Uprate additional State Pension and protected payments by CPI, not the flat rate #1941

additional_state_pension uprates the part of the reported State Pension above the flat rate by the flat rate's own uprating: the full rate for the year over the data year's (policyengine_uk/variables/gov/dwp/additional_state_pension.py, the uprating term). The flat rates follow the triple lock, but in law the amounts above them rise with prices.

Law

The Social Security Benefits Up-rating Order 2026 (SI 2026/148), made under sections 150 and 151A of the Social Security Administration Act 1992:

• art. 4(2) raises basic pension from £176.45 to £184.90 (+4.8%), and art. 6(1) raises the full rate of the new State Pension from £230.25 to £241.30 (+4.8%).
• art. 4(3): "additional pensions in long-term benefits calculated by reference to any final relevant year earlier than the tax year 2025-2026", deferral increases and related sums "shall in each case be increased by 3.8 per cent".
• art. 6(3)(b): the amounts of new State Pension "at the transitional rate which exceed the full rate" (protected payments) "shall ... be increased by 3.8 per cent".

https://www.legislation.gov.uk/uksi/2026/148/article/4/made and /article/6/made. 3.8% is the September 2025 CPI. The reviewer of #1922 reports the same pattern in 2025-26: 1.7% for additional pension and protected payment against 4.1% for the flat rates.

Size

DWP's Spring Forecast 2026 tables put State Second Pension spending at £19.9bn, Graduated Retirement Benefit at £1.9bn and protected payments at £1.2bn in 2024-25. Uprating these by the flat rate instead of CPI overstates them by a growing amount each year. That is a rough estimate, not yet measured: measure it with full before/after microsimulation runs.

Fix

• Uprate the excess over the flat rate by the index DWP uses for CPI uprating, e.g. gov.benefit_uprating_cpi (lagged September CPI, already used for other DWP benefits). Check its values against the 3.8% (2026-27) and 1.7% (2025-26) outturns.
• The flat-rate part keeps the flat-rate ratio.
• policyengine_uk/tests/test_state_pension_components.py (from Split additional State Pension by the period's State Pension type #1922) keeps the add-on's uprating in one helper, add_on_uprating, so only that helper needs changing.
• Axiom parity: SI 2026/148 arts 4(3) and 6(3) are not encoded in rulespec-uk (uk/regulations/uksi/2026/148/article/4.yaml and 6.yaml encode only the flat rates). Encode them or queue them per the parity rule.
• Changes to published aggregates need sign-off.