Charge in-unit non-dependants in the local Council Tax Reduction schemes

[MaxGhenis]

Stacked on #2017 (in-unit-non-dep-deductions), for issue #2014. #2017 charges non-dependants inside the claimant's own benefit unit in UC, HB and the national CTR schemes, and keeps the five English working-age local schemes on their old aggregation behind a mask. This PR brings those schemes (Merton, Kingston upon Thames, Newham, Westminster, Oxford) into line with the law and with #2017's national variable.

The law

Each council's 2026-27 scheme reproduces the Default Scheme's (SI 2012/2886) definitions:

• Merton, Kingston upon Thames and Newham: paras 4, 6 and 9 and the working-age deduction paragraph (30, 30A, 30B).
• Westminster: its summary says the Default Scheme is the basis, and its Appendix A gives the 2026-27 scale.
• Oxford: paras 42-47 and the Annex 1 glossary.

What follows from the texts:

• Who is a non-dependant. Anyone who normally resides with the applicant (para 9), except:

  • a member of the applicant's family (para 6: the partner, or a child under 16 or qualifying young person the applicant is responsible for);
  • a joint occupier;
  • a commercial lodger;
  • a charity-engaged carer.

  So an adult in the applicant's own benefit unit who is not the partner, a child or a qualifying young person is a non-dependant (a 19-year-old who has left education, say, or an adult son). So is each adult in another family. Westminster's guidance agrees ("anyone over 18 living in your home"; partners and children the claimant gets Child Benefit for excepted). So does Oxford's glossary.

• Banding (para 30(1)-(2), para 10). A non-dependant aged 18 or over is banded on his normal gross weekly income, if in remunerative work on their own hours. For a couple, regard is had to the couple's joint income (para 30(4); Oxford para 44).

• Couples (para 30(3)). A couple pays one deduction, the higher of the two. Merton and Kingston upon Thames add "(other than where there is an award of universal credit)"; Newham's 30B(3), Westminster and Oxford have no such exception.

• Joint occupiers (para 30(5)). A non-dependant of two or more jointly liable people is apportioned equally between them.

• Exemptions (para 30(8)). Being "on income support, state pension credit, an income-based jobseeker's allowance or an income-related employment and support allowance", or entitled to UC calculated on no earned income, exempts the person on the award. Oxford applies these to pensioner applicants only (para 47), as before.

Every 2026-27 deduction parameter for the five councils matches its scheme text.

Changes

policyengine_uk/variables/gov/local_authorities/council_tax_reduction/_legacy.py:

• local_non_dep_deductions now has the same shape as Charge non-dependant deductions for non-dependants within the claimant's benefit unit #2017's council_tax_reduction_non_dep_deductions: claims × joint-liability share × (the household's pool of non-dependant deductions, less the claiming family's own claimant, partner and children). The pool counts:

  • the higher member of each non-dependant couple (each member in Merton and Kingston upon Thames when the couple has UC);
  • every other eligible person separately.

  It replaces "the household total minus the family's own maximum", which never charged the applicant's own in-unit non-dependants. It also drops Charge non-dependant deductions for non-dependants within the claimant's benefit unit #2017's temporary in_other_non_liable_family mask from the aggregation. Eligibility still uses that helper.

• normal_gross_income_non_dep_deduction now:

  • bands a claimant or partner on the couple's joint gross income and anyone else on their own, not on the whole benefit unit's income;
  • uses each person's own hours, not the benefit unit's maximum;
  • gates the income-related benefit and UC exemptions to the award's claimant and partner;
  • computes the UC limb on the couple's earned income.

These are #2007's fixes 1 and 3 (couple-only joint income; benefit-receipt exemptions gated to the claimant and partner), carried into the local schemes. They use the same names and semantics as the composition branch in-unit-non-dep-deductions-on-2007.

#2017's Newham case "a jointly liable sharer does not bear the head's own in-unit son" pinned the temporary mask. It now apportions the son's £979.68 half to each jointly liable family (para 30(5)), as #2017's national variable already does.

Open interpretation, flagged with #2007's author. Para 30(8)(c) exempts a non-dependant "entitled to an award of universal credit where the award is calculated on the basis that the person does not have any earned income". This PR reads a joint award on the couple's earnings (today's local behaviour). #2007 reads it on each person's own earnings for the national schemes and HB. Neither of us found guidance that settles it. It only binds for a non-dependant UC couple where exactly one member earns.

Invariants

For every input:

1. Oracle. Each council's individual and family deductions equal an independent calculation from its 2026-27 scale (Hypothesis property 6). Every other council's variables are zero.
2. Independence. The earnings of an adult who is not a claimant or partner change no one else's deduction (property 7).
3. Conservation. Across jointly liable applicants, each non-dependant's deduction is borne once in full (by claims × share). Property 6's oracle implies this.
4. No self-charge. A claiming family is never charged for its own claimant, partner or children. Asserted by Charge non-dependant deductions for non-dependants within the claimant's benefit unit #2017's eligibility property and the oracle's own-family subtraction.

Properties 6 and 7 both fail on #2017's head (the previous aggregation), so they bind. Property 7 adds a guaranteed binding family to every generated household.

Tests

• New YAML (local_scheme_non_dependants.yaml, 19 cases): each council's own in-unit son, a lone parent plus her working son in another family, gated Income Support, JSA and UC exemptions, Merton's UC couple on its own hours, a joint-income couple, the applicant PIP exemption, a full-time student, a 19-year-old who has left education, and Oxford's pensioner-only exemptions. Every expectation is hand-computed from the scheme text.
• Local and in-unit YAML suites: 90 passed (gov/local_authorities, benefit_unit_non_dependants.yaml).
• Property tests: the four existing ones pass; the two new ones pass.

Impact

Real Microsimulation of the released enhanced FRS 2024-25 (uk-data 1.57.4, sha256 03fe15e40a4c), 2026, #2017 head 270412d → 570f163:

• no change to any household;
• council tax reduction £2,360.32m in both runs;
• household net income and poverty unchanged.

Every enhanced-FRS household has local_authority = MAIDSTONE (the variable's default; the dataset carries no local authority), so the five local schemes never run on survey data. The fix changes household-level calculations in those councils only.

Axiom

axiom: TheAxiomFoundation/rulespec-uk#426 queued. The Kingston upon Thames, Merton and Newham modules derive each non-dependant's deduction from their own income and hours, which is encoded-correct given its inputs. They take who is a non-dependant, the exemption and the household total as inputs. Westminster defers Appendix A, and Oxford has no module.

Coordination

• Assess each Council Tax Reduction claim under its own family's scheme and exemption #2015 (ctr-per-family-scheme) rewrites the same two helpers. Whoever lands second, the agreed resolution is:
  • where(applicant_exempt, 0, claims * share * non_dependants);
  • Assess each Council Tax Reduction claim under its own family's scheme and exemption #2015's in_scheme_area for working_age;
  • drop claimant_exempt;
  • keep this PR's couple and own income, own hours and gated exemptions.
• Give Council Tax Reduction to the household head's family, not the oldest adult's #2009 changes council_tax_reduction_claimant_benunit, which the pool reads. Its eligibility line follows Charge non-dependant deductions for non-dependants within the claimant's benefit unit #2017's resolution.
• Apply Housing Benefit and national Council Tax Reduction non-dependant deductions as legislated #2007 doesn't touch _legacy.py.

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