Means-test the Council Tax Reduction applicant and partner, not the benefit unit's claimant

[MaxGhenis]

Stacked on #2009 (ctr-claimant-household-head at 4b84ea0), which is stacked on #2006 and #1896. Merge #2009 first; the diff here is this PR's changes only.

Summary

Council Tax Reduction (CTR) is means-tested on the applicant's own income and their partner's. The applicant is the person liable for the council tax. The model means-tested the benefit unit's claimant and partner (is_claimant_or_partner) instead, and the two are not always the same people.

The case that shows it (#2009 review r3, finding N3): an 80-year-old grandmother heads a household in Scotland and shares a benefit unit with two 17-year-old parents and their baby. #1896's parent-couple rule makes the young parents the benefit unit's claimant and partner. #2009 makes the grandmother the liable person, so the family claims through her. The means test still read the young couple:

Grandmother's private pension	CTR before	CTR after
£0	£1,800	£1,800
£100,000	£1,800	£0

This was already so on #2009's base (b065a48), so #2009 did not introduce it.

What changes

Two new variables say who is assessed:

• council_tax_reduction_head_applies_alone (BenUnit): the household head is liable for the council tax but is not the benefit unit's claimant or partner.
• is_council_tax_reduction_applicant_or_partner (Person):
  • where the family's liable person is the benefit unit's claimant or partner, the claimant and partner, whatever their ages (a partner under 18 is not liable, but their income counts);
  • where the head applies alone, the head only;
  • for a family with no liable person, which cannot claim, the claimant and partner, so nothing changes for it.

Every part of the assessment that read the generic claimant, or the whole benefit unit, now reads the applicant:

Part of the assessment	Before, for the grandmother	After
Applicable income	the young couple's income, plus their Child Benefit	her own income
Personal allowance	a couple's, on the couple's ages, plus the pension-age couple amount (£25,880 a year)	a single pensioner's (£13,312)
Premiums	the couple's disability and caring	her own, at the single rates (council_tax_reduction_premiums)
Non-dependant exemption	not exempt if she is blind (a Merton probe deducted £270.40)	exempt
Income-based benefit passport	the couple's Income Support, JSA or ESA passported her	not passported by their award
Universal Credit route (Merton, Kingston, Newham, Westminster, Oxford)	the couple's award put her on the Universal Credit figures	her own income and allowance
Oxford's Child Benefit deduction	took the couple's Child Benefit off her income	nothing to take off
Pensioner scheme	any pension-age member of the head's family	a pension-age applicant or partner

Where the head applies alone, she is assessed as a single person with no partner or children. The benefit unit's own awards (Child Benefit, Income Support, income-based JSA, income-related ESA, Universal Credit, tax credits) and its children belong to its claimant and partner.

Families whose applicant is the benefit unit's claimant are unchanged, with one exception: the pension-age allowance and the pensioner scheme no longer switch on a pension-age member who is neither the applicant nor the partner.

Law

• The applicant is the person liable for the council tax: SSI 2012/319 reg 14(1) ("A person who is liable to pay council tax under section 75 of the Act"), and the classes in SI 2012/2885 Sch 1 paras 2-3, WSI 2013/3029 regs 22-25 and SSI 2021/249 reg 13 (Give Council Tax Reduction to the household head's family, not the oldest adult's #2009).
• The means test is the applicant's and the partner's:
  • SSI 2012/319 reg 21(1): "The income and capital of an applicant's partner is to be treated as income and capital of the applicant"; regs 23 and 26 calculate "the income and capital of the applicant".
  • SI 2012/2885 Sch 1 para 11: "The income and capital of— (a) an applicant; and (b) any partner of that applicant".
  • WSI 2013/3029 Sch 1 para 5 and Sch 6 para 7: the same words.
  • SSI 2021/249 reg 36: the same rule. Reg 37, like SI 2012/2885 Sch 1 para 12, counts a non-dependant's income only where there is an arrangement to take advantage of the scheme.
• The applicable amount is the applicant's: SI 2012/2885 Sch 1 para 6, SSI 2012/319 reg 20, WSI 2013/3029 Sch 1 para 1.
• The exemption from non-dependant deductions is "the applicant or his partner": SI 2012/2885 Sch 1 para 8(6).
• A pensioner is a person who "has attained the qualifying age for state pension credit": SI 2012/2885 reg 3.

Invariants

These hold for every input, and the property tests check them:

1. The liable head is assessed. A liable household head is always an applicant or partner of their own family. Raising the head's own private pension raises their family's applicable income.
2. Monotone. Raising the head's pension never raises any family's simulated reduction.
3. High income. With £250,000 more pension the head's family gets no simulated reduction, unless an income-based benefit passports it or its applicant has a Universal Credit award.
4. Ordinary families unchanged. Where the head does not apply alone, the applicant and partner are exactly the benefit unit's claimant and partner. Where the head applies alone, the head is the only applicant, and a young parent's earnings do not move the family's applicable income.
5. Two inputs, one answer (differential). A head of any age who shares a benefit unit with parents aged 16 or 17 and their baby gets the same applicable income, applicable amount, premiums, passport, scheme, exemption and reduction as the same head entered as her own benefit unit, with the young family as another, in all eight schemes. Two limits keep the two inputs comparable. Nobody takes up Universal Credit or Income Support (a head inside another couple's benefit unit cannot be given her own award; the couple's awards are supplied as inputs). The parents are under 18 (adult ones would be her non-dependants, and the model charges a deduction only when they are in another benefit unit).

Tests

• YAML, applicant_or_partner.yaml, 11 end-to-end cases. None sets applicable income. 9 fail on Give Council Tax Reduction to the household head's family, not the oldest adult's #2009's head (4b84ea0); the other 2 pin behaviour that should not change (a partner under 18's income counts).

  • The grandmother with no income gets £1,800, and with a £100,000 pension gets £0.
  • In England her £20,000 pension gives £759.60: 1,800 − (20,000 − 1,486 tax − 13,312) × 20%.
  • The young couple's income-based JSA does not passport her.
  • Her premiums follow her own disability, at the single rates.
  • A blind grandmother head has no non-dependant deduction in Merton.
  • The young couple's Universal Credit does not put her on Newham's Universal Credit route, and does not remove her tariff income from capital in Kingston upon Thames.
  • A pension-age relative who is not the applicant or partner does not make a young couple's claim a pensioner's.

• Hypothesis, test_council_tax_reduction_applicant_properties.py: 2 tests, the 5 invariants above, across all eight modelled schemes.

• Mutation checks. Ten mutations each put back one piece of the old behaviour, and every one makes a test fail:

  • the differential property catches seven: premiums, passport, the benefit unit's benefits and children, couple status, exemption, the Universal Credit route and Oxford's Child Benefit deduction;
  • the liable-head property catches applicable income reading the generic claimant;
  • YAML cases catch the other two: Kingston's tariff income, and the pensioner flag reading any member of the head's family.

  The first draft of the differential missed the Universal Credit route, because its savings and bills rarely gave a part-withdrawn award. Its strategy now weights toward those values.

• Regression. All 113 cases in the 12 YAML files that read Council Tax Reduction, its council schemes or the premiums pass. So do Give Council Tax Reduction to the household head's family, not the oldest adult's #2009's and Give sharers, boarders and lodgers their own rent, tenure and LHA category #2006's property tests (test_council_tax_reduction_claimant_properties.py, test_rent_of_sharers_boarders_and_lodgers_properties.py). CI runs the full suite.

Enhanced FRS impact

Real Microsimulation runs, base 4b84ea0 against this branch at 905d0d9, on the same file each time. Later commits change only tests. Aggregates only.

Dataset	Year	CTR, base	CTR, branch	Households that change
Enhanced FRS 2024-25 (e433e532)	2026	£2,360.98m	£2,360.98m	none
Enhanced FRS 2024-25 (e433e532)	2025	£2,304.29m	£2,304.29m	none
Payer build (ed1a2991, uk-data#512)	2026	£2,182.18m	£2,182.18m	none

The reduction is identical for every household in all three runs. So are applicable income, applicable amount, the passport, the pensioner flag, the exemption, net income and poverty status. No household in the microdata has a head who applies alone, and none has a pension-age member of the head's family who is not the applicant or partner. The FRS forms a benefit unit from one adult or a couple and their dependent children, so it does not put parents into a grandparent's benefit unit. The fix therefore changes household calculations entered that way, and no population estimate.

Not in this PR

• Non-dependant deductions for adults in the applicant's own benefit unit. Where the head applies alone and the young couple are 18 or over, they are her non-dependants, and this PR stops counting their income without charging a deduction. Charge non-dependant deductions for non-dependants within the claimant's benefit unit #2017 adds in-unit non-dependants; it should identify them with is_council_tax_reduction_applicant_or_partner for CTR (see Coordination).
• A head's partner or child who is not the benefit unit's claimant, partner or child. The model has no relationship input to find them, so a head who applies alone is single.
• The head's own means-tested benefits. A working-age head who applies alone may have her own Universal Credit, which the model does not calculate inside another couple's benefit unit.
• Children's income. The means test still counts the income of the applicant's children, as before (SSI 2012/319 reg 21(3) and SSI 2021/249 reg 36(2) exclude it).

Coordination

• Assess each Council Tax Reduction claim under its own family's scheme and exemption #2015 (per-family scheme and exemption, also on Give Council Tax Reduction to the household head's family, not the oldest adult's #2009) now reads is_council_tax_reduction_applicant_or_partner in its per-family pensioner and exemption flags, and carries byte-identical copies of this PR's two identity variables. It deletes council_tax_reduction_household_has_pensioner and council_tax_reduction_household_has_non_dep_exemption, which this PR edits. Whichever lands second keeps Assess each Council Tax Reduction claim under its own family's scheme and exemption #2015's per-family variables, and points the two applicant_or_partner.yaml cases that read the household flags at them.
• Apply the benefit cap to mixed-age couples on Universal Credit, and keep them off pensioner Council Tax Reduction #1944 and Give Scotland and Wales working-age council tax reduction their own rules (UC routes, passport, child amounts) #1966 (council_tax_reduction_pensioner): the same swap.
• Charge non-dependant deductions for non-dependants within the claimant's benefit unit #2017 (in-unit non-dependants): for CTR the family's own members are the applicant, the partner and their children, so the non-dependants in a head-alone family are the young couple, not the head.
• Apply Housing Benefit and national Council Tax Reduction non-dependant deductions as legislated #2007 (national exemption): read the applicant or partner.
• Read one resolved household head in every programme #2062 (one resolved household head) deletes council_tax_reduction_household_head, which council_tax_reduction_head_applies_alone, is_council_tax_reduction_applicant_or_partner and the identity property test read. Its is_resolved_household_head has the same formula, so whichever lands second renames it in those three files.
• Give pension-age HB and CTR the severe disability premium Pension Credit uses #1938, Give the legacy severe disability premium its statutory conditions (stacked on #1896) #1946, Use one carer attribution for the legacy severe disability premium and the Pension Credit addition (stacked on #1946 and #1951) #1977, Pay the legacy carer premium per qualifying claimant or partner; IS carer route through them only #2001 (premiums): council_tax_reduction_premiums uses benefits_premiums for ordinary families, so their changes flow through. Its head-alone path mirrors the single-claimant rules, and invariant 5 fails if the two drift apart.

axiom: uk/regulations/uksi/2012/2885/schedule/1/paragraph/11 encoded-correct (tests applicant_income_and_capital_are_calculated, partner_income_and_capital_are_calculated, household_member_is_not_treated_without_polygamous_marriage) | Scotland and Wales (SSI 2012/319 reg 21, SSI 2021/249 reg 36, WSI 2013/3029 Sch 1 para 5 and Sch 6 para 7) TheAxiomFoundation/rulespec-uk#427 queued

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