Uprate the Scottish Carer Supplement with the benefit uprating index

[MaxGhenis]

Fixes #1957

What

gov.social_security_scotland.carer_support_payment.supplement is the Scottish Carer Supplement (SSI 2023/302 reg. 16(1A)). It had no uprating metadata, so the model held it at £11.70 a week (£608.40 a year) from 2027-28 onward. Meanwhile the Carer Support Payment component beside it (rate.yaml, reg. 16(1)) rose with gov.benefit_uprating_cpi.

This PR gives the supplement the same uprating: gov.benefit_uprating_cpi and cites the statutory uprating duty. Nothing changes for 2026-27 or earlier: the supplement stays £0 before 2026 and £11.70 in 2026-27.

The parameter edits:

• supplement.yaml adds references to Social Security (Scotland) Act 2018 ss. 86A and 86B and to the 2026-27 s. 86A report (section 3 and the table of 2026-27 rates).
• It drops a dead link: socialsecurity.gov.scot/news-events/news/new-payment-rates-from-april-2026 returns 404.
• rate.yaml, which already uprated, gains the same s. 86B and report references.

Law and uprating policy

Read on legislation.gov.uk and gov.scot on 2026-10-02.

• SSI 2026/170 reg. 12(3). SSI 2026/170 reg. 12(3) is in force from 5 April 2026 (reg. 1(3)). It substitutes three amounts in reg. 16:

  • (a) £83.30 becomes £86.45.
  • (b) £11.29 becomes £11.70.
  • (c) £10.00 becomes £10.40 (the Carer Additional Person Payment).

  Each is the old amount × 1.038, rounded to the nearest 5p. For the supplement, 11.29 × 1.038 = 11.719, which rounds to 11.70.

• The statutory uprating duty, s. 86B. Social Security (Scotland) Act 2018 s. 86B is the "Duty to uprate assistance" (as amended from 10 May 2025 by the Social Security (Amendment) (Scotland) Act 2025). It requires Ministers, before the end of each financial year, to "bring forward legislation to replace any relevant figure ... which is, in their opinion, materially below its inflation-adjusted level with a figure of at least that level (subject to any rounding they think appropriate)".

  • s. 86A(4) defines that level as f × (1 + i/100).
  • s. 86A(5) leaves Ministers to decide the "relevant prices".
  • A "relevant figure" (s. 86A(3)(a)(i)) includes figures in regulations under Chapter 2 of Part 2. Carer's assistance is s. 28, in that Chapter, so reg. 16(1) and (1A) are both relevant figures.

• Ministers' measure is September CPI. The 2026-27 s. 86A report is Social Security Assistance in Scotland: up-rating for inflation in 2026-27, laid 2 February 2026.

  • Section 1.6: all assistance under the 2018 Act "is to be up-rated to the inflation-adjusted level" (s. 86B).
  • Sections 2.1 and 3.1: the measure is "the 12 months to September rate of CPI", with rates "rounded to the nearest five pence".
  • Table 1: the supplement goes from £11.29 to £11.70, alongside the Carer Support Payment component (£83.30 to £86.45).

• Alternatives considered:

  • No uprating (main's behaviour) is contrary to s. 86B.
  • The s. 81 Carer's Allowance Supplement formula (an uprated JSA rate minus Carer's Allowance) does not govern reg. 16(1A): s. 81 is in Part 3 and sets the supplement for Carer's Allowance recipients. Applied anyway, it would give 12.00 / 12.25 / 12.50 / 12.75 a week for 2027-28 to 2030-31, within 5p of this branch.
  • Chained uprating with 5p rounding gives 11.95 / 12.20 / 12.45 / 12.70.
  • The prospective Children (Care, Care Experience and Services Planning) (Scotland) Act 2026 s. 37 applies ss. 86A-86B to adoption allowances and kinship care rates only, so it does not affect carer's assistance.

Model index

gov.benefit_uprating_cpi is the model's index for "the previous September's CPI, applied in April". It is already used by the Carer Support Payment component, Carer's Allowance and other DWP and Scottish benefit parameters. Two limits:

• It is a proxy. Today it lags OBR calendar-year CPI by a year, so it approximates September CPI rather than reproducing it. For example, its April 2026 step is 3.4% (2025 calendar-year CPI), where the actual September 2025 rate was 3.8%.
• No 5p rounding. As for every other uprated parameter, the model does not round projected rates, so 2027-28 is £11.969 a week rather than £11.95.

The unmerged branch benefit-uprating-september-cpi re-bases gov.benefit_uprating_cpi on true September CPI. Because the supplement points at the shared index, it will follow automatically. Whichever of the two merges second will need to refresh this PR's 2027 and 2028 YAML expectations, which hard-code the index values.

gov.economic_assumptions.indices.obr.consumer_price_index, which scottish_child_payment and pawhp use, would apply year-Y CPI to the April of year Y. That is a year early: 2.0% rather than 2.3% for 2027-28.

Values

gov.benefit_uprating_cpi under the OBR March 2026 EFO:

Year	Index
2026	184.3803432
2027	188.6206824
2028	192.3927984

Year	Weekly, main	Weekly, branch	Annual, branch
2026-27	£11.70	£11.70	£608.40
2027-28	£11.70	£11.969	£622.39
2028-29	£11.70	£12.208	£634.84
2029-30	£11.70	£12.453	£647.53
2030-31	£11.70	£12.702	£660.48

Invariants and tests

policyengine_uk/tests/policy/baseline/gov/social_security_scotland/scottish_carer_supplement.yaml has four cases:

• 2026: £608.40.
• 2027: 11.70 * 188.6206824 / 184.3803432 * 52 = £622.39. The Carer Support Payment component rises by the same factor.
• 2028: £634.84.
• 2027, a carer in England: £0.

On main the 2027 and 2028 cases fail. A mutant that freezes 2027 at £608.40 also fails.

policyengine_uk/tests/test_scottish_carer_supplement_uprating.py tests these invariants for every year 2027-2039, the last year of the uprating index:

1. Differential. The supplement and the Carer Support Payment component grow by the same factor from April 2026.
2. Index. That factor is gov.benefit_uprating_cpi relative to 2026, so the supplement never falls while the index does not.
3. Metamorphic (Hypothesis, derandomized). Setting CPI growth for year Y−1 to g in [0, 12%] and rebuilding the parameters scales both components by 1 + g in year Y, to within the 5-decimal rounding of the index.

Run locally on the rebased branch:

• the new and related YAML tests at 55173abc8, after merging main (tests/policy/baseline/gov/social_security_scotland, carer_support_payment.yaml, pension_credit_carer_income.yaml, uc_unearned_benefits.yaml, carers_allowance_overlapping_benefits.yaml): 102 passed;
• at 55173abc8, the new pytest file, test_carer_support_payment.py and test_pension_credit_carer_income_properties.py: 22 passed;
• ruff format --check and ruff check: clean;
• the full YAML policy suite at 2bdb860a8: 1,656 passed, 0 failed (policyengine-core test policyengine_uk/tests/policy -c policyengine_uk).

Enhanced FRS impact

There were two real microsimulation runs on the published Enhanced FRS 2024-25 (policyengine-uk-data-private tag 1.57.4, the latest tag; enhanced_frs_2024_25.h5, sha256 03fe15e4…8e68d4):

• main at 2cbf20cea;
• this branch at 55173abc8, the PR merged with that main.

Each run covers 2027-28 to 2030-31, with totals summed in float64. No figure is scaled or interpolated. Two earlier pairs gave the same figures to the precision shown, except UC in 2028-29 (+£0.027m on 3c48247eb):

• 3c48247eb against the first branch commit;
• 84f5ad43a against 2bdb860a8, before main brought in Reduce Carer's Allowance and Carer Support Payment by overlapping benefits #1994.

#1994 pays the supplement only where Carer Support Payment is still above £0 after the overlapping-benefit reduction, and it removed none of the 100 sample recipients.

Year	Weekly rate: main → branch	Supplement spending	Income tax	Universal Credit	Household net income (all in Scotland)	Recipients (weighted)
2027-28	£11.70 → £11.97	+£1.59m	+£0.18m	+£0.015m	+£1.43m	113.9k
2028-29	£11.70 → £12.21	+£3.02m	+£0.34m	+£0.028m	+£2.71m	114.3k
2029-30	£11.70 → £12.45	+£4.49m	+£0.51m	+£0.042m	+£4.03m	114.8k
2030-31	£11.70 → £12.70	+£6.01m	+£0.77m	+£0.071m	+£5.31m	115.3k

Supplement spending on main is £69.3m to £70.2m a year over 2027-28 to 2030-31. On this branch it is £70.9m in 2027-28, rising to £76.2m in 2030-31.

Per-record checks, every year:

• Who gains. 100 sample people (113.9k-115.3k weighted), all in Scotland and all receiving Carer Support Payment, get the supplement. Each gains exactly 52 × the change in the weekly rate. 96 households gain and none loses. Nothing outside Scotland changes.
• Accounting. Per household, the change in net income equals the change in benefits minus the change in tax, to within £0.02 (float32 storage).
• Income tax. The supplement is chargeable to income tax (SI 2026/93), so tax rises for the 30-32 recipients who pay it.
• Universal Credit. UC rises for 6 benefit units because uc_earned_income deducts the unit's whole tax, including income tax on the supplement, from earnings. That is UC earned income deducts the whole benefit unit's tax, including tax on pensions and other unearned income #1942, open in Deduct only each person's own tax and NI on earnings from UC earned income #1949. UC Regs reg. 66 does not count the supplement as unearned income.
• Unchanged. Pension Credit, Housing Benefit, Council Tax Reduction and tax credits do not change. The income lists for Pension Credit (guarantee_credit/income.yaml), Housing Benefit and CTR leave the supplement out, as SPC Regs reg. 15(1)(ri) and HB Regs Sch 5 para 75 require.
• Poverty. BHC and deep poverty do not change in any year. AHC poverty changes only in 2029-30, when one sample household leaves it. That is −4.3k people in Scotland (−0.07pp of Scotland's population; −0.006pp UK).

An independent recomputation from the per-record arrays reproduced every figure above (144 checks, 0 failures). The scripts (dataset_impact.py, compare_impact.py) and outputs are kept outside the repo.

Axiom

axiom: uk:regulations/ssi/2023/302/16#scottish_carer_supplement_weekly_rate encoded-correct | TheAxiomFoundation/rulespec-uk#431 queued

• Encoded-correct. The enacted rate this PR projects from is reg. 16(1A), £11.70 from 2026-04-05. TheAxiomFoundation/rulespec-uk uk/regulations/ssi/2023/302/16.yaml encodes it, and its companion case carer_support_payment_2026_weekly_amounts in 16.test.yaml checks it.
• Queued. The statutory rule behind the new uprating, Social Security (Scotland) Act 2018 ss. 86A-86B, has no Axiom module, and both corpus rows return 404. Encode Social Security (Scotland) Act 2018 ss.86A and 86B (inflation-adjusted level of devolved assistance and the duty to uprate it) (PE parity: PolicyEngine/policyengine-uk#2105) TheAxiomFoundation/rulespec-uk#431 queues it as a dispatch-ready pe-parity issue. This follows the precedent of SSAA 1992 s150 (rulespec-uk#367) for DWP uprating.
• 2027-28 onward diverges by design. Axiom holds enacted law, so it carries £11.70 forward until a 2027-28 SSI exists, while PolicyEngine projects £11.969. The Use legislative references for Child Tax Credit #431 module's inflation_adjusted_level is the comparable figure.
• Pre-existing gap in Axiom's reg 16. The module has no version before 2026-04-05, so it lacks £83.30 for 2025-26 and £11.29 for 15 March to 4 April 2026. Neither value is touched by this PR. The re-encode is noted on rulespec-uk#398, which already re-encodes reg 16.

Review

Five independent checkers each tried to refute the change: law and index choice, model mechanics (hand derivation of the index), impact arithmetic, code review and Axiom parity. None refuted it. Their should-fix points are addressed in this head:

• cite s. 86B, the duty to uprate, not only s. 86A;
• say the index is a proxy for September CPI and that the model does not round;
• derandomize the Hypothesis test and drop a strict-monotonicity assertion that depended on the forecast;
• replace the dead link.

Out of scope, flagged separately:

• Scottish Child Payment and PAWHP uprate with same-year CPI;
• the gov.benefit_uprating_cpi description says "September CPIH";
• the Carer's Allowance 2025-26 rate is £83.29 where the published rate is £83.30;
• the Carer Additional Person Payment (reg. 16(1B)) is not modelled.

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