Add Indiana supplemental homestead credit (SEA 1, 2025)

[DTrim99]

Summary

Adds Indiana's Supplemental Homestead Credit, created by Senate Enrolled Act 1 (2025) (P.L.68-2025), SEC. 74, adding IC 6-1.1-20.6-7.7.

The credit is the lesser of 10% of the homestead property tax liability or $300, for property taxes first due and payable in 2026 and after. There is no age or income limit; it applies to owner-occupied homesteads. It is modeled as min(0.10 × real_estate_taxes, $300), so renters (with no real estate taxes) receive $0.

Changes

• New variable in_supplemental_homestead_credit (TaxUnit, defined_for = StateCode.IN) = min(rate × real_estate_taxes, cap).
• New parameters under gov.states.in.tax.property.supplemental_homestead_credit: rate (10%) and cap ($300), both from 2026.
• Wired into the 2026 block of state_property_tax_credits.yaml, alongside the existing in_over_65_property_tax_credit.

Scope

SEA 1 created several property-tax credits. This PR adds the flagship supplemental homestead credit (universal to homesteaders, no income test). For context on the others:

• The Over-65 $150 credit (IC 6-1.1-51.3-1) is already implemented as in_over_65_property_tax_credit (Add Indiana over-65 property tax credit #8308).
• The Blind/Disabled $125 credit (IC 6-1.1-51.3-2) is a possible follow-up.
• The SEA 1 veteran credits were repealed by HEA 1427-2025 (veterans keep their pre-existing deductions), so they are intentionally not modeled.

Tests

in_supplemental_homestead_credit.yaml covers: below-cap ($2,000 → $200), the $3,000 liability boundary (→ $300 cap), above-cap ($5,000 → $300), renter with no property tax (→ $0), and non-IN resident (→ $0).

References

• Senate Enrolled Act 1 (2025), enrolled text — SEC. 74, IC 6-1.1-20.6-7.7(c)
• Indiana DLGF memo — Legislation Affecting Deductions, Exemptions, and Credits

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ff39625) to head (7cb204e).
⚠️ Report is 16 commits behind head on main.

Additional details and impacted files

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unittests	100.00% <100.00%> (ø)

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[DTrim99]

Independent review + fixes applied

Independent review confirmed the credit is regulatorily correct (IC 6-1.1-20.6-7.7 = lesser of 10% of homestead liability or $300, 2026+, no age/income test, no phase-in). No critical issues. Applied the findings:

🟢 Suggestions — applied

• Added a formula NOTE that real_estate_taxes is a proxy for the homestead-only liability (PE has no homestead-specific real-estate-tax variable; the effect is bounded by the $300 cap).
• Trimmed the rate.yaml description ("up to a cap") and the reference-title glosses down to the section citation.
• Adopted Case N test naming with absolute_error_margin.

⚠️ Scope note (confirmed)

state_property_tax_credits feeds only taxsim_state_property_tax_credit (TAXSIM parity) — it does not currently flow into household_net_income. So this credit, like the sibling in_over_65_property_tax_credit, is modeled for TAXSIM parity and does not change modeled net income. Pre-existing/systemic, not a defect in this PR; flagging for visibility.