HBAI and market income should read property income after finance costs #2390

Since #2172 (2.123.0) property_income is documented as landlords' profit before the property allowance and before dwelling-related finance costs, which are relieved as the s. 274A tax reduction, and the legacy means tests (Housing Benefit, Income Support, tax credits, Council Tax Reduction, Pension Credit) read property_income_after_finance_costs.

hbai_household_net_income, market_income and household_market_income still add property_income. HBAI's published basis is FRS property income as reported, after show card K6, which lists mortgage payments and loan interest, so it sits after interest. On datasets that fill property_finance_costs (PolicyEngine/microcosm#1145 does, on both the FRS and the SPI channels), market income therefore sits above the published basis by the finance costs, and HBAI net income by the finance costs less the extra tax, which income_tax already carries.

Proposal: the three variables add property_income_after_finance_costs instead of property_income. The allowance route is unaffected (property_income_after_finance_costs equals property_income when there are no finance costs), and taxable_property_income and the relief are unchanged.

Size on Microcosm UK (the #1145 candidate, policyengine-uk 2.123.0, 2025, aggregates only): the FRS landlords' imputed finance costs lift hbai_household_net_income by £2.95bn (0.20%) and household_market_income by £3.29bn against the as-reported alternative, and move the BHC poverty rate by 0.02 points and the AHC rate by 0.04 points. The SPI channel carries a further £6.8bn of finance costs on the same before-interest basis, so the total HBAI gap is larger than the FRS figure.

References: PolicyEngine/microcosm#1145 (item 1 of PolicyEngine/microcosm#1145 (comment)), #2172, PolicyEngine/microcosm#1106.