Implement Maine PTFC unknown-utilities 15% rule (Schedule PTFC line 5c)

[DTrim99]

Surfaced by PolicyEngine/policyengine-taxsim#1126 (ME renter with rent that includes utilities).

Problem

Maine's property tax fairness credit treats 15% of rent as "rent constituting property taxes," after first excluding any heat/utilities included in the rent. Per 2022 Schedule PTFC/STFC line 5c: if the rent includes utilities and the amount is known, subtract it; if the rent includes utilities and the amount is not known, subtract 15% of gross rent instead.

me_property_tax_fairness_credit_countable_rent only handled the known-amount case (utilities_included_in_rent * utility_expense) and its comment noted the unknown-amount branch was "not implemented." So a filer whose rent includes utilities but who did not itemize a utility amount received no utility exclusion at all, overstating the countable rent (and the credit).

Fix

• Implement the line-5c fallback: when utilities_included_in_rent is true and the utility amount is unknown (utility_expense is 0), deduct 15% of gross rent before applying the 15% rent-constituting-property-tax rate.
• Add parameter gov.states.me.tax.income.credits.fairness.property_tax.rate.utilities_included_in_rent (0.15) for the utility fraction.
• Add a countable-rent test for the unknown-amount case.

Example

Rent $17,139 with utilities included, amount unknown: utility portion = 15% × 17,139 = 2,571; countable rent = 15% × (17,139 − 2,571) = 2,185, matching the Maine worksheet (previously PE used the full 15% × 17,139 = 2,571). The known-amount and utilities-not-included cases are unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a6acf1f) to head (cafacda).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

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[PavelMakarchuk]

The implementation mechanics check out — all 34 tests in the ME PTFC suite pass on the branch, the new parameter loads with correct metadata, utilities_included_in_rent defaults to false so microsim defaults are unaffected, no partner contract tests are touched, and the worksheet math in the new test (127.5 = (1000 − 150) × 0.15) is internally consistent with the quoted form text.

Requesting changes for two findings on the rewritten formula, detailed inline:

1. utility_expense is the wrong "amount known" discriminator — it's the household's general SNAP-style utility expense total, not the utility portion of rent, so the new 15% fallback only fires when the household reports zero utility expenses of any kind, and separately-paid utilities get subtracted from rent.
2. No clamp when utilities exceed rent — countable rent can go negative and offsets real_estate_taxes downstream; line 5d on the form can never be negative.

Caveat: maine.gov was unreachable from the review environment, so the line-5c rule was verified against the PR's quoted form text and worksheet example rather than the form PDF itself.

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Generated by Claude Code

[PavelMakarchuk]

utility_expense is the wrong discriminator for "is the utility portion of rent known?". It's the SPM-unit total of heating/cooling, gas, electricity, trash, and water expenses the household pays (the same variable SNAP uses) — not "the portion of rent attributable to utilities." That has two effects:

1. The new 15% fallback is unreachable for any household that reports any utility expense, since utility_expenses > 0 then reads as "amount known."
2. Separately-paid utilities are subtracted from rent as if they were included in it (this subtraction predates the PR, but the PR newly overloads the same variable as the known/unknown flag).

Concrete case: a ME renter with $12,000 rent that includes heat (portion unknown, so line 5c's 15% rule should apply) who also pays $600/yr of electricity separately gets amount_known = True and countable rent of (12,000 − 600) × 0.15 = $1,710 instead of the form's (12,000 − 1,800) × 0.15 = $1,530. A dedicated variable for the utilities-included-in-rent amount (rather than reusing the household's general utility expenses) would make the known/unknown split match the form.

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Generated by Claude Code

[PavelMakarchuk]

The known-amount branch has no clamp: when utility_expenses exceeds rent, net_rent goes negative and so does countable rent. Example: rent = 2,400, utilities_included_in_rent = true, utility_expense = 3,600 (plausible for low/subsidized rent with high utilities) → net_rent = −1,200 → countable rent = −180. On the real form, line 5d (5a minus 5c) can never be negative because 5c is a portion of 5a, and downstream me_property_tax_fairness_credit_countable_rent_property_tax adds this negative value to real_estate_taxes, understating the credit for part-year owner/renters.

Since this PR rewrote exactly these lines, a min_(utility_expenses, rent) on the deductible (or max_(net_rent, 0)) would close it.

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