Add Tennessee elderly property tax relief

[daphnehanse11]

Summary

• Add Tennessee Property Tax Relief for Elderly Homeowners.
• Parameterize age, residential assessment rate, income limits, and qualifying property value cap.
• Include 2025 and 2026 values: income limit 37,530 in 2025 and 38,470 in 2026; property value cap 32,700 for both years.
• Add focused eligibility and amount tests.
• Keep the rule household-calculator-only for now; this PR does not wire the relief into refundable or property-tax-credit aggregates while assessed-property-value data readiness is unresolved.

Modeling notes

• Uses real_estate_taxes to preserve local tax-rate differences.
• Converts the published full-market-value cap to an assessed-value cap using Tennessee's 25% residential assessment rate.
• Uses adjusted gross income as the modeled tax-unit income proxy.

Tests

• PYTHONPATH=. /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/states/tn/tax/property/elderly_property_tax_relief -c policyengine_us
• /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/ruff check policyengine_us/variables/gov/states/tn/tax/property/elderly_property_tax_relief

Closes #8203

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (fbace93) to head (6650d99).
⚠️ Report is 18 commits behind head on main.

Additional details and impacted files

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[MaxGhenis]

Converted this to draft for the same data-readiness reason as #8303, but scoped to property tax data. This PR relies on assessed_property_value, which is currently only a PolicyEngine-US input variable and is not present/imputed in policyengine-us-data; rent and real_estate_taxes are data-backed, but assessed value is not. The rule tests are still useful, but baseline wiring into refundable/property-tax-credit aggregates should wait until we either add an assessed-value data/imputation path or revise the formula to rely only on data-backed inputs.

[hua7450]

🔍 Program Review — Tennessee Elderly Property Tax Relief

Tennessee implements Tenn. Code Ann. § 67-5-702 property tax relief for low-income elderly homeowners (age ≥ 65) by prorating real-estate taxes over the relief-eligible portion of assessed value. The implementation is structurally sound and fully parameter-driven — eligibility operators, the 25% assessment mechanism, and the proration math all faithfully encode § 67-5-702. One confirmed CRITICAL value error remains, plus several should-address robustness, scoping, and test-coverage gaps.

Scope: PR changes only — 10 files (4 params, 3 variables, 2 tests, 1 changelog). Branch is 0 behind main.

Source Documents

Document	Link
2025 TN Comptroller Tax Relief Brochure (TY2025 values, p.2)	https://comptroller.tn.gov/content/dam/cot/pa/documents/tax-relief/TaxReliefBrochure.pdf#page=2
Tenn. Code Ann. §§ 67-5-701–704 (§ 702 on p.3)	https://comptroller.tn.gov/content/dam/cot/pa/documents/tax-relief/tax%20relief%20tca.pdf#page=3
Robertson County Trustee — TY2026 values ($33,600 / $38,470)	https://www.robertsoncountytn.gov/local_government/trustee/tax_relief.php

---

🔴 Critical (Must Fix)

C1. 2026 property-value cap is wrong — should be $33,600, not $32,700
policyengine_us/parameters/gov/states/tn/tax/property/elderly_property_tax_relief/property_value_cap.yaml

values:
  2025-01-01: 32_700
  2026-01-01: 32_700   # WRONG — stale 2025 value frozen into 2026

§ 67-5-702(a)(3)(B) requires the cap to be CPI-adjusted every tax year. $32,700 is the TY2025 value. The TY2026 cap is $33,600, verified against primary sources:

• Robertson County Trustee (program administrator): "payment of the tax on an 2026 appraised property value of up to $33,600."
• Corroborated by Nashville/Davidson, Blount County, and the AARP-TN guide; consistent with the parallel income-limit rise ($37,530 → $38,470) and the statute's mandated annual adjustment ($32,700 × ~1.0275 ≈ $33,600).
• The lone $32,700-for-2026 source (UT-MTAS) was last reviewed 12/11/2025, before the 2026 figures were set — it is stale.

Fix (one change, three coupled edits):

  2026-01-01: 33_600   # TY2026 COLA-adjusted cap

• Assessed cap $8,175 → $8,400 (= $33,600 × 25%)
• tn_elderly_property_tax_relief.yaml Case 1 (period 2026): relief $327 → $336, comment → "33,600 × 25% = 8,400"
• Repoint the 2026 reference to a 2026-specific source (Robertson County trustee; the Comptroller's 2026 brochure isn't posted until Oct 2026)
• ⚠️ Keep the 2026-01-01 entry — once corrected it is no longer a duplicate.

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🟡 Should Address

	Finding	Location
S1	Unguarded / assessed_property_value → 0/0 = nan under vectorized microsim (defined_for masks output, not execution). Latent now (household-only), but SC/MS/KY all guard it. Fix: / max_(assessed_property_value, 1). (Rated CRITICAL by the code-pattern validator.)	tn_elderly_property_tax_relief.py:27
S2	Assessed value + taxes summed over the whole tax unit incl. dependents; statute ties relief to the owner. SC/MS/KY scope to head/joint-spouse	...relief.py:19,25; ..._eligible.py:24-25
S3	Income uses adjusted_gross_income proxy — omits untaxed SS / muni interest vs statutory "income from all sources"; variable has no reference or explanatory comment	..._income.py:12
S4	2026 income limit (38_470, value correct) cites the Tax Freeze doc for a Tax Relief param — program/document mismatch; prefer the brochure or § 67-5-702(a)(2)	income_limit.yaml:13-14
S5	Param PDF hrefs lack #page= anchors (correct: brochure #page=2, statute § 702 #page=3)	all 4 param files
S6	assessment_rate cites § 67-5-801 (general); the operative 25% is in § 67-5-702(b)(2) — tighten primary citation	assessment_rate.yaml
S7	2025 income limit only tested at $37,531 ($1 over); no at-/below-limit case and no 2025 positive-benefit case	both test yamls
S8	Untested: assessed-value-at-cap boundary ($8,400 after C1); benefit var for a non-TN household; eligibility >0 gates as false	both test yamls

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🟢 Suggestions

• G1. assessment_rate.yaml secondary ref is non-.gov Justia — prefer the .gov statute PDF as primary (pairs with S6).

---

PDF Audit: 5 confirmed correct, 1 confirmed mismatch

Parameter	Repo	Source	Status
age_threshold	65	65	✅
assessment_rate	0.25	25% (§ 702(b)(2))	✅
income_limit 2025 / 2026	37_530 / 38_470	$37,530 / $38,470	✅
property_value_cap 2025	32_700	$32,700 market	✅
property_value_cap 2026	32_700	$33,600	❌ mismatch

Formula integrity confirmed: cap stored as MARKET value, assessment_rate applied exactly once.

⚖️ Review Severity: REQUEST_CHANGES

One confirmed CRITICAL value error (2026 cap), with the assessed-cap and 2026-test-case updates required as part of the same fix.