a non-dependant from another family between the
+# jointly liable people.
+- name: CTR, jointly liable families share the deduction for another family's adult
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      head:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+      sharer:
+        age: 48
+        is_claimant_or_partner: true
+      friend:
+        age: 30
+        is_claimant_or_partner: true
+        council_tax_reduction_individual_non_dep_deduction: 300
+    benunits:
+      benunit_1:
+        members: [head]
+      benunit_2:
+        members: [sharer]
+        liable_for_share_of_household_rent: true
+      benunit_3:
+        members: [friend]
+    households:
+      household:
+        members: [head, sharer, friend]
+        rent: 12_000
+  output:
+    council_tax_reduction_non_dep_deductions: [150, 150, 0]
+
+# SI 2012/2885 reg 9(2)(e) excludes only the person liable to pay the
+# applicant, so a lodger's adult son who pays nothing himself is the
+# applicant's non-dependant.
+- name: CTR, a lodger's adult son is the applicant's non-dependant
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      head:
+        age: 60
+        is_household_head: true
+        is_claimant_or_partner: true
+      lodger:
+        age: 45
+        is_claimant_or_partner: true
+        rent_paid_as_lodger: 5_200
+      lodger_son:
+        age: 21
+        is_claimant_or_partner: false
+        council_tax_reduction_individual_non_dep_deduction: 300
+    benunits:
+      benunit_1:
+        members: [head]
+      benunit_2:
+        members: [lodger, lodger_son]
+    households:
+      household:
+        members: [head, lodger, lodger_son]
+        rent: 9_000
+  output:
+    council_tax_reduction_individual_non_dep_deduction_eligible: [false, false, true]
+    council_tax_reduction_non_dep_deductions: [300, 0]
+
+# Only a family that claims bears deductions; the non-dependant's own family
+# does not.
+- name: CTR, a non-dependant family with an adult member bears no deductions itself
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      head:
+        age: 70
+        is_household_head: true
+        is_claimant_or_partner: true
+      daughter:
+        age: 45
+        is_claimant_or_partner: true
+        council_tax_reduction_individual_non_dep_deduction: 200
+      grandson:
+        age: 23
+        is_claimant_or_partner: false
+        council_tax_reduction_individual_non_dep_deduction: 100
+    benunits:
+      benunit_1:
+        members: [head]
+      benunit_2:
+        members: [daughter, grandson]
+    households:
+      household:
+        members: [head, daughter, grandson]
+  output:
+    council_tax_reduction_individual_non_dep_deduction_eligible: [false, true, true]
+    council_tax_reduction_non_dep_deductions: [300, 0]
+
+# The English working-age local schemes keep their own aggregation, which
+# charges only other families' non-dependants. Widening the shared
+# eligibility must not make a jointly liable sharer bear the head's own son.
+- name: Local CTR (Newham), a jointly liable sharer does not bear the head's own in-unit son
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    people:
+      head:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+      son:
+        age: 25
+        is_claimant_or_partner: false
+        employment_income: 20_000
+        weekly_hours: 37.5
+      sharer:
+        age: 48
+        is_claimant_or_partner: true
+    benunits:
+      benunit_1:
+        members: [head, son]
+        would_claim_council_tax_reduction: true
+      benunit_2:
+        members: [sharer]
+        liable_for_share_of_household_rent: true
+        would_claim_council_tax_reduction: true
+    households:
+      household:
+        members: [head, son, sharer]
+        rent: 12_000
+        region: LONDON
+        local_authority: NEWHAM
+        tenure_type: RENT_PRIVATELY
+        council_tax: 1_800
+  output:
+    council_tax_reduction_joint_liability_share: [0.5, 0.5]
+    newham_council_tax_reduction_non_dep_deductions: [0, 0]
+    newham_council_tax_reduction: [630, 630]
+
+# SI 2012/2885 reg 9(2)(a): the applicant's own family are not its
+# non-dependants, so an individual deduction set for the applicant (here as
+# an input) is never charged to the applicant's own claim.
+- name: CTR, an applicant is never charged a deduction for itself
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      head:
+        age: 50
+        is_household_head: true
+        is_claimant_or_partner: true
+        council_tax_reduction_individual_non_dep_deduction: 300
+      friend:
+        age: 30
+        is_claimant_or_partner: true
+        council_tax_reduction_individual_non_dep_deduction: 200
+    benunits:
+      benunit_1:
+        members: [head]
+      benunit_2:
+        members: [friend]
+    households:
+      household:
+        members: [head, friend]
+        rent: 12_000
+  output:
+    council_tax_reduction_claimant_benunit: [true, false]
+    council_tax_reduction_non_dep_deductions: [200, 0]
diff --git a/policyengine_uk/tests/policy/baseline/finance/benefit/family/housing_benefit/non_dep_deduction/housing_benefit_individual_non_dep_deduction_eligible.yaml b/policyengine_uk/tests/policy/baseline/finance/benefit/family/housing_benefit/non_dep_deduction/housing_benefit_individual_non_dep_deduction_eligible.yaml
index 0307c37986..3d8f2812d9 100644
--- a/policyengine_uk/tests/policy/baseline/finance/benefit/family/housing_benefit/non_dep_deduction/housing_benefit_individual_non_dep_deduction_eligible.yaml
+++ b/policyengine_uk/tests/policy/baseline/finance/benefit/family/housing_benefit/non_dep_deduction/housing_benefit_individual_non_dep_deduction_eligible.yaml
@@ -1,7 +1,7 @@
 - name: Eligible
   period: 2025
   input:
-    benunit_is_rent_liable: false
+    is_non_dependant_of_household_head: true
     age: 18
   output:
     housing_benefit_individual_non_dep_deduction_eligible: true
@@ -9,7 +9,7 @@
 - name: Age ineligible
   period: 2025
   input:
-    benunit_is_rent_liable: false
+    is_non_dependant_of_household_head: true
     age: 17
   output:
     housing_benefit_individual_non_dep_deduction_eligible: false
@@ -21,3 +21,23 @@
     age: 18
   output:
     housing_benefit_individual_non_dep_deduction_eligible: false
+
+# HB Regs 2006 reg 3: a member of the claimant's own benefit unit who is not
+# the claimant, partner or a child or young person.
+- name: Non-dependant within the benefit unit
+  period: 2025
+  input:
+    is_non_dependant_of_household_head: false
+    is_benefit_unit_non_dependant_for_legacy_benefits: true
+    age: 19
+  output:
+    housing_benefit_individual_non_dep_deduction_eligible: true
+
+- name: Non-dependant within the benefit unit, under 18
+  period: 2025
+  input:
+    is_non_dependant_of_household_head: false
+    is_benefit_unit_non_dependant_for_legacy_benefits: true
+    age: 17
+  output:
+    housing_benefit_individual_non_dep_deduction_eligible: false
diff --git a/policyengine_uk/tests/policy/baseline/finance/benefit/family/universal_credit/housing_costs_element/non_dep_deduction/uc_individual_non_dep_deduction.yaml b/policyengine_uk/tests/policy/baseline/finance/benefit/family/universal_credit/housing_costs_element/non_dep_deduction/uc_individual_non_dep_deduction.yaml
index 1f7f3eada3..75d21ea9ce 100644
--- a/policyengine_uk/tests/policy/baseline/finance/benefit/family/universal_credit/housing_costs_element/non_dep_deduction/uc_individual_non_dep_deduction.yaml
+++ b/policyengine_uk/tests/policy/baseline/finance/benefit/family/universal_credit/housing_costs_element/non_dep_deduction/uc_individual_non_dep_deduction.yaml
@@ -6,6 +6,8 @@
       person:
         age: 22
         uc_non_dep_deduction_exempt: false
+        # A member of another family who is not liable for rent.
+        is_non_dependant_of_household_head: true
     benunits:
       benunit:
         members: person
@@ -21,6 +23,7 @@
       person:
         age: 22
         uc_non_dep_deduction_exempt: true
+        is_non_dependant_of_household_head: true
     benunits:
       benunit:
         members: person
@@ -36,6 +39,8 @@
       person:
         age: 20
         uc_non_dep_deduction_exempt: false
+        # A member of another family who is not liable for rent.
+        is_non_dependant_of_household_head: true
     benunits:
       benunit:
         members: person
diff --git a/policyengine_uk/tests/policy/baseline/finance/benefit/family/universal_credit/housing_costs_element/non_dep_deduction/uc_individual_non_dep_deduction_eligible.yaml b/policyengine_uk/tests/policy/baseline/finance/benefit/family/universal_credit/housing_costs_element/non_dep_deduction/uc_individual_non_dep_deduction_eligible.yaml
index 11fe57ddce..99888fb21a 100644
--- a/policyengine_uk/tests/policy/baseline/finance/benefit/family/universal_credit/housing_costs_element/non_dep_deduction/uc_individual_non_dep_deduction_eligible.yaml
+++ b/policyengine_uk/tests/policy/baseline/finance/benefit/family/universal_credit/housing_costs_element/non_dep_deduction/uc_individual_non_dep_deduction_eligible.yaml
@@ -4,7 +4,7 @@
   input:
     age: 21
     uc_non_dep_deduction_exempt: false
-    benunit_is_rent_liable: false
+    is_non_dependant_of_household_head: true
   output:
     uc_individual_non_dep_deduction_eligible: true
 
@@ -14,6 +14,40 @@
   input:
     age: 20
     uc_non_dep_deduction_exempt: false
-    benunit_is_rent_liable: false
+    is_non_dependant_of_household_head: true
+  output:
+    uc_individual_non_dep_deduction_eligible: false
+
+- name: Exempt
+  period: 2020
+  absolute_error_margin: 0
+  input:
+    age: 21
+    uc_non_dep_deduction_exempt: true
+    is_non_dependant_of_household_head: true
+  output:
+    uc_individual_non_dep_deduction_eligible: false
+
+# UC Regs 2013 Sch 4 para 9(1)(c): a member of the renter's own benefit unit
+# who is not the claimant, partner or a qualifying young person.
+- name: Non-dependant within the benefit unit
+  period: 2020
+  absolute_error_margin: 0
+  input:
+    age: 25
+    uc_non_dep_deduction_exempt: false
+    is_non_dependant_of_household_head: false
+    is_benefit_unit_non_dependant_for_universal_credit: true
+  output:
+    uc_individual_non_dep_deduction_eligible: true
+
+- name: The renter is not their own non-dependant
+  period: 2020
+  absolute_error_margin: 0
+  input:
+    age: 25
+    uc_non_dep_deduction_exempt: false
+    is_non_dependant_of_household_head: false
+    is_benefit_unit_non_dependant_for_universal_credit: false
   output:
     uc_individual_non_dep_deduction_eligible: false
diff --git a/policyengine_uk/tests/test_benefit_unit_non_dependant_properties.py b/policyengine_uk/tests/test_benefit_unit_non_dependant_properties.py
new file mode 100644
index 0000000000..45a5ab4714
--- /dev/null
+++ b/policyengine_uk/tests/test_benefit_unit_non_dependant_properties.py
@@ -0,0 +1,428 @@
+"""Property-based tests for non-dependants within a benefit unit.
+
+A benefit unit can include members aged 16 or over who are neither the
+claimant or partner nor a qualifying young person: in law they are
+non-dependants of the renter (UC Regs 2013 Sch 4 para 9; HB Regs 2006 reg 3;
+CTR SI 2012/2885 reg 9 and equivalents). Invariants, for any generated
+population of households:
+
+1. Oracle: each family's Universal Credit, Housing Benefit and Council Tax
+   Reduction non-dependant deductions, and who is eligible for an individual
+   deduction, equal an independent calculation from the generated household
+   structure: who is a non-dependant, of whom, and how a non-dependant of
+   several joint occupiers is apportioned (by rent or liability share,
+   whichever family the non-dependant is in).
+2. Conservation: every Housing Benefit non-dependant deduction is borne in
+   full across the joint occupiers, and once more by a boarder or lodger for
+   a non-dependant in its own benefit unit; each Council Tax Reduction
+   claiming family bears every non-dependant in proportion to its
+   joint-liability share; every Universal Credit contribution is borne by
+   exactly one family unless that family's renter is exempt.
+3. Exemptions: a family whose renter is exempt under Sch 4 para 15 has no
+   Universal Credit deductions; no deduction is charged for a non-dependant
+   under 21 (UC) or under 18 (HB, CTR), or for a qualifying young person.
+4. No-op: in households without non-dependants inside a benefit unit, the
+   deductions equal the previous formulas, which charged only members of
+   other families.
+5. Bedrooms: every Universal Credit non-dependant within a benefit unit has a
+   bedroom in the size criteria (para 10(1)(c)), so the deduction and the
+   bedroom use one definition.
+"""
+
+import numpy as np
+from hypothesis import HealthCheck, given, settings
+from hypothesis import strategies as st
+
+from policyengine_uk import Simulation
+
+# Situation inputs without a period key are set for 2025, so the properties
+# are checked in 2025, when the contribution was £93.02 a month.
+YEAR = 2025
+UC_CONTRIBUTION = 93.02 * 12
+PROPERTY_SETTINGS = settings(
+    max_examples=10,
+    deadline=None,
+    derandomize=True,
+    suppress_health_check=[HealthCheck.too_slow, HealthCheck.data_too_large],
+)
+# Members of a benefit unit who are not the claimant or partner.
+#   dependant: a qualifying young person the claimant is responsible for;
+#   looked_after: a qualifying young person placed by a local authority, for
+#     whom no one in the family is responsible;
+#   non_dependant: aged 16 or over and not a qualifying young person.
+KINDS = ["dependant", "looked_after", "non_dependant"]
+ROLES = ["sharer", "lodger", "non_dependant"]
+
+
+@st.composite
+def extra_member(draw):
+    kind = draw(st.sampled_from(KINDS))
+    age = draw(st.integers(16, 19) if kind != "non_dependant" else st.integers(16, 60))
+    return dict(
+        kind=kind,
+        age=age,
+        earnings=draw(st.sampled_from([0, 2_600, 15_600, 30_000])),
+        hours=draw(st.sampled_from([0, 10, 37.5])),
+        attendance_allowance=draw(st.booleans()),
+    )
+
+
+@st.composite
+def family(draw, role):
+    # The household head's family has the household's oldest members, so it
+    # is unambiguously the head's (and the council tax claimant's) family.
+    adult_age = st.integers(61, 90) if role == "head" else st.integers(25, 59)
+    return dict(
+        role=role,
+        adult_ages=draw(st.lists(adult_age, min_size=1, max_size=2)),
+        extras=draw(st.lists(extra_member(), min_size=0, max_size=2)),
+        child=draw(st.booleans()),
+        renter_pip=draw(st.booleans()),
+        payment=draw(st.floats(1_000, 10_000, allow_nan=False)),
+    )
+
+
+@st.composite
+def household(draw):
+    head = draw(family("head"))
+    others = [
+        draw(family(draw(st.sampled_from(ROLES))))
+        for _ in range(draw(st.integers(0, 2)))
+    ]
+    return dict(
+        families=[head] + others,
+        rent=draw(st.floats(3_000, 20_000, allow_nan=False)),
+        tenure=draw(st.sampled_from(["RENT_FROM_COUNCIL", "RENT_PRIVATELY"])),
+    )
+
+
+population = st.lists(household(), min_size=1, max_size=5)
+
+
+def build(population, convert=None):
+    """One situation for the whole population, and a record per person and
+    per family for the oracle. convert maps extra members' kinds to others."""
+    people, benunits, homes = {}, {}, {}
+    person_rows, family_rows = [], []
+    for h, house in enumerate(population):
+        members = []
+        for f, fam in enumerate(house["families"]):
+            name = f"h{h}_f{f}"
+            ids = []
+            head_family = f == 0
+            # The head's family has the household's oldest member.
+            for i, age in enumerate(fam["adult_ages"]):
+                pid = f"{name}_adult_{i}"
+                pip = fam["renter_pip"] and i == 0
+                people[pid] = {
+                    "age": age,
+                    "is_claimant_or_partner": True,
+                    "is_household_head": head_family and i == 0,
+                    "pip_dl": 3_000.0 if pip else 0.0,
+                }
+                ids.append(pid)
+                person_rows.append(
+                    dict(
+                        household=h,
+                        family=len(family_rows),
+                        kind="adult",
+                        age=age,
+                        disability_benefit=pip,
+                    )
+                )
+            for i, extra in enumerate(fam["extras"]):
+                kind = (convert or {}).get(extra["kind"], extra["kind"])
+                pid = f"{name}_extra_{i}"
+                young = kind != "non_dependant"
+                people[pid] = {
+                    "age": min(extra["age"], 19) if young else extra["age"],
+                    "is_claimant_or_partner": False,
+                    "is_household_head": False,
+                    "is_qualifying_young_person_for_universal_credit": young,
+                    "is_child_or_qualifying_young_person_for_child_benefit": young,
+                    "is_looked_after_by_local_authority": kind == "looked_after",
+                    "employment_income": float(extra["earnings"]),
+                    "weekly_hours": float(extra["hours"]),
+                    "attendance_allowance": (
+                        3_000.0 if extra["attendance_allowance"] else 0.0
+                    ),
+                }
+                ids.append(pid)
+                person_rows.append(
+                    dict(
+                        household=h,
+                        family=len(family_rows),
+                        kind=kind,
+                        age=people[pid]["age"],
+                        disability_benefit=extra["attendance_allowance"],
+                    )
+                )
+            if fam["child"]:
+                pid = f"{name}_child"
+                people[pid] = {
+                    "age": 8,
+                    "is_claimant_or_partner": False,
+                    "is_household_head": False,
+                }
+                ids.append(pid)
+                person_rows.append(
+                    dict(
+                        household=h,
+                        family=len(family_rows),
+                        kind="child",
+                        age=8,
+                        disability_benefit=False,
+                    )
+                )
+            benunit = {"members": ids}
+            if fam["role"] == "sharer":
+                benunit["liable_for_share_of_household_rent"] = True
+            if fam["role"] == "lodger":
+                people[ids[0]]["rent_paid_as_lodger"] = fam["payment"]
+            benunits[name] = benunit
+            members.extend(ids)
+            family_rows.append(
+                dict(
+                    household=h,
+                    role=fam["role"],
+                    renter_pip=fam["renter_pip"],
+                    n_liable=len(fam["adult_ages"]),
+                )
+            )
+        homes[f"h{h}"] = {
+            "members": members,
+            "rent": house["rent"],
+            "tenure_type": house["tenure"],
+            "brma": "MAIDSTONE",
+        }
+    situation = {"people": people, "benunits": benunits, "households": homes}
+    return situation, person_rows, family_rows
+
+
+def calc(sim, variable, map_to=None):
+    return np.asarray(sim.calculate(variable, YEAR, map_to=map_to), dtype=float)
+
+
+def oracle(person_rows, family_rows, uc, hb, ctr, pension_credit, claims, ctr_share):
+    """Expected deductions per family from the generated structure.
+
+    uc, hb and ctr are each person's individual deduction from the model;
+    the oracle decides independently who is whose non-dependant.
+    """
+    n = len(family_rows)
+    exp_uc, exp_hb, exp_ctr = np.zeros(n), np.zeros(n), np.zeros(n)
+    expected_uc_individual = np.zeros(len(person_rows))
+    households = sorted({f["household"] for f in family_rows})
+    for h in households:
+        fams = [i for i, f in enumerate(family_rows) if f["household"] == h]
+        head = fams[0]
+        # Families liable for the household's rent: the head's and sharers'.
+        liable = [i for i in fams if family_rows[i]["role"] in ("head", "sharer")]
+        total_liable = sum(family_rows[i]["n_liable"] for i in liable)
+        share = {
+            i: (family_rows[i]["n_liable"] / total_liable if i in liable else 0.0)
+            for i in fams
+        }
+        for p, row in enumerate(person_rows):
+            if row["household"] != h:
+                continue
+            f = row["family"]
+            role = family_rows[f]["role"]
+            other_family = role == "non_dependant"
+            own_family = row["kind"] == "non_dependant"
+            # UC: a member of a family not liable for rent is the head's
+            # non-dependant; a non-QYP member aged 16+ of any other family is
+            # their own family's.
+            # Para 16: Attendance Allowance or PIP daily living, or Pension
+            # Credit, which only a claimant or partner receives.
+            uc_exempt = row["disability_benefit"] or (
+                row["kind"] == "adult" and pension_credit[f] > 0
+            )
+            uc_non_dependant = other_family or own_family
+            if uc_non_dependant and row["age"] >= 21 and not uc_exempt:
+                expected_uc_individual[p] = UC_CONTRIBUTION
+            bearer = head if other_family else f
+            if uc_non_dependant and not family_rows[bearer]["renter_pip"]:
+                exp_uc[bearer] += uc[p]
+            # HB: every non-dependant, in any family, is pooled across the
+            # joint occupiers by rent share; a boarder or lodger, who is not
+            # a joint occupier, also bears its own in full.
+            if other_family or own_family:
+                for i in fams:
+                    exp_hb[i] += share[i] * hb[p]
+                if own_family and role == "lodger":
+                    exp_hb[f] += hb[p]
+            # CTR: every non-dependant, a lodger's included, is pooled across
+            # the claiming families by joint-liability share, except that a
+            # family's own claimant or partner is not its non-dependant.
+            if other_family or own_family:
+                for i in fams:
+                    if i == f and not own_family:
+                        continue
+                    exp_ctr[i] += claims[i] * ctr_share[i] * ctr[p]
+    return exp_uc, exp_hb, exp_ctr, expected_uc_individual
+
+
+def run(population, convert=None):
+    situation, person_rows, family_rows = build(population, convert)
+    sim = Simulation(situation=situation)
+    values = dict(
+        uc=calc(sim, "uc_individual_non_dep_deduction"),
+        hb=calc(sim, "household_benefits_individual_non_dep_deduction"),
+        ctr=calc(sim, "council_tax_reduction_individual_non_dep_deduction"),
+        pension_credit=calc(sim, "pension_credit"),
+        claims=calc(sim, "council_tax_reduction_claimant_benunit"),
+        ctr_share=calc(sim, "council_tax_reduction_joint_liability_share"),
+    )
+    return sim, person_rows, family_rows, values
+
+
+@PROPERTY_SETTINGS
+@given(population)
+def test_deductions_match_the_oracle(population):
+    sim, person_rows, family_rows, v = run(population)
+    exp_uc, exp_hb, exp_ctr, exp_uc_individual = oracle(person_rows, family_rows, **v)
+    # 1. Oracle.
+    assert np.allclose(v["uc"], exp_uc_individual, atol=0.01)
+    assert np.allclose(calc(sim, "uc_non_dep_deductions"), exp_uc, atol=0.01)
+    assert np.allclose(
+        calc(sim, "housing_benefit_non_dep_deductions"), exp_hb, atol=0.01
+    )
+    assert np.allclose(
+        calc(sim, "council_tax_reduction_non_dep_deductions"), exp_ctr, atol=0.01
+    )
+
+    # HB and CTR eligibility: a non-dependant aged 18 or over, either in a
+    # family not liable for rent or within their own benefit unit.
+    def eligible(own_family_roles):
+        return np.array(
+            [
+                row["age"] >= 18
+                and (
+                    family_rows[row["family"]]["role"] == "non_dependant"
+                    or (
+                        row["kind"] == "non_dependant"
+                        and family_rows[row["family"]]["role"] in own_family_roles
+                    )
+                )
+                for row in person_rows
+            ]
+        )
+
+    # Any family's own non-dependants, a boarder's or lodger's included.
+    for variable in (
+        "housing_benefit_individual_non_dep_deduction_eligible",
+        "council_tax_reduction_individual_non_dep_deduction_eligible",
+    ):
+        assert np.array_equal(
+            calc(sim, variable) > 0,
+            eligible(("head", "sharer", "lodger", "non_dependant")),
+        )
+
+
+@PROPERTY_SETTINGS
+@given(population)
+def test_conservation_and_exemptions(population):
+    sim, person_rows, family_rows, v = run(population)
+    ages = np.array([row["age"] for row in person_rows])
+    kinds = np.array([row["kind"] for row in person_rows])
+    # 2. Conservation. HB: each deduction is borne in full across the joint
+    # occupiers, and a lodger's own non-dependant once more by the lodger.
+    hb_families = sim.map_result(
+        calc(sim, "housing_benefit_non_dep_deductions"), "benunit", "household"
+    )
+    lodgers_own = np.array(
+        [
+            row["kind"] == "non_dependant"
+            and family_rows[row["family"]]["role"] == "lodger"
+            for row in person_rows
+        ]
+    )
+    hb_people = sim.map_result(v["hb"] * (1 + lodgers_own), "person", "household")
+    assert np.allclose(hb_families, hb_people, atol=0.01)
+    # CTR: every non-dependant is borne by the claiming families in
+    # proportion to their joint-liability shares (which follow the
+    # regulations' per-person wording, so they need not sum to one).
+    claim_shares = sim.map_result(v["claims"] * v["ctr_share"], "benunit", "household")
+    ctr_families = sim.map_result(
+        calc(sim, "council_tax_reduction_non_dep_deductions"), "benunit", "household"
+    )
+    ctr_people = sim.map_result(v["ctr"], "person", "household")
+    assert np.allclose(ctr_families, claim_shares * ctr_people, atol=0.01)
+    # UC: each contribution is borne once, except where the bearer is exempt.
+    uc_families = sim.map_result(
+        calc(sim, "uc_non_dep_deductions"), "benunit", "household"
+    )
+    uc_people = sim.map_result(v["uc"], "person", "household")
+    assert np.all(uc_families <= uc_people + 0.01)
+    # 3. Exemptions.
+    renter_exempt = calc(sim, "uc_non_dep_deductions_renter_exempt") > 0
+    assert np.all(calc(sim, "uc_non_dep_deductions")[renter_exempt] == 0)
+    assert np.all(v["uc"][ages < 21] == 0)
+    assert np.all(v["hb"][ages < 18] == 0)
+    assert np.all(v["ctr"][ages < 18] == 0)
+    # Nobody in a family liable for rent is charged except its own
+    # non-dependants.
+    not_non_dependant = kinds != "non_dependant"
+    liable_family = np.array(
+        [family_rows[row["family"]]["role"] != "non_dependant" for row in person_rows]
+    )
+    for amounts in (v["uc"], v["hb"], v["ctr"]):
+        assert np.all(amounts[not_non_dependant & liable_family] == 0)
+
+
+@PROPERTY_SETTINGS
+@given(population)
+def test_no_op_without_non_dependants_in_a_benefit_unit(population):
+    every_kind_dependant = {kind: "dependant" for kind in KINDS}
+    sim, person_rows, family_rows, v = run(population, every_kind_dependant)
+    # 4. The previous formulas: only members of other families, charged to
+    # the head's family (UC), by rent share (HB) or by claim share (CTR).
+    person_household = np.array([row["household"] for row in person_rows])
+    family_household = np.array([row["household"] for row in family_rows])
+    other = calc(sim, "is_non_dependant_of_household_head") > 0
+
+    def other_families(amounts):
+        by_household = np.bincount(
+            person_household,
+            weights=amounts * other,
+            minlength=family_household.max() + 1,
+        )
+        return by_household[family_household]
+
+    head_family = np.array([row["role"] == "head" for row in family_rows])
+    renter_exempt = calc(sim, "uc_non_dep_deductions_renter_exempt") > 0
+    assert np.allclose(
+        calc(sim, "uc_non_dep_deductions"),
+        np.where(renter_exempt, 0, head_family * other_families(v["uc"])),
+        atol=0.01,
+    )
+    assert np.allclose(
+        calc(sim, "housing_benefit_non_dep_deductions"),
+        calc(sim, "share_of_household_rent") * other_families(v["hb"]),
+        atol=0.01,
+    )
+    assert np.allclose(
+        calc(sim, "council_tax_reduction_non_dep_deductions"),
+        v["claims"] * v["ctr_share"] * other_families(v["ctr"]),
+        atol=0.01,
+    )
+
+
+@PROPERTY_SETTINGS
+@given(population)
+def test_every_non_dependant_in_a_benefit_unit_has_a_bedroom(population):
+    sim, person_rows, family_rows, v = run(population)
+    # 5. Para 10(1)(b)-(c): a bedroom for each responsible qualifying young
+    # person and each non-dependant within the unit; the rest of the count
+    # (renter, children, other families' non-dependants) is unchanged when
+    # those members are made dependants instead.
+    in_unit = calc(sim, "is_benefit_unit_non_dependant_for_universal_credit")
+    kinds = np.array([row["kind"] for row in person_rows])
+    assert np.array_equal(in_unit > 0, kinds == "non_dependant")
+    rooms = calc(sim, "LHA_allowed_bedrooms")
+    as_dependants, *_ = run(population, {"non_dependant": "dependant"})
+    rooms_as_dependants = calc(as_dependants, "LHA_allowed_bedrooms")
+    # A non-dependant has a bedroom as a responsible qualifying young person
+    # would, so the count is the same either way.
+    assert np.array_equal(rooms, rooms_as_dependants)
diff --git a/policyengine_uk/tests/test_rent_of_sharers_boarders_and_lodgers_properties.py b/policyengine_uk/tests/test_rent_of_sharers_boarders_and_lodgers_properties.py
index c3b3efca5a..823c384da4 100644
--- a/policyengine_uk/tests/test_rent_of_sharers_boarders_and_lodgers_properties.py
+++ b/policyengine_uk/tests/test_rent_of_sharers_boarders_and_lodgers_properties.py
@@ -12,9 +12,11 @@
 3. Tenure: a boarder's or lodger's family rents privately, is never in social
    housing, and is LHA-eligible.
 4. Non-dependants: a family liable for rent is never a non-dependant; only
-   the household head's family has Universal Credit non-dependant
-   deductions; a boarder's or lodger's family has no Housing Benefit or
-   Council Tax Reduction non-dependant deductions.
+   the household head's family has Universal Credit deductions for
+   non-dependants in other families; a boarder's or lodger's family has no
+   Housing Benefit or Council Tax Reduction deductions for them. (The
+   generated families have no non-dependants within their own benefit unit,
+   which test_benefit_unit_non_dependant_properties covers.)
 5. Category: the Universal Credit LHA category does not depend on the
    household input is_shared_accommodation; the Housing Benefit category is
    the shared rate for any family entitled to one bedroom that lacks
diff --git a/policyengine_uk/variables/gov/dwp/housing_benefit/non_dep_deduction/housing_benefit_individual_non_dep_deduction_eligible.py b/policyengine_uk/variables/gov/dwp/housing_benefit/non_dep_deduction/housing_benefit_individual_non_dep_deduction_eligible.py
index 37ea75a311..164a42b907 100644
--- a/policyengine_uk/variables/gov/dwp/housing_benefit/non_dep_deduction/housing_benefit_individual_non_dep_deduction_eligible.py
+++ b/policyengine_uk/variables/gov/dwp/housing_benefit/non_dep_deduction/housing_benefit_individual_non_dep_deduction_eligible.py
@@ -5,10 +5,24 @@ class housing_benefit_individual_non_dep_deduction_eligible(Variable):
     value_type = bool
     entity = Person
     label = "eligible person for the Housing Benefit non-dependent deduction"
+    documentation = (
+        "A non-dependant aged 18 or over: either a member of another family "
+        "who is not liable for rent (see is_non_dependant_of_household_head) "
+        "or a member of their own benefit unit who is not the claimant, "
+        "partner or a child or young person (see "
+        "is_benefit_unit_non_dependant_for_legacy_benefits)."
+    )
     definition_period = YEAR
+    reference = (
+        "https://www.legislation.gov.uk/uksi/2006/213/regulation/3",
+        "https://www.legislation.gov.uk/uksi/2006/213/regulation/74",
+        "https://www.legislation.gov.uk/uksi/2006/214/regulation/55",
+    )
 
     def formula(person, period, parameters):
-        rent_liable = person.benunit("benunit_is_rent_liable", period)
         p = parameters(period).gov.dwp.housing_benefit.non_dep_deduction
+        non_dependant = person("is_non_dependant_of_household_head", period) | person(
+            "is_benefit_unit_non_dependant_for_legacy_benefits", period
+        )
         age_eligible = person("age", period) >= p.age_threshold
-        return age_eligible & ~rent_liable
+        return non_dependant & age_eligible
diff --git a/policyengine_uk/variables/gov/dwp/housing_benefit/non_dep_deduction/housing_benefit_non_dep_deductions.py b/policyengine_uk/variables/gov/dwp/housing_benefit/non_dep_deduction/housing_benefit_non_dep_deductions.py
index e5e9e3a933..66fa7799c8 100644
--- a/policyengine_uk/variables/gov/dwp/housing_benefit/non_dep_deduction/housing_benefit_non_dep_deductions.py
+++ b/policyengine_uk/variables/gov/dwp/housing_benefit/non_dep_deduction/housing_benefit_non_dep_deductions.py
@@ -5,26 +5,53 @@ class housing_benefit_non_dep_deductions(Variable):
     value_type = float
     entity = BenUnit
     label = "non-dependent deductions"
+    documentation = (
+        "Deductions for the claimant's non-dependants: members of other "
+        "families of the household who are not liable for rent, and members "
+        "of a family liable for the rent who are not its claimant, partner or "
+        "a child or young person. Each non-dependant is apportioned between "
+        "joint occupiers by their shares of the rent, whichever family they "
+        "are in. A boarder or lodger also bears the whole deduction for a "
+        "non-dependant in its own benefit unit."
+    )
     definition_period = YEAR
     unit = GBP
     reference = (
         "https://www.legislation.gov.uk/uksi/2006/213/regulation/3",
         "https://www.legislation.gov.uk/uksi/2006/213/regulation/74",
+        "https://www.legislation.gov.uk/uksi/2006/214/regulation/3",
         "https://www.legislation.gov.uk/uksi/2006/214/regulation/55",
+        "https://assets.publishing.service.gov.uk/media/5a7ce2b840f0b6629523c64c/hbgm-a5-calculating-benefit.pdf",
     )
 
     def formula(benunit, period, parameters):
         # Deductions are made for non-dependants residing with the claimant
         # (HB Regs 2006 reg 74; HB (SPC) Regs 2006 reg 55). Joint occupiers,
         # boarders, lodgers and the landlord's household are not
-        # non-dependants (reg 3(2)(d)-(e), 3(4)). A non-dependant of more
-        # than one joint occupier is apportioned between them by their shares
-        # of the payments (reg 74(5); SPC reg 55(5)), so each family liable
-        # for the household's rent bears its share; a boarder or lodger bears
-        # none.
+        # non-dependants (reg 3(2)(d)-(e), 3(4)).
         person = benunit.members
-        deductions = person(
-            "household_benefits_individual_non_dep_deduction", period
-        ) * person("is_non_dependant_of_household_head", period)
+        deductions = person("household_benefits_individual_non_dep_deduction", period)
+        other_family = person("is_non_dependant_of_household_head", period)
+        # A member of a rent-liable family who is not in its claimant's
+        # family under reg 3(2)(a) and (c) is a non-dependant too.
+        own_family = (
+            person("is_benefit_unit_non_dependant_for_legacy_benefits", period)
+            & ~other_family
+        )
+        # A non-dependant normally resides with each joint occupier (reg 3(1)
+        # and (4)), whichever family they belong to: another family's adult,
+        # a joint occupier's own adult son, or a boarder's or lodger's (reg
+        # 3(2)(e)(i) excludes only the person liable to pay the claimant).
+        # One who is a non-dependant of more than one joint occupier is
+        # apportioned between them by their shares of the payments (reg
+        # 74(5); SPC reg 55(5)).
         share = benunit("share_of_household_rent", period)
-        return share * benunit.max(person.household.sum(deductions))
+        from_joint_occupiers = share * benunit.max(
+            person.household.sum(deductions * (other_family | own_family))
+        )
+        # A boarder or lodger is not a joint occupier with the householder,
+        # so it also bears the whole deduction for a non-dependant in its own
+        # benefit unit. The model charges it for no one else.
+        joint_occupier = share > 0
+        from_own_family = where(joint_occupier, 0, benunit.sum(deductions * own_family))
+        return from_joint_occupiers + from_own_family
diff --git a/policyengine_uk/variables/gov/dwp/is_benefit_unit_non_dependant_for_legacy_benefits.py b/policyengine_uk/variables/gov/dwp/is_benefit_unit_non_dependant_for_legacy_benefits.py
new file mode 100644
index 0000000000..7819dc646c
--- /dev/null
+++ b/policyengine_uk/variables/gov/dwp/is_benefit_unit_non_dependant_for_legacy_benefits.py
@@ -0,0 +1,46 @@
+from policyengine_uk.model_api import *
+
+
+class is_benefit_unit_non_dependant_for_legacy_benefits(Variable):
+    value_type = bool
+    entity = Person
+    label = "Non-dependant within the benefit unit for Housing Benefit and Council Tax Reduction"
+    documentation = (
+        "A member of the benefit unit who is neither the claimant or partner "
+        "nor a child or young person: someone aged 16 or over who is not a "
+        "Child Benefit qualifying young person. The benefit unit follows the "
+        "survey's family, which can include an adult child or a 16 to 19 year "
+        "old who is not a qualifying young person. Under the Housing Benefit "
+        "and Council Tax Reduction schemes such a person is not in the "
+        "claimant's family (SSCBA 1992 s.137(1), with young person defined "
+        "by Child Benefit) but normally resides with the claimant, so is a "
+        "non-dependant. A child or young person is either in the family or, "
+        "if placed with the claimant by a local authority, excluded from "
+        "being a non-dependant. Such a person's own benefit awards are not "
+        "modelled, so the exemptions for a non-dependant on an income-related "
+        "benefit or on Universal Credit with no earned income (HB reg 74(8), "
+        "CTR Sch 1 para 8(8)) never apply to them."
+    )
+    definition_period = YEAR
+    reference = (
+        "https://www.legislation.gov.uk/uksi/2006/213/regulation/3",
+        "https://www.legislation.gov.uk/uksi/2006/213/regulation/19",
+        "https://www.legislation.gov.uk/uksi/2006/213/regulation/21",
+        "https://www.legislation.gov.uk/uksi/2006/214/regulation/3",
+        "https://www.legislation.gov.uk/uksi/2012/2885/regulation/9",
+        "https://www.legislation.gov.uk/wsi/2013/3029/regulation/9",
+        "https://www.legislation.gov.uk/ssi/2021/249/regulation/8",
+        "https://www.legislation.gov.uk/ssi/2012/319/regulation/3",
+        "https://www.legislation.gov.uk/ukpga/1992/4/section/137",
+    )
+
+    def formula(person, period, parameters):
+        # HB Regs 2006 and HB (SPC) Regs 2006 reg 3(2)(a), (c) with regs 19
+        # and 21(3); SI 2012/2885 and WSI 2013/3029 reg 9(2)(a), (c); SSI
+        # 2021/249 reg 8(2)(a)-(b); SSI 2012/319 reg 3. A Child Benefit child
+        # is under 16.
+        claimant_or_partner = person("is_claimant_or_partner", period)
+        child_or_young_person = person(
+            "is_child_or_qualifying_young_person_for_child_benefit", period
+        )
+        return ~claimant_or_partner & ~child_or_young_person
diff --git a/policyengine_uk/variables/gov/dwp/is_benefit_unit_non_dependant_for_universal_credit.py b/policyengine_uk/variables/gov/dwp/is_benefit_unit_non_dependant_for_universal_credit.py
new file mode 100644
index 0000000000..bc6b71d000
--- /dev/null
+++ b/policyengine_uk/variables/gov/dwp/is_benefit_unit_non_dependant_for_universal_credit.py
@@ -0,0 +1,36 @@
+from policyengine_uk.model_api import *
+
+
+class is_benefit_unit_non_dependant_for_universal_credit(Variable):
+    value_type = bool
+    entity = Person
+    label = "Non-dependant within the benefit unit for Universal Credit"
+    documentation = (
+        "A member of the benefit unit, aged 16 or over, who is neither the "
+        "claimant or partner nor a qualifying young person. The benefit unit "
+        "follows the survey's family, which can include an adult child or a "
+        "16 to 19 year old who is not a qualifying young person. In Universal "
+        "Credit law such a person is not in the claimant's family but is a "
+        "non-dependant of the renter: they normally live in the accommodation "
+        "and are none of the persons excluded by Sch 4 para 9(2). A qualifying "
+        "young person the renter is responsible for is a member of the "
+        "renter's extended benefit unit, and one no one is responsible for, "
+        "for example one looked after by a local authority, is excluded by "
+        "para 9(2)(g). A child, under 16, is always one or the other."
+    )
+    definition_period = YEAR
+    reference = (
+        "https://www.legislation.gov.uk/uksi/2013/376/schedule/4/paragraph/9",
+        "https://www.legislation.gov.uk/uksi/2013/376/regulation/4",
+        "https://www.legislation.gov.uk/uksi/2013/376/regulation/5",
+        "https://www.legislation.gov.uk/ukpga/2012/5/section/40",
+    )
+
+    def formula(person, period, parameters):
+        # UC Regs 2013 Sch 4 para 9(1)(a)-(b) and (2)(a), (g); WRA 2012 s.40.
+        not_a_child = ~person("is_child_for_universal_credit", period)
+        claimant_or_partner = person("is_claimant_or_partner", period)
+        qualifying_young_person = person(
+            "is_qualifying_young_person_for_universal_credit", period
+        )
+        return not_a_child & ~claimant_or_partner & ~qualifying_young_person
diff --git a/policyengine_uk/variables/gov/dwp/universal_credit/housing_costs_element/non_dep_deduction/uc_individual_non_dep_deduction_eligible.py b/policyengine_uk/variables/gov/dwp/universal_credit/housing_costs_element/non_dep_deduction/uc_individual_non_dep_deduction_eligible.py
index abd9869256..d47928de45 100644
--- a/policyengine_uk/variables/gov/dwp/universal_credit/housing_costs_element/non_dep_deduction/uc_individual_non_dep_deduction_eligible.py
+++ b/policyengine_uk/variables/gov/dwp/universal_credit/housing_costs_element/non_dep_deduction/uc_individual_non_dep_deduction_eligible.py
@@ -5,14 +5,27 @@ class uc_individual_non_dep_deduction_eligible(Variable):
     value_type = bool
     entity = Person
     label = "Eligible person for the Universal Credit non-dependent deduction"
+    documentation = (
+        "A non-dependant of a renter, aged 21 or over and not otherwise "
+        "exempt: either a member of another family who is not liable for rent "
+        "(see is_non_dependant_of_household_head) or a member of their own "
+        "benefit unit who is not the claimant, partner or a qualifying young "
+        "person (see is_benefit_unit_non_dependant_for_universal_credit)."
+    )
     definition_period = YEAR
+    reference = (
+        "https://www.legislation.gov.uk/uksi/2013/376/schedule/4/paragraph/9",
+        "https://www.legislation.gov.uk/uksi/2013/376/schedule/4/paragraph/13",
+        "https://www.legislation.gov.uk/uksi/2013/376/schedule/4/paragraph/16",
+    )
 
     def formula(person, period, parameters):
-        not_rent_liable = ~person.benunit("benunit_is_rent_liable", period)
         p = parameters(
             period
         ).gov.dwp.universal_credit.elements.housing.non_dep_deduction
-        age = person("age", period)
-        age_eligible = age >= p.age_threshold
+        non_dependant = person("is_non_dependant_of_household_head", period) | person(
+            "is_benefit_unit_non_dependant_for_universal_credit", period
+        )
+        age_eligible = person("age", period) >= p.age_threshold
         exempt = person("uc_non_dep_deduction_exempt", period)
-        return ~exempt & age_eligible & not_rent_liable
+        return non_dependant & age_eligible & ~exempt
diff --git a/policyengine_uk/variables/gov/dwp/universal_credit/housing_costs_element/non_dep_deduction/uc_non_dep_deduction_exempt.py b/policyengine_uk/variables/gov/dwp/universal_credit/housing_costs_element/non_dep_deduction/uc_non_dep_deduction_exempt.py
index 432cfb0936..512d2e32b3 100644
--- a/policyengine_uk/variables/gov/dwp/universal_credit/housing_costs_element/non_dep_deduction/uc_non_dep_deduction_exempt.py
+++ b/policyengine_uk/variables/gov/dwp/universal_credit/housing_costs_element/non_dep_deduction/uc_non_dep_deduction_exempt.py
@@ -11,13 +11,18 @@ class uc_non_dep_deduction_exempt(Variable):
         "receiving Pension Credit, the middle or higher rate of the DLA care "
         "component, the PIP daily living component or Attendance Allowance, or "
         "who is entitled to Carer's Allowance (underlying entitlement included, "
-        "so the carer test is is_carer_for_benefits rather than receipt)."
+        "so the carer test is is_carer_for_benefits rather than receipt). "
+        "Pension Credit is awarded to a benefit unit's claimant and partner, "
+        "so another adult in the unit is not receiving it."
     )
     reference = "https://www.legislation.gov.uk/uksi/2013/376/schedule/4/paragraph/16"
 
     def formula(person, period, parameters):
+        receives_pension_credit = person("is_claimant_or_partner", period) & (
+            person.benunit("pension_credit", period) > 0
+        )
         return (
-            (person.benunit("pension_credit", period) > 0)
+            receives_pension_credit
             | person("dla_sc_middle_plus", period)
             | (person("pip_dl", period) > 0)
             | (person("attendance_allowance", period) > 0)
diff --git a/policyengine_uk/variables/gov/dwp/universal_credit/housing_costs_element/non_dep_deduction/uc_non_dep_deductions.py b/policyengine_uk/variables/gov/dwp/universal_credit/housing_costs_element/non_dep_deduction/uc_non_dep_deductions.py
index eca4abcd2a..20509cd891 100644
--- a/policyengine_uk/variables/gov/dwp/universal_credit/housing_costs_element/non_dep_deduction/uc_non_dep_deductions.py
+++ b/policyengine_uk/variables/gov/dwp/universal_credit/housing_costs_element/non_dep_deduction/uc_non_dep_deductions.py
@@ -5,23 +5,42 @@ class uc_non_dep_deductions(Variable):
     value_type = float
     entity = BenUnit
     label = "Universal Credit non-dependent deductions"
+    documentation = (
+        "Housing cost contributions for the renter's non-dependants: those in "
+        "other families of the household, for the household head's family "
+        "only, and those within the renter's own benefit unit. None if the "
+        "renter or a joint renter is exempt."
+    )
     definition_period = YEAR
     unit = GBP
     reference = (
         "https://www.legislation.gov.uk/uksi/2013/376/schedule/4/paragraph/9",
         "https://www.legislation.gov.uk/uksi/2013/376/schedule/4/paragraph/13",
+        "https://www.legislation.gov.uk/uksi/2013/376/schedule/4/paragraph/15",
     )
 
     def formula(benunit, period, parameters):
         # A housing cost contribution is deducted for each non-dependant in
         # the renter's extended benefit unit (UC Regs 2013 Sch 4 para 13).
-        # Non-dependants live outside the household head's family and are
-        # not liable for rent, and only the household head's family has them
-        # (para 9(2)(d)-(f)): a sharer, boarder or lodger has none from the
-        # household head's family.
         person = benunit.members
-        deductions = person("uc_individual_non_dep_deduction", period) * person(
-            "is_non_dependant_of_household_head", period
-        )
+        deductions = person("uc_individual_non_dep_deduction", period)
+        # Non-dependants from other families live outside the household
+        # head's family and are not liable for rent. A non-dependant counts
+        # in one claim only (para 9(2)(f)), which the model gives to the
+        # household head's family, so a sharer, boarder or lodger has none
+        # from other families.
+        other_family = person("is_non_dependant_of_household_head", period)
         head_family = benunit.any(person("is_household_head", period))
-        return head_family * benunit.max(person.household.sum(deductions))
+        from_other_families = head_family * benunit.max(
+            person.household.sum(deductions * other_family)
+        )
+        # A member of the renter's own benefit unit who is not the claimant,
+        # partner or a qualifying young person is a non-dependant of the
+        # renter (para 9(1)(c) and (2)(a), (g)).
+        own_family = (
+            person("is_benefit_unit_non_dependant_for_universal_credit", period)
+            & ~other_family
+        )
+        from_own_family = benunit.sum(deductions * own_family)
+        renter_exempt = benunit("uc_non_dep_deductions_renter_exempt", period)
+        return where(renter_exempt, 0, from_other_families + from_own_family)
diff --git a/policyengine_uk/variables/gov/dwp/universal_credit/housing_costs_element/non_dep_deduction/uc_non_dep_deductions_renter_exempt.py b/policyengine_uk/variables/gov/dwp/universal_credit/housing_costs_element/non_dep_deduction/uc_non_dep_deductions_renter_exempt.py
new file mode 100644
index 0000000000..187b0ec1ab
--- /dev/null
+++ b/policyengine_uk/variables/gov/dwp/universal_credit/housing_costs_element/non_dep_deduction/uc_non_dep_deductions_renter_exempt.py
@@ -0,0 +1,31 @@
+from policyengine_uk.model_api import *
+
+
+class uc_non_dep_deductions_renter_exempt(Variable):
+    value_type = bool
+    entity = BenUnit
+    label = "Universal Credit renter exempt from all housing cost contributions"
+    documentation = (
+        "No housing cost contribution is deducted for any non-dependant if the "
+        "renter, or either joint renter of a couple, is blind or receives "
+        "Attendance Allowance, the middle or highest rate of the DLA care "
+        "component or the PIP daily living component. Scottish disability "
+        "assistance (adult and pension age disability payments, child "
+        "disability payment, Scottish adult DLA) has no separate input, and "
+        "entitlement suspended in hospital is not modelled."
+    )
+    definition_period = YEAR
+    reference = "https://www.legislation.gov.uk/uksi/2013/376/schedule/4/paragraph/15"
+
+    def formula(benunit, period, parameters):
+        # UC Regs 2013 Sch 4 para 15: "joint renters" are joint claimants
+        # (para 1(2)), so the claimant and partner.
+        person = benunit.members
+        renter = person("is_claimant_or_partner", period)
+        qualifying = (
+            person("is_blind", period)
+            | person("dla_sc_middle_plus", period)
+            | (person("attendance_allowance", period) > 0)
+            | (person("pip_dl", period) > 0)
+        )
+        return benunit.any(renter & qualifying)
diff --git a/policyengine_uk/variables/gov/local_authorities/council_tax_reduction/_legacy.py b/policyengine_uk/variables/gov/local_authorities/council_tax_reduction/_legacy.py
index 966d230ff7..fd35957f53 100644
--- a/policyengine_uk/variables/gov/local_authorities/council_tax_reduction/_legacy.py
+++ b/policyengine_uk/variables/gov/local_authorities/council_tax_reduction/_legacy.py
@@ -4,6 +4,20 @@
 )
 
 
+def in_other_non_liable_family(person, period):
+    """Whether each person is in a family that is neither the oldest adult's
+    nor liable for the rent. This stands in for SI 2012/2885 reg 9(2)(a) and
+    (d)-(e): such a family is outside the claimant's family, and not jointly
+    liable or paying the claimant. It is a proxy: a family jointly liable for
+    the council tax but paying no rent is wrongly included, which #2009's
+    claimant-family test fixes. The eligibility for a non-dependant deduction
+    and the local schemes' aggregation both use this, so they stay on one
+    definition."""
+    return ~person.benunit("benunit_contains_household_head", period) & ~person.benunit(
+        "benunit_is_rent_liable", period
+    )
+
+
 def is_full_time_student_non_dep(person, period):
     return (person("current_education", period) != EducationType.NOT_IN_EDUCATION) | (
         person("in_HE", period)
@@ -74,7 +88,14 @@ def local_non_dep_deductions(
     individual_deduction_variable,
     one_deduction_for_uc_couples=True,
 ):
-    deductions = benunit.members(individual_deduction_variable, period)
+    # The local schemes still charge only the non-dependants of other
+    # families. Non-dependants within a family's own benefit unit, now
+    # eligible for the national schemes, are left out here until each
+    # council's aggregation is brought into line.
+    person = benunit.members
+    deductions = person(individual_deduction_variable, period) * (
+        in_other_non_liable_family(person, period)
+    )
     deduction_for_benunit = benunit.max(deductions)
     if not one_deduction_for_uc_couples:
         has_uc = benunit("universal_credit", period) > 0
diff --git a/policyengine_uk/variables/gov/local_authorities/council_tax_reduction/council_tax_reduction_individual_non_dep_deduction_eligible.py b/policyengine_uk/variables/gov/local_authorities/council_tax_reduction/council_tax_reduction_individual_non_dep_deduction_eligible.py
index 459598f8d3..2c39ee3daa 100644
--- a/policyengine_uk/variables/gov/local_authorities/council_tax_reduction/council_tax_reduction_individual_non_dep_deduction_eligible.py
+++ b/policyengine_uk/variables/gov/local_authorities/council_tax_reduction/council_tax_reduction_individual_non_dep_deduction_eligible.py
@@ -1,4 +1,7 @@
 from policyengine_uk.model_api import *
+from policyengine_uk.variables.gov.local_authorities.council_tax_reduction._legacy import (
+    in_other_non_liable_family,
+)
 
 
 class council_tax_reduction_individual_non_dep_deduction_eligible(Variable):
@@ -6,23 +9,29 @@ class council_tax_reduction_individual_non_dep_deduction_eligible(Variable):
     entity = Person
     label = "eligible person for CTR non-dependent deduction"
     documentation = (
-        "An adult outside the claimant's family who is not liable for rent. "
-        "Someone jointly and severally liable for the council tax with the "
-        "claimant (a joint tenant or other sharer of the rent) and someone "
-        "liable to the claimant on a commercial basis (a boarder or lodger) "
-        "is not a non-dependant."
+        "A non-dependant aged 18 or over: an adult outside the claimant's "
+        "family who is not liable for rent, or a member of any family's "
+        "benefit unit who is not its claimant, partner or a child or young "
+        "person (see is_benefit_unit_non_dependant_for_legacy_benefits), "
+        "including a boarder's or lodger's adult son. Someone jointly and "
+        "severally liable for the council tax with the claimant (a joint "
+        "tenant or other sharer of the rent) and a boarder or lodger who pays "
+        "the claimant are not non-dependants."
     )
     definition_period = YEAR
     reference = (
         "https://www.legislation.gov.uk/uksi/2012/2885/regulation/9",
-        "https://www.legislation.gov.uk/wsi/2013/3029",
-        "https://www.legislation.gov.uk/ssi/2021/249",
+        "https://www.legislation.gov.uk/wsi/2013/3029/regulation/9",
+        "https://www.legislation.gov.uk/ssi/2021/249/regulation/8",
+        "https://www.legislation.gov.uk/ssi/2012/319/regulation/3",
     )
 
     def formula(person, period, parameters):
-        # SI 2012/2885 reg 9(2)(d)-(e) and the Welsh and Scottish equivalents.
-        return (
-            (person("age", period) >= 18)
-            & ~person.benunit("benunit_contains_household_head", period)
-            & ~person.benunit("benunit_is_rent_liable", period)
-        )
+        # SI 2012/2885 reg 9(2)(a), (c)-(e) and the Welsh and Scottish
+        # equivalents; deductions are for non-dependants aged 18 or over.
+        other_family = in_other_non_liable_family(person, period)
+        # A member of any family's benefit unit who is not in its claimant's
+        # family. Reg 9(2)(e) excludes only the person liable to pay the
+        # applicant, so a boarder's or lodger's adult son is a non-dependant.
+        in_unit = person("is_benefit_unit_non_dependant_for_legacy_benefits", period)
+        return (person("age", period) >= 18) & (other_family | in_unit)
diff --git a/policyengine_uk/variables/gov/local_authorities/council_tax_reduction/council_tax_reduction_non_dep_deductions.py b/policyengine_uk/variables/gov/local_authorities/council_tax_reduction/council_tax_reduction_non_dep_deductions.py
index 6241abf4c7..b2589bfae4 100644
--- a/policyengine_uk/variables/gov/local_authorities/council_tax_reduction/council_tax_reduction_non_dep_deductions.py
+++ b/policyengine_uk/variables/gov/local_authorities/council_tax_reduction/council_tax_reduction_non_dep_deductions.py
@@ -5,16 +5,36 @@ class council_tax_reduction_non_dep_deductions(Variable):
     value_type = float
     entity = BenUnit
     label = "CTR non-dependent deductions"
+    documentation = (
+        "Deductions for a claiming family's non-dependants, whether in its "
+        "own benefit unit or another family's, apportioned equally between "
+        "the jointly liable people."
+    )
     definition_period = YEAR
     unit = GBP
+    reference = (
+        "https://www.legislation.gov.uk/uksi/2012/2885/regulation/9",
+        "https://www.legislation.gov.uk/uksi/2012/2885/schedule/1/paragraph/8",
+    )
 
     def formula(benunit, period, parameters):
-        deductions = benunit.members(
+        person = benunit.members
+        deductions = person(
             "council_tax_reduction_individual_non_dep_deduction", period
         )
-        deductions_in_household = benunit.max(benunit.members.household.sum(deductions))
-        deductions_in_benunit = benunit.sum(deductions)
-        # A non-dependant of two or more jointly liable people is apportioned
-        # equally between them (SI 2012/2885 Sch 1 para 8(5)).
+        claims = benunit("council_tax_reduction_claimant_benunit", period)
+        # Every eligible non-dependant normally resides with each liable
+        # person (SI 2012/2885 reg 9(1)): another family's adult, an
+        # applicant's own adult son, or a boarder's or lodger's (reg 9(2)(e)
+        # excludes only the person liable to pay the applicant). A
+        # non-dependant of two or more jointly liable people is apportioned
+        # equally between them (Sch 1 para 8(5)). Members of the applicant's
+        # own family are not its non-dependants (reg 9(2)(a)).
+        own_family_member = ~person(
+            "is_benefit_unit_non_dependant_for_legacy_benefits", period
+        )
+        non_dependants = benunit.max(person.household.sum(deductions)) - benunit.sum(
+            deductions * own_family_member
+        )
         share = benunit("council_tax_reduction_joint_liability_share", period)
-        return (deductions_in_household - deductions_in_benunit) * share
+        return claims * share * non_dependants