Count only the claimant's and partner's income in the legacy means tests

[MaxGhenis]

Fixes #2385

Summary

Housing Benefit, Income Support, tax credit and Council Tax Reduction applicable income counted the family's own children's and young persons' income, and subtracted their tax, National Insurance and pension contributions. Each formula's comment said the law counts only the claimant's and partner's, and that "dropping dependants' own income is a follow-up". This PR does that.

On main (e01f2f5), 2026-27, England, a lone parent with a 17-year-old at school who earns £8,000 (YAML Case 1):

Variable	main	This PR
housing_benefit_applicable_income	£8,000	£0
income_support_applicable_income	£6,960	£0
tax_credits_applicable_income	£8,000	£0
council_tax_reduction_applicable_income	£8,000	£0

It is needed by #2299 (Child Benefit routes for young people who have left education), whose review ran a working 18-year-old school-leaver: once rightly a young person, their £20,000 of earnings entered the parent's Housing Benefit, Income Support and Council Tax Reduction means tests.

The law

• Housing Benefit. SSCBA 1992 s.136(1) treats a family member's income as the claimant's "except in prescribed circumstances". HB Regs 2006 reg. 25(3): "The income and capital of a child or young person shall not be treated as the income and capital of the claimant." HB (SPC) Regs 2006 reg. 23(3): the same words.
• Income Support. IS Regs 1987 reg. 23(2): "Subject to the following provisions of this Part, the income paid to, or in respect of, and capital of, a child or young person who is a member of the claimant's family shall not be treated as the income or capital of the claimant."
• Tax credits. Tax Credits (Definition and Calculation of Income) Regs 2002 reg. 3(1): "The manner in which income of a claimant or, in the case of a joint claim, the aggregate income of the claimants, is to be calculated [...]"; each step adds up the income "of the claimant, or, in the case of a joint claim, of the claimants".
• Council Tax Reduction. The applicant's and partner's income: SI 2012/2885 Sch. 1 para. 11 and WSI 2013/3029 Sch. 1 para. 5 ("The income and capital of— (a) an applicant; and (b) any partner of that applicant, is to be calculated [...]"); SSI 2012/319 reg. 21(3) and SSI 2021/249 reg. 36(2): "The income and capital of a child or young person is not to be treated as the income and capital of the applicant."

Change

The four formulas' member mask is now is_claimant_or_partner, where it was that or the programme's own child or young person. That covers the income components, the personal benefits, tax, National Insurance and pension contributions they add up. Unchanged: who is a child or young person, the allowances for them, and the benefit-unit awards these formulas add (Child Benefit, Income Support, the claimant-or-partner JSA and ESA, Universal Credit, tax credits). Council Tax Reduction keeps the household head who applies alone as #2288 left them: counted when they are a Child Benefit young person. Counting every such applicant is #2078's change, which overlaps this reader.

Invariants

1. A child's or young person's own income, tax and personal benefits never change any of the four (nor Universal Credit's or Pension Credit's income, which already followed the rule).
2. The claimant's and partner's income still counts in full.
3. Anyone outside the family still never counts (the existing property).

Tests

• Properties, test_means_test_income_properties.py: new test_income_of_children_and_young_persons_never_counts (invariant 1), on generated families with a child under 16 or a young person at school, each with up to £50,000 of earnings, pension and savings income, with every family on and off Universal Credit, across all seven income totals. It first checks that the dependant is each legacy programme's own child or young person, so the property cannot pass by excluding them. The existing outside-member property (invariant 3) is unchanged.
• YAML, new means_tests_young_person_income.yaml, three cases hand-computed (2026-27), each failing on main:

Case	Family	Expected
1	lone parent, no income; 17-year-old at school earning £8,000	all four applicable incomes £0
2	as 1, parent earning £5,000	HB £5,000 − £25 × 52 = £3,700; IS £5,000 − £20 × 52 = £3,960; tax credits and CTR £5,000
3	pension-age HB: grandparent with a £20,000 pension, grandchild of 17 earning £8,000	HB and CTR £20,000 − £1,486 income tax = £18,514 (on main £26,514)

• Changed expectations. council_tax_reduction_young_person.yaml Cases 2 and 3 counted the young person's £2,000 of earnings (£3,406.60); they now give Child Benefit alone, £1,406.60. test_council_tax_reduction_young_person_properties.py invariant 6: a non-applicant young person's earnings never count; the applicant's always do.

Mutation checks at fafd220, each applied alone in a copy of the branch (mutation/prG2.txt):

Mutation	YAML cases that fail	Property test
HB counts dependants again	1, 2, 3	fails
IS counts dependants again	1, 2	fails
Tax credits count dependants again	1, 2	fails
CTR counts dependants again	1, 2, 3, and the Scottish CTR file's Cases 2 and 3	fails
CTR drops the household head who applies alone	the Scottish CTR file's Cases 9 and 10	passes (no head applicant in its families)

Run locally at fafd220:

• policyengine-core test on the 103 YAML files that mention Housing Benefit, Income Support, tax credits or Council Tax Reduction (the new one included): 1,140 passed. Before the two changed CTR expectations, those 2 cases were the only failures.
• pytest on the 18 Python test files that read the four totals: 105 passed.
• The new YAML file on main e01f2f5: all 3 cases fail (HB applicable income £8,000, £11,700 and £26,514 against £0, £3,700 and £18,514).
• ruff format --check and ruff check on policyengine_uk: clean.

The full suites run in CI.

Microsimulation impact

Real runs on Enhanced FRS 2024-25 (uk-data 1.58.0, sha256 5ba399ea5ab9a6c7), main e01f2f5 against this branch at fafd220, one run each for 2024-25 to 2030-31, exact comparison of every record (impact/prG2/ in the session folder).

Household incomes change only in 2024-25, for 2 records (343 weighted households), +£42,197 in working tax credit (mean £123). No household loses, and no Housing Benefit, Income Support, Council Tax Reduction, Universal Credit or Pension Credit award changes in any year: few families in the data have a dependant with income of their own, and those that do are not on these benefits.

The applicable incomes move (£ million a year; records that differ in brackets):

Variable	2024-25	2026-27	2030-31
tax_credits_applicable_income	−840.1 (555)	−913.1 (557)	−1,049.4 (557)
housing_benefit_applicable_income	−0.42 (6)	−0.34 (8)	+0.38 (12)
income_support_applicable_income	−0.46 (4)	−0.34 (8)	+0.38 (12)
council_tax_reduction_applicable_income	−0.42 (6)	−0.34 (8)	+0.38 (12)

Tax credits count investment income at step one, so children's savings and the like drop out there in many more records than in the other three. In the other three, the change can be upward: where a dependant paid income tax on income those formulas never counted, main subtracted the tax from the family's income. In 2030-31 each such rise equals the dependant's own income tax (£337.18, £110.64, £484.63, £2,199.95), and the one fall is a 19-year-old's own social security income of £4,880.24 (impact/prG2/dependants_2030.txt).

Reproducer: policyengine_uk/tests/test_means_test_income_properties.py::test_income_of_children_and_young_persons_never_counts

axiom: SI 2012/2885 Sch 1 para 11 encoded-correct (uk/regulations/uksi/2012/2885/schedule/1/paragraph/11.yaml, companion test household_member_is_not_treated_without_polygamous_marriage); HB Regs 2006 reg 25, HB (SPC) Regs 2006 reg 23, IS Regs 1987 reg 23, SI 2002/2006 reg 3, WSI 2013/3029 Sch 1 para 5, SSI 2012/319 reg 21 and SSI 2021/249 reg 36 TheAxiomFoundation/rulespec-uk#463 queued (blocker stated there: no module cites the rows, and a billed encoder run needs approval)

🤖 Generated with Claude Code