Model North Dakota Homestead Property Tax Credit

## Summary

Follow-up to #8199 and #8314: model North Dakota's homeowner-side Homestead Property Tax Credit separately from the Renter's Refund.

## Current Policy Values

- Eligibility: age 65 or older, or permanently and totally disabled.
- Income limit: $70,000, including spouse and dependent income for the calendar year preceding the assessment date.
- Asset limit: current Tax Commissioner guidance says there is no asset limitation for homeowners.
- Homeowner requirements: the applicant must live at and have an ownership interest in the property; co-ownership percentage applies to the credit.
- Credit schedule:
- Income from $0 to $40,000: taxable value reduced by 100%, capped at a $9,000 taxable-value reduction / $200,000 true-and-full-value reduction.
- Income from $40,001 to $70,000: taxable value reduced by 50%, capped at a $4,500 taxable-value reduction / $100,000 true-and-full-value reduction.

## Modeling Limitations To Resolve

- The credit reduces taxable value rather than directly issuing a refund. PolicyEngine has `assessed_property_value` and `real_estate_taxes`, but not a North Dakota taxable-value input or mill-rate decomposition for translating the taxable-value reduction into a tax reduction.
- The income test uses income from all sources after eligible medical expense deductions, so a program-specific income variable may be more accurate than federal AGI alone.
- The permanent-and-total disability standard is stricter than a generic disability indicator.
- Ownership share, temporary absence, farm residence exclusion, revocable/irrevocable trust ownership, portability after sale, death-year termination, and interaction ordering with the Disabled Veteran's Credit all need explicit modeling decisions.
- Mobile-home owners on rented lots can interact with the renter refund exception, so the renter/homeowner boundary should be documented in tests.

## Sources

- North Dakota Office of State Tax Commissioner, Homestead Property Tax Credit: https://www.tax.nd.gov/homestead-property-tax-credit
- North Dakota Office of State Tax Commissioner, Homestead Property Tax Credit and Renter's Refund: https://www.tax.nd.gov/homestead-property-tax-credit-and-renters-refund
- North Dakota Office of State Tax Commissioner, Property Tax Credits for North Dakota Homeowners and Renters: https://www.tax.nd.gov/sites/www/files/documents/guidelines/homestead-veterans-renters/credits-for-nd-homeowners-renters-guideline.pdf