diff --git a/changelog.d/ny-pre-tcja-mortgage-interest.fixed.md b/changelog.d/ny-pre-tcja-mortgage-interest.fixed.md
new file mode 100644
index 00000000000..b9181cd44d1
--- /dev/null
+++ b/changelog.d/ny-pre-tcja-mortgage-interest.fixed.md
@@ -0,0 +1 @@
+Apply New York's pre-TCJA mortgage debt limits ($1,000,000 acquisition and $100,000 home equity debt) to its itemized interest deduction from 2018, with new home value and non-acquisition debt inputs.
diff --git a/policyengine_us/parameters/gov/states/ny/tax/income/deductions/itemized/interest/home_acquisition_debt_limit.yaml b/policyengine_us/parameters/gov/states/ny/tax/income/deductions/itemized/interest/home_acquisition_debt_limit.yaml
new file mode 100644
index 00000000000..02e2f769e7e
--- /dev/null
+++ b/policyengine_us/parameters/gov/states/ny/tax/income/deductions/itemized/interest/home_acquisition_debt_limit.yaml
@@ -0,0 +1,29 @@
+description: New York limits the home acquisition debt on which mortgage interest is deductible to this amount, reduced by any grandfathered debt, based on filing status.
+SINGLE:
+  2018-01-01: 1_000_000
+JOINT:
+  2018-01-01: 1_000_000
+SEPARATE:
+  2018-01-01: 500_000
+HEAD_OF_HOUSEHOLD:
+  2018-01-01: 1_000_000
+SURVIVING_SPOUSE:
+  2018-01-01: 1_000_000
+metadata:
+  propagate_metadata_to_children: true
+  breakdown: filing_status
+  unit: currency-USD
+  period: year
+  label: New York home acquisition debt limit
+  reference:
+    # N.Y. Tax Law 615(a) computes New York itemized deductions as they
+    # existed immediately before P.L. 115-97, so the IRC 163(h)(3)(B)(ii)
+    # limit applies rather than the IRC 163(h)(3)(F) limit for later debt.
+    - title: New York Tax Law 615(a)
+      href: https://www.nysenate.gov/legislation/laws/TAX/615
+    - title: "L.2018, chapter 59 (S7509C), Part JJ, sections 3 and 7 #page 42-43"
+      href: https://legislation.nysenate.gov/pdf/bills/2017/S7509C#page=42
+    - title: 26 U.S. Code 163(h)(3)(B)(ii) (2017 edition)
+      href: https://www.govinfo.gov/content/pkg/USCODE-2017-title26/html/USCODE-2017-title26-subtitleA-chap1-subchapB-partVI-sec163.htm
+    - title: New York State Department of Taxation and Finance, Itemized deductions (2025), Interest you paid
+      href: https://www.tax.ny.gov/pit/file/itemized-deductions-2025.htm#interest
diff --git a/policyengine_us/parameters/gov/states/ny/tax/income/deductions/itemized/interest/home_equity_debt_limit.yaml b/policyengine_us/parameters/gov/states/ny/tax/income/deductions/itemized/interest/home_equity_debt_limit.yaml
new file mode 100644
index 00000000000..edf2e20e399
--- /dev/null
+++ b/policyengine_us/parameters/gov/states/ny/tax/income/deductions/itemized/interest/home_equity_debt_limit.yaml
@@ -0,0 +1,29 @@
+description: New York limits the home equity debt on which mortgage interest is deductible to this amount, based on filing status.
+SINGLE:
+  2018-01-01: 100_000
+JOINT:
+  2018-01-01: 100_000
+SEPARATE:
+  2018-01-01: 50_000
+HEAD_OF_HOUSEHOLD:
+  2018-01-01: 100_000
+SURVIVING_SPOUSE:
+  2018-01-01: 100_000
+metadata:
+  propagate_metadata_to_children: true
+  breakdown: filing_status
+  unit: currency-USD
+  period: year
+  label: New York home equity debt limit
+  reference:
+    # N.Y. Tax Law 615(a) computes New York itemized deductions as they
+    # existed immediately before P.L. 115-97, so the IRC 163(h)(3)(C)(ii)
+    # limit applies and IRC 163(h)(3)(F) does not suspend it.
+    - title: New York Tax Law 615(a)
+      href: https://www.nysenate.gov/legislation/laws/TAX/615
+    - title: "L.2018, chapter 59 (S7509C), Part JJ, sections 3 and 7 #page 42-43"
+      href: https://legislation.nysenate.gov/pdf/bills/2017/S7509C#page=42
+    - title: 26 U.S. Code 163(h)(3)(C)(ii) (2017 edition)
+      href: https://www.govinfo.gov/content/pkg/USCODE-2017-title26/html/USCODE-2017-title26-subtitleA-chap1-subchapB-partVI-sec163.htm
+    - title: "2017 IRS Publication 936, Home equity debt limit and Table 1 #page 10-11"
+      href: https://www.irs.gov/pub/irs-prior/p936--2017.pdf#page=10
diff --git a/policyengine_us/tests/core/test_ny_mortgage_debt_limit_invariants.py b/policyengine_us/tests/core/test_ny_mortgage_debt_limit_invariants.py
new file mode 100644
index 00000000000..b3a1e2a20c3
--- /dev/null
+++ b/policyengine_us/tests/core/test_ny_mortgage_debt_limit_invariants.py
@@ -0,0 +1,290 @@
+"""Invariants for New York's pre-TCJA mortgage debt limits.
+
+N.Y. Tax Law 615(a) computes New York itemized deductions as they existed
+immediately before P.L. 115-97, so from 2018 New York keeps the IRC
+163(h)(3)(B)-(D) limits: grandfathered debt, incurred on or before October
+13, 1987, has no dollar limit and reduces the $1,000,000 acquisition-debt
+limit ($500,000 married filing separately); acquisition debt above that limit
+and non-acquisition debt count as home equity debt up to $100,000 ($50,000),
+and on each home only up to its fair market value less the debt on it
+treated as acquisition or grandfathered debt. A fair market value of 0 means
+unknown: the home counts as worth at least the debt on it, so only the dollar
+limit applies.
+
+Hypothesis draws two homes, each with an acquisition balance and vintage,
+non-acquisition debt and a fair market value, around every cutoff and limit,
+and checks _ny_qualified_loan_limit:
+
+1. Differential: it equals an independent restatement in the statute's
+   order, which places the acquisition limit on each home by a separate loop.
+   The restatement shares the acquisition limit between homes the same way
+   (in proportion to each home's other acquisition debt), so it cannot catch
+   a wrong sharing rule.
+2. Bounds: grandfathered debt <= New York limit <= total debt, and the New
+   York limit is at least PE's federal limit for the same acquisition loans.
+3. Defaults: with no fair market value and no non-acquisition debt (the
+   input defaults), the limit is line 6 plus the acquisition debt above it,
+   up to the home equity debt limit. The Hawaii helper gives the same
+   limit.
+4. Order: swapping the two homes never changes the limit.
+5. Monotone: a higher positive fair market value never lowers the limit.
+6. Unknown value: setting every fair market value to 0 (unknown) never
+   lowers the limit.
+
+For _ny_allowed_interest, PE's federal share for the same loans splits
+interest on two homes with no non-acquisition debt into the federally
+deductible and non-deductible parts. The New York result lies between the
+federal deduction and the interest paid, and equals the federal deduction
+when federal law disallows nothing. The federal bound follows from the
+limits, not from a floor.
+
+For _ny_deductible_share: the share is at least the unrounded ratio and at
+least the ratio rounded half up to three places, at most one, and exceeds the
+unrounded ratio by no more than .0005.
+
+The test reads the limits from the New York and federal parameters for
+every filing status in 2018, 2025 and 2026. The properties cannot pass
+trivially: the differential compares against a separately written per-home
+calculation, and the default and bound checks fail if the fair market value
+test, the home equity limit or the acquisition limit is dropped or reversed.
+
+These are for-all properties of the helpers, which YAML cannot state. The
+policy outcomes are checked in ny_mortgage_interest_deduction.yaml.
+"""
+
+import math
+from fractions import Fraction
+
+import numpy as np
+from hypothesis import example, given, settings
+from hypothesis import strategies as st
+
+from policyengine_us.system import system
+from policyengine_us.variables.gov.states.hi.tax.income.deductions.itemized.hi_mortgage_interest_deduction import (
+    _hi_qualified_loan_limit,
+)
+from policyengine_us.variables.gov.states.ny.tax.income.taxable_income.deductions.itemized.ny_mortgage_interest_deduction import (
+    _ny_allowed_interest,
+    _ny_deductible_share,
+    _ny_qualified_loan_limit,
+)
+from policyengine_us.variables.household.expense.tax_unit.mortgage_interest_structure import (
+    _debt_by_vintage,
+    _qualified_loan_limit,
+)
+
+GRANDFATHERED_YEAR = 1987
+PRE_TCJA_YEAR = 2017
+FILING_STATUSES = (
+    "SINGLE",
+    "JOINT",
+    "SEPARATE",
+    "HEAD_OF_HOUSEHOLD",
+    "SURVIVING_SPOUSE",
+)
+TAX_YEARS = (2018, 2025, 2026)
+
+
+def parameter_limits(year, filing_status):
+    p = system.parameters(f"{year}-01-01")
+    ny = p.gov.states.ny.tax.income.deductions.itemized.interest
+    federal = p.gov.irs.deductions.itemized.interest.mortgage
+    return (
+        (
+            float(ny.home_acquisition_debt_limit[filing_status]),
+            float(ny.home_equity_debt_limit[filing_status]),
+        ),
+        (
+            float(federal.pre_tcja_cap[filing_status]),
+            float(federal.cap[filing_status]),
+        ),
+    )
+
+
+LIMITS = {
+    (year, filing_status): parameter_limits(year, filing_status)
+    for year in TAX_YEARS
+    for filing_status in FILING_STATUSES
+}
+TOLERANCE = 0.01
+
+YEARS = (0, 1960, 1987, 1988, 2017, 2018, 2025)
+BOUNDARY_AMOUNTS = (
+    0,
+    1,
+    50_000,
+    100_000,
+    375_000,
+    500_000,
+    550_000,
+    750_000,
+    1_000_000,
+    1_100_000,
+)
+amounts = st.one_of(
+    st.sampled_from(BOUNDARY_AMOUNTS),
+    st.integers(0, 3_000_000),
+).map(float)
+# (acquisition balance, origination year, non-acquisition debt, fair market value)
+homes = st.tuples(amounts, st.sampled_from(YEARS), amounts, amounts)
+
+
+def grandfathered(year):
+    return 0 < year <= GRANDFATHERED_YEAR
+
+
+def model_limit(pair, limits):
+    grandfathered_debts = [
+        np.array([balance if grandfathered(year) else 0.0])
+        for balance, year, _, _ in pair
+    ]
+    other_debts = [
+        np.array([0.0 if grandfathered(year) else balance])
+        for balance, year, _, _ in pair
+    ]
+    return float(
+        _ny_qualified_loan_limit(
+            grandfathered_debts,
+            other_debts,
+            [np.array([home[2]]) for home in pair],
+            [np.array([home[3]]) for home in pair],
+            *limits,
+        )[0]
+    )
+
+
+def statute(pair, limits):
+    """IRC 163(h)(3)(B)(ii), (C) and (D) before P.L. 115-97, home by home."""
+    acquisition_limit, home_equity_limit = limits
+    old = sum(balance for balance, year, _, _ in pair if grandfathered(year))
+    other = sum(balance for balance, year, _, _ in pair if not grandfathered(year))
+    # (D)(ii): grandfathered debt reduces the $1,000,000 limit.
+    other_allowed = min(other, max(0.0, acquisition_limit - old))
+    home_equity = 0.0
+    for balance, year, non_acquisition, fair_market_value in pair:
+        if grandfathered(year):
+            on_home, excess = balance, 0.0
+        else:
+            on_home = other_allowed * balance / other if other > 0 else 0.0
+            excess = balance - on_home
+        # (C)(i): no more than fair market value less acquisition debt. A
+        # value of 0 means unknown and sets no such limit.
+        room = (
+            max(0.0, fair_market_value - on_home) if fair_market_value > 0 else math.inf
+        )
+        home_equity += min(excess + non_acquisition, room)
+    # (C)(ii): the aggregate home equity limit.
+    return old + other_allowed + min(home_equity, home_equity_limit)
+
+
+def federal_limit(pair, limits):
+    debt = _debt_by_vintage(
+        [np.array([home[0]]) for home in pair],
+        [np.array([home[1]]) for home in pair],
+        GRANDFATHERED_YEAR,
+        PRE_TCJA_YEAR,
+    )
+    return float(_qualified_loan_limit(*debt, *limits)[0])
+
+
+SETTINGS = dict(max_examples=400, deadline=None, derandomize=True)
+
+
+def test_new_york_limits_are_the_statutory_amounts():
+    for (year, filing_status), (ny_limits, _) in LIMITS.items():
+        expected = (
+            (500_000, 50_000) if filing_status == "SEPARATE" else (1_000_000, 100_000)
+        )
+        assert ny_limits == expected, (year, filing_status)
+
+
+@settings(**SETTINGS)
+@given(first=homes, second=homes, status=st.sampled_from(sorted(LIMITS)))
+def test_new_york_qualified_loan_limit_properties(first, second, status):
+    ny_limits, federal_limits = LIMITS[status]
+    pair = [first, second]
+    limit = model_limit(pair, ny_limits)
+    old = sum(home[0] for home in pair if grandfathered(home[1]))
+    total = sum(home[0] + home[2] for home in pair)
+
+    # 1. Differential.
+    assert abs(limit - statute(pair, ny_limits)) <= TOLERANCE
+    # 2. Bounds.
+    assert old - TOLERANCE <= limit <= total + TOLERANCE
+    assert limit >= federal_limit(pair, federal_limits) - TOLERANCE
+    # 3. Defaults: no fair market value and no non-acquisition debt.
+    acquisition_limit, home_equity_limit = ny_limits
+    defaults = [(home[0], home[1], 0.0, 0.0) for home in pair]
+    acquisition = sum(home[0] for home in pair)
+    line_6 = min(max(old, acquisition_limit), acquisition)
+    expected = line_6 + min(acquisition - line_6, home_equity_limit)
+    assert abs(model_limit(defaults, ny_limits) - expected) <= TOLERANCE
+    hawaii = float(
+        _hi_qualified_loan_limit(
+            np.array([old]), np.array([acquisition - old]), *ny_limits
+        )[0]
+    )
+    assert abs(expected - hawaii) <= TOLERANCE
+    # 4. Order.
+    assert abs(limit - model_limit([second, first], ny_limits)) <= TOLERANCE
+    # 5. Monotone in a positive fair market value.
+    richer = [
+        (home[0], home[1], home[2], home[3] + 250_000 if home[3] > 0 else 0.0)
+        for home in pair
+    ]
+    assert model_limit(richer, ny_limits) >= limit - TOLERANCE
+    # 6. An unknown value (0) never lowers the limit.
+    unknown = [(home[0], home[1], home[2], 0.0) for home in pair]
+    assert model_limit(unknown, ny_limits) >= limit - TOLERANCE
+
+
+@settings(**SETTINGS)
+@given(
+    interest=amounts,
+    first=homes,
+    second=homes,
+    status=st.sampled_from(sorted(LIMITS)),
+)
+def test_new_york_allowed_interest_without_non_acquisition_debt(
+    interest, first, second, status
+):
+    ny_limits, federal_limits = LIMITS[status]
+    pair = [(home[0], home[1], 0.0, home[3]) for home in (first, second)]
+    acquisition = sum(home[0] for home in pair)
+    if acquisition > 0:
+        federal_share = min(1.0, federal_limit(pair, federal_limits) / acquisition)
+    else:
+        federal_share = 1.0
+    deductible = interest * federal_share
+    non_deductible = interest - deductible
+    share = _ny_deductible_share(
+        np.array([model_limit(pair, ny_limits)]), np.array([acquisition])
+    )
+    allowed = float(
+        _ny_allowed_interest(
+            np.array([deductible]),
+            np.array([non_deductible]),
+            np.array([0.0]),
+            share,
+        )[0]
+    )
+    assert deductible - TOLERANCE <= allowed <= interest + TOLERANCE
+    if non_deductible == 0:
+        assert allowed == deductible
+
+
+@settings(**SETTINGS)
+@given(limit=amounts, debt=amounts)
+@example(limit=1_001.0, debt=2_000.0)
+def test_new_york_deductible_share_is_the_larger_method(limit, debt):
+    debt = max(debt, 1.0)
+    limit = min(limit, debt)
+    share = float(_ny_deductible_share(np.array([limit]), np.array([debt]))[0])
+    ratio = limit / debt
+    # Rounded half up to three places, computed exactly.
+    exact = Fraction(int(limit), int(debt))
+    rounded = math.floor(exact * 1_000 + Fraction(1, 2)) / 1_000
+    assert share >= ratio - 1e-12
+    assert share >= rounded - 1e-12
+    assert share <= 1 + 1e-12
+    assert share - ratio <= 0.0005 + 1e-12
diff --git a/policyengine_us/tests/policy/baseline/gov/states/ny/tax/income/taxable_income/deductions/itemized/ny_mortgage_interest_deduction.yaml b/policyengine_us/tests/policy/baseline/gov/states/ny/tax/income/taxable_income/deductions/itemized/ny_mortgage_interest_deduction.yaml
new file mode 100644
index 00000000000..5ebb83c3d0d
--- /dev/null
+++ b/policyengine_us/tests/policy/baseline/gov/states/ny/tax/income/taxable_income/deductions/itemized/ny_mortgage_interest_deduction.yaml
@@ -0,0 +1,289 @@
+# N.Y. Tax Law 615(a) computes New York itemized deductions as they existed
+# immediately before P.L. 115-97, so from 2018 New York keeps the pre-TCJA
+# mortgage limits of IRC 163(h)(3)(B)-(D). IRS Publication 936 (2017),
+# Table 1 (PDF page 11):
+#   G = grandfathered debt, 1987 or earlier; A = other acquisition debt
+#   line 6 = min(max(G, $1,000,000), G + A)
+#   line 7 = min(home equity debt, $100,000), where home equity debt is the
+#            acquisition debt above line 6 plus non-acquisition debt, limited
+#            on each home to its room: its fair market value less the debt on
+#            it counted in line 6 (a value of 0 means unknown and sets no
+#            limit)
+#   line 8 = line 6 + line 7
+#   line 11 = the larger of line 8 / total debt rounded to three places and
+#             the unrounded ratio
+#   line 12 = interest x line 11
+# Married filing separately halves the limits. PolicyEngine's federal
+# deduction uses the $750,000 limit for debt from 2018 or later, no home
+# equity debt and the exact ratio.
+
+- name: New York keeps the $1,000,000 acquisition-debt limit
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    state_code: NY
+    filing_status: SINGLE
+    home_mortgage_interest: 36_000
+    first_home_mortgage_balance: 900_000
+    first_home_mortgage_origination_year: 2020
+    ny_first_home_fair_market_value: 1_200_000
+    ny_first_home_non_acquisition_debt: 0
+  output:
+    # Federal: $36,000 x $750,000 / $900,000 = $30,000.
+    deductible_mortgage_interest_tax_unit: 30_000
+    # New York: the $1,000,000 limit covers all $900,000. Main allowed the
+    # federal $30,000.
+    ny_mortgage_interest_deduction: 36_000
+    ny_interest_deduction: 36_000
+    ny_itemized_deductions_max: 36_000
+
+- name: Acquisition debt above the limit counts as home equity debt
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    state_code: NY
+    filing_status: SINGLE
+    home_mortgage_interest: 40_000
+    first_home_mortgage_balance: 1_500_000
+    first_home_mortgage_origination_year: 2020
+    ny_first_home_fair_market_value: 2_000_000
+    ny_first_home_non_acquisition_debt: 0
+  output:
+    # Federal: $40,000 x $750,000 / $1,500,000 = $20,000.
+    deductible_mortgage_interest_tax_unit: 20_000
+    # New York: ($1,000,000 + $100,000) / $1,500,000 = .7333. The worksheet's
+    # .733 rounds down, so the unrounded share applies: $40,000 x 11/15.
+    ny_mortgage_interest_deduction: 29_333.33
+
+- name: Separate returns halve both limits
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    state_code: NY
+    filing_status: SEPARATE
+    home_mortgage_interest: 20_000
+    first_home_mortgage_balance: 600_000
+    first_home_mortgage_origination_year: 2020
+    ny_first_home_fair_market_value: 800_000
+    ny_first_home_non_acquisition_debt: 0
+  output:
+    # Federal: $20,000 x $375,000 / $600,000 = $12,500.
+    deductible_mortgage_interest_tax_unit: 12_500
+    # New York: ($500,000 + $50,000) / $600,000 = .9167. The worksheet's .917
+    # is the larger share: $20,000 x .917.
+    ny_mortgage_interest_deduction: 18_340
+
+- name: Without a fair market value, only the dollar limit caps home equity debt
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    state_code: NY
+    filing_status: SINGLE
+    home_mortgage_interest: 40_000
+    first_home_mortgage_balance: 1_500_000
+    first_home_mortgage_origination_year: 2020
+  output:
+    deductible_mortgage_interest_tax_unit: 20_000
+    # New York: a value of 0 means unknown, so the home has no room limit.
+    # Line 7 = min($500,000, $100,000). ($1,000,000 + $100,000) / $1,500,000
+    # = .7333 (worksheet .733); $40,000 x 11/15.
+    ny_mortgage_interest_deduction: 29_333.33
+
+- name: Fair market value limits home equity debt
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    state_code: NY
+    filing_status: SINGLE
+    home_mortgage_interest: 40_000
+    first_home_mortgage_balance: 1_500_000
+    first_home_mortgage_origination_year: 2020
+    ny_first_home_fair_market_value: 1_040_000
+  output:
+    # Room for home equity debt: $1,040,000 - $1,000,000 = $40,000.
+    # New York: $1,040,000 / $1,500,000 = .6933 (worksheet .693);
+    # $40,000 x 1.04 / 1.5.
+    ny_mortgage_interest_deduction: 27_733.33
+
+- name: Non-acquisition debt counts as home equity debt up to the limit
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    state_code: NY
+    filing_status: JOINT
+    home_mortgage_interest: 35_000
+    first_home_mortgage_balance: 500_000
+    first_home_mortgage_origination_year: 2015
+    ny_first_home_fair_market_value: 1_000_000
+    ny_first_home_non_acquisition_debt: 200_000
+  output:
+    # PE's federal calculation does not see the non-acquisition debt.
+    deductible_mortgage_interest_tax_unit: 35_000
+    # New York: ($500,000 + $100,000) / $700,000 = .8571 (worksheet .857);
+    # $35,000 x 6/7.
+    ny_mortgage_interest_deduction: 30_000
+
+- name: Non-acquisition debt on the second home counts as home equity debt
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    state_code: NY
+    filing_status: JOINT
+    home_mortgage_interest: 30_000
+    first_home_mortgage_balance: 400_000
+    first_home_mortgage_origination_year: 2015
+    ny_second_home_non_acquisition_debt: 200_000
+  output:
+    # PE's federal calculation does not see the non-acquisition debt.
+    deductible_mortgage_interest_tax_unit: 30_000
+    # New York: ($400,000 + $100,000) / $600,000 = .8333 (worksheet .833);
+    # $30,000 x 5/6.
+    ny_mortgage_interest_deduction: 25_000
+
+- name: Grandfathered debt reduces the acquisition-debt limit
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    state_code: NY
+    filing_status: JOINT
+    home_mortgage_interest: 50_000
+    first_home_mortgage_balance: 400_000
+    first_home_mortgage_origination_year: 1986
+    second_home_mortgage_balance: 800_000
+    second_home_mortgage_origination_year: 2020
+    ny_first_home_fair_market_value: 500_000
+    ny_second_home_fair_market_value: 900_000
+  output:
+    # Federal: line 6 = $400,000 of grandfathered debt; the later debt
+    # raises the limit only to $750,000; $50,000 x $750,000 / $1,200,000.
+    deductible_mortgage_interest_tax_unit: 31_250
+    # New York: line 6 = max($400,000, $1,000,000) = $1,000,000, of which
+    # $600,000 is the second home's; its other $200,000 is home equity debt,
+    # within its room of $900,000 - $600,000 = $300,000 and limited to
+    # $100,000. ($1,000,000 + $100,000) / $1,200,000 = .9167. The worksheet's
+    # .917 is the larger share: $50,000 x .917.
+    ny_mortgage_interest_deduction: 45_850
+
+- name: Each home's equity room limits its own excess debt
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    state_code: NY
+    filing_status: JOINT
+    home_mortgage_interest: 60_000
+    first_home_mortgage_balance: 900_000
+    first_home_mortgage_origination_year: 2020
+    second_home_mortgage_balance: 600_000
+    second_home_mortgage_origination_year: 2021
+    ny_first_home_fair_market_value: 3_000_000
+    ny_second_home_fair_market_value: 400_000
+  output:
+    # Line 6 = $1,000,000, shared 9:6: $600,000 on the first home and
+    # $400,000 on the second. The first home's other $300,000 fits in its
+    # room; the second home has no room, so its other $200,000 does not
+    # count. Line 7 = min($300,000, $100,000).
+    # ($1,000,000 + $100,000) / $1,500,000 = .7333 (worksheet .733);
+    # $60,000 x 11/15.
+    ny_mortgage_interest_deduction: 44_000
+
+- name: A directly supplied deduction passes through when nothing is disallowed
+  period: 2025
+  input:
+    state_code: NY
+    deductible_mortgage_interest: 12_000
+  output:
+    ny_mortgage_interest_deduction: 12_000
+
+- name: A directly supplied deduction passes through even with balances
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    state_code: NY
+    deductible_mortgage_interest: 12_000
+    first_home_mortgage_balance: 1_500_000
+    first_home_mortgage_origination_year: 2020
+  output:
+    # With no home_mortgage_interest, non_deductible_mortgage_interest_tax_unit
+    # is 0: it takes home_mortgage_interest_tax_unit plus the points, less
+    # deductible_mortgage_interest_tax_unit, and does not read the supplied
+    # person-level amount. So the supplied $12,000 passes through. Without the
+    # pass-through, the worksheet would limit it again: $12,000 x 11/15 = $8,800.
+    ny_mortgage_interest_deduction: 12_000
+
+- name: Without a fair market value, home equity debt keeps its interest
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    state_code: NY
+    filing_status: SINGLE
+    home_mortgage_interest: 20_000
+    first_home_mortgage_balance: 300_000
+    first_home_mortgage_origination_year: 2020
+    ny_first_home_non_acquisition_debt: 50_000
+  output:
+    # PE's federal calculation does not see the $50,000.
+    deductible_mortgage_interest_tax_unit: 20_000
+    # New York: line 8 = $300,000 + min($50,000, $100,000) = $350,000, equal
+    # to line 9, so the share is 1. A value of 0 once gave this home no room
+    # and $17,140.
+    ny_mortgage_interest_deduction: 20_000
+
+- name: Before 2018 New York uses the federal amount
+  period: 2017
+  absolute_error_margin: 0.01
+  input:
+    state_code: NY
+    filing_status: SINGLE
+    home_mortgage_interest: 40_000
+    first_home_mortgage_balance: 1_500_000
+    first_home_mortgage_origination_year: 2010
+    ny_first_home_fair_market_value: 2_000_000
+  output:
+    # Federal: $40,000 x $1,000,000 / $1,500,000.
+    deductible_mortgage_interest_tax_unit: 26_666.67
+    ny_mortgage_interest_deduction: 26_666.67
+
+- name: When the worksheet rounds down, New York uses the unrounded share
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    state_code: NY
+    filing_status: SINGLE
+    home_mortgage_interest: 60_000
+    first_home_mortgage_balance: 1_200_000
+    first_home_mortgage_origination_year: 2015
+    ny_first_home_fair_market_value: 1_000_000
+  output:
+    # Federal: $60,000 x $1,000,000 / $1,200,000 = $50,000.
+    # New York: room = $1,000,000 - $1,000,000 = $0, so line 7 is 0;
+    # $1,000,000 / $1,200,000 = .8333. The worksheet's .833 gives $49,980,
+    # the unrounded share $50,000, which applies.
+    deductible_mortgage_interest_tax_unit: 50_000
+    ny_mortgage_interest_deduction: 50_000
+
+- name: An exact tie in the worksheet's share rounds up
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    state_code: NY
+    filing_status: SINGLE
+    home_mortgage_interest: 80_000
+    first_home_mortgage_balance: 2_000_000
+    first_home_mortgage_origination_year: 2020
+    ny_first_home_fair_market_value: 1_001_000
+  output:
+    # Federal: $80,000 x $750,000 / $2,000,000 = $30,000.
+    deductible_mortgage_interest_tax_unit: 30_000
+    # New York: room = $1,001,000 - $1,000,000 = $1,000. $1,001,000 /
+    # $2,000,000 = .5005, an exact tie, so the worksheet's share is .501.
+    ny_mortgage_interest_deduction: 40_080
+
+- name: Mortgage interest outside New York is zero
+  period: 2025
+  input:
+    state_code: NJ
+    home_mortgage_interest: 36_000
+    first_home_mortgage_balance: 900_000
+    first_home_mortgage_origination_year: 2020
+  output:
+    ny_mortgage_interest_deduction: 0
diff --git a/policyengine_us/variables/gov/states/hi/tax/income/deductions/itemized/hi_mortgage_interest_deduction.py b/policyengine_us/variables/gov/states/hi/tax/income/deductions/itemized/hi_mortgage_interest_deduction.py
index 67a8036904a..7c2e0662d13 100644
--- a/policyengine_us/variables/gov/states/hi/tax/income/deductions/itemized/hi_mortgage_interest_deduction.py
+++ b/policyengine_us/variables/gov/states/hi/tax/income/deductions/itemized/hi_mortgage_interest_deduction.py
@@ -19,8 +19,8 @@ def _hi_qualified_loan_limit(
     Debt above the acquisition-debt limit may qualify as home equity debt, up
     to the home equity debt limit (line 7; Publication 936 (2017), Figure A,
     note 3). The home equity debt limit is also capped by each home's fair
-    market value less its acquisition debt; PolicyEngine has no home value
-    input, so every home is treated as worth at least its mortgages.
+    market value less its acquisition debt; PolicyEngine has no Hawaii home
+    value input, so it counts every home as worth at least its mortgages.
     """
     total_debt = grandfathered_debt + other_debt
     # Lines 3-6.
diff --git a/policyengine_us/variables/gov/states/ny/tax/income/taxable_income/deductions/itemized/ny_interest_deduction.py b/policyengine_us/variables/gov/states/ny/tax/income/taxable_income/deductions/itemized/ny_interest_deduction.py
new file mode 100644
index 00000000000..24ba2108855
--- /dev/null
+++ b/policyengine_us/variables/gov/states/ny/tax/income/taxable_income/deductions/itemized/ny_interest_deduction.py
@@ -0,0 +1,30 @@
+from policyengine_us.model_api import *
+
+
+class ny_interest_deduction(Variable):
+    value_type = float
+    entity = TaxUnit
+    label = "New York interest deduction"
+    unit = USD
+    definition_period = YEAR
+    documentation = (
+        "Interest New York allows as an itemized deduction: the federal "
+        "interest_deduction with New York's pre-TCJA home mortgage interest "
+        "in place of the federal home mortgage interest."
+    )
+    reference = (
+        "https://www.nysenate.gov/legislation/laws/TAX/615",
+        "https://www.tax.ny.gov/pit/file/itemized-deductions-2025.htm#interest",
+    )
+    defined_for = StateCode.NY
+
+    def formula(tax_unit, period, parameters):
+        # Starting from interest_deduction keeps an interest total supplied
+        # directly (deductible_interest_expense); only the federal mortgage
+        # component is swapped for New York's.
+        federal_interest = tax_unit("interest_deduction", period)
+        federal_mortgage_interest = add(
+            tax_unit, period, ["deductible_mortgage_interest"]
+        )
+        ny_mortgage_interest = tax_unit("ny_mortgage_interest_deduction", period)
+        return federal_interest - federal_mortgage_interest + ny_mortgage_interest
diff --git a/policyengine_us/variables/gov/states/ny/tax/income/taxable_income/deductions/itemized/ny_itemized_deductions_max.py b/policyengine_us/variables/gov/states/ny/tax/income/taxable_income/deductions/itemized/ny_itemized_deductions_max.py
index c217b06ddea..0b23e8bd25f 100644
--- a/policyengine_us/variables/gov/states/ny/tax/income/taxable_income/deductions/itemized/ny_itemized_deductions_max.py
+++ b/policyengine_us/variables/gov/states/ny/tax/income/taxable_income/deductions/itemized/ny_itemized_deductions_max.py
@@ -21,12 +21,15 @@ class ny_itemized_deductions_max(Variable):
     - Medical expenses deductible above 10% of AGI (not the 7.5% floor)
     - Miscellaneous deductions still allowed with 2% AGI floor
     - Casualty losses not limited to federally declared disasters
+    - Mortgage interest under the pre-TCJA acquisition and home equity
+      debt limits (ny_interest_deduction)
     - NY-specific college tuition deduction addition per § 615(d)
     """
     adds = [
         # Federal deductions that don't differ from pre-TCJA
         "charitable_deduction",
-        "interest_deduction",
+        # Federal interest components with NY's pre-TCJA mortgage interest
+        "ny_interest_deduction",
         # NY-specific pre-TCJA versions
         "ny_medical_expense_deduction",
         "ny_salt_deduction",
diff --git a/policyengine_us/variables/gov/states/ny/tax/income/taxable_income/deductions/itemized/ny_mortgage_interest_deduction.py b/policyengine_us/variables/gov/states/ny/tax/income/taxable_income/deductions/itemized/ny_mortgage_interest_deduction.py
new file mode 100644
index 00000000000..a61b4eb00ae
--- /dev/null
+++ b/policyengine_us/variables/gov/states/ny/tax/income/taxable_income/deductions/itemized/ny_mortgage_interest_deduction.py
@@ -0,0 +1,197 @@
+from policyengine_us.model_api import *
+
+
+def _ny_qualified_loan_limit(
+    grandfathered_debts,
+    other_debts,
+    non_acquisition_debts,
+    fair_market_values,
+    home_acquisition_debt_limit,
+    home_equity_debt_limit,
+):
+    """Pre-TCJA IRC 163(h)(3)(B)-(D), which N.Y. Tax Law 615(a) preserves.
+
+    Each argument except the limits is a list with one array per home.
+
+    Grandfathered debt, incurred on or before October 13, 1987, has no dollar
+    limit but reduces the acquisition-debt limit for other acquisition debt
+    (2017 Publication 936, Table 1, lines 1-6). Acquisition debt above that
+    limit, and debt that was never acquisition debt, may qualify as home
+    equity debt (Figure A, note 3). On each home, home equity debt cannot
+    exceed the home's fair market value less the debt on it treated as
+    acquisition or grandfathered debt (IRC 163(h)(3)(C)(i)); a value of 0
+    means unknown and sets no such limit. Across homes, home equity debt
+    cannot exceed the home equity debt limit (line 7).
+
+    PE shares the limit on other acquisition debt among homes in proportion
+    to each home's other acquisition debt; the statute's aggregate limit does
+    not say how to place it on a particular home.
+    """
+    grandfathered_debt = sum(grandfathered_debts)
+    other_debt = sum(other_debts)
+    acquisition_debt = grandfathered_debt + other_debt
+    # Lines 3-6: debt treated as acquisition or grandfathered debt.
+    line_6 = min_(
+        max_(grandfathered_debt, home_acquisition_debt_limit), acquisition_debt
+    )
+    other_debt_treated = line_6 - grandfathered_debt
+    home_equity_debt = 0
+    for grandfathered, other, non_acquisition, fair_market_value in zip(
+        grandfathered_debts, other_debts, non_acquisition_debts, fair_market_values
+    ):
+        other_share = np.divide(
+            other,
+            other_debt,
+            out=np.zeros_like(other_debt),
+            where=other_debt > 0,
+        )
+        treated = grandfathered + other_debt_treated * other_share
+        excess_acquisition = other - other_debt_treated * other_share
+        # 0 means unknown: treat the home as worth at least the debt on it,
+        # as the Hawaii calculation does.
+        equity_room = where(
+            fair_market_value > 0, max_(fair_market_value - treated, 0), np.inf
+        )
+        home_equity_debt = home_equity_debt + min_(
+            excess_acquisition + non_acquisition, equity_room
+        )
+    # Line 7.
+    line_7 = min_(home_equity_debt, home_equity_debt_limit)
+    # Line 8.
+    return line_6 + line_7
+
+
+def _ny_deductible_share(qualified_loan_limit, total_debt):
+    """2017 Publication 936, Table 1, line 11, or the unrounded ratio.
+
+    Until regulations issue, the House report on OBRA 1987 allows any
+    reasonable method of allocating interest on debt above the limits. The
+    IRS accepts the worksheet, which rounds the ratio to three places, and
+    the regulation's simplified method, which uses the unrounded ratio
+    (Treas. Reg. 1.163-10T(d); CCA 201201017). A taxpayer can use either, so
+    the larger share applies; PE rounds half up. The share is one when the
+    household reports no balance, as in the federal calculation. PE computes the
+    ratio in double precision so that single precision cannot move it
+    across a rounding boundary.
+    """
+    limit = np.asarray(qualified_loan_limit, dtype=np.float64)
+    debt = np.asarray(total_debt, dtype=np.float64)
+    ratio = np.divide(limit, debt, out=np.ones_like(debt), where=debt > 0)
+    # Round the scaled ratio first. In binary, 1,001,000 / 2,000,000 x 1,000
+    # comes to 500.49999999999994, so the floor alone rounds this exact tie down.
+    worksheet = np.floor(np.round(ratio * 1_000, 9) + 0.5) / 1_000
+    return np.maximum(worksheet, ratio)
+
+
+def _ny_allowed_interest(
+    deductible, non_deductible, non_acquisition_debt, deductible_share
+):
+    """Worksheet lines 10 and 12, with PE's federal pass-through.
+
+    Interest paid (line 10) is the federally deductible and non-deductible
+    parts together, the raw interest rather than the federally allowed
+    amount. When federal law disallows none of the interest and the
+    household reports no non-acquisition debt, the federal amount passes
+    through, which keeps a deduction supplied directly. Otherwise line 12
+    applies. New York's limits never fall below the federal ones for the
+    same loans, so without non-acquisition debt the result never falls
+    below PE's federal amount.
+    """
+    worksheet = (deductible + non_deductible) * deductible_share
+    uses_worksheet = (non_deductible > 0) | (non_acquisition_debt > 0)
+    return where(uses_worksheet, worksheet, deductible)
+
+
+class ny_mortgage_interest_deduction(Variable):
+    value_type = float
+    entity = TaxUnit
+    label = "New York home mortgage interest deduction"
+    unit = USD
+    definition_period = YEAR
+    documentation = (
+        "Home mortgage interest New York allows. N.Y. Tax Law 615(a) computes "
+        "New York itemized deductions as they existed immediately before "
+        "P.L. 115-97, so from 2018 New York keeps interest on up to "
+        "$1,000,000 of home acquisition debt plus $100,000 of home equity "
+        "debt ($500,000 and $50,000 for married filing separately), rather "
+        "than the federal $750,000 limit for debt incurred after December 15, "
+        "2017 and the federal suspension of home equity interest. Before 2018 "
+        "New York uses the federal amount."
+    )
+    reference = (
+        "https://www.nysenate.gov/legislation/laws/TAX/615",
+        # L.2018, chapter 59, Part JJ, sections 3 and 7. PDF pages 42-43.
+        "https://legislation.nysenate.gov/pdf/bills/2017/S7509C#page=42",
+        "https://www.tax.ny.gov/pit/file/itemized-deductions-2025.htm#interest",
+        # 26 U.S.C. 163(h)(3)(B)-(D), 2017 edition.
+        "https://www.govinfo.gov/content/pkg/USCODE-2017-title26/html/USCODE-2017-title26-subtitleA-chap1-subchapB-partVI-sec163.htm",
+        # 2017 Publication 936: Figure A note 3, home acquisition debt, home
+        # equity debt and its limit, grandfathered debt, and Table 1 with its
+        # line 7 instructions.
+        # PDF pages 3, 9-11, 13
+        "https://www.irs.gov/pub/irs-prior/p936--2017.pdf#page=3",
+        # Treasury Regulation 1.163-10T(d) and (g): the unrounded simplified
+        # method and the taxpayer's choice of method. PDF pages 3-4, 6.
+        "https://www.govinfo.gov/content/pkg/CFR-2025-title26-vol3/pdf/CFR-2025-title26-vol3-sec1-163-10T.pdf#page=3",
+        # CCA 201201017 (nonprecedential): any reasonable allocation method.
+        # PDF pages 4-5.
+        "https://www.irs.gov/pub/irs-wd/1201017.pdf#page=4",
+    )
+    defined_for = StateCode.NY
+
+    def formula(tax_unit, period, parameters):
+        # Before 2018, New York's itemized deductions follow the federal ones.
+        return add(tax_unit, period, ["deductible_mortgage_interest"])
+
+    def formula_2018(tax_unit, period, parameters):
+        balances = [
+            tax_unit("first_home_mortgage_balance", period).astype(np.float64),
+            tax_unit("second_home_mortgage_balance", period).astype(np.float64),
+        ]
+        origination_years = [
+            tax_unit("first_home_mortgage_origination_year", period),
+            tax_unit("second_home_mortgage_origination_year", period),
+        ]
+        non_acquisition_debts = [
+            tax_unit("ny_first_home_non_acquisition_debt", period).astype(np.float64),
+            tax_unit("ny_second_home_non_acquisition_debt", period).astype(np.float64),
+        ]
+        fair_market_values = [
+            tax_unit("ny_first_home_fair_market_value", period).astype(np.float64),
+            tax_unit("ny_second_home_fair_market_value", period).astype(np.float64),
+        ]
+        p_irs = parameters(period).gov.irs.deductions.itemized.interest.mortgage
+        # The shared origination-year convention: a loan from 1987 or earlier
+        # is grandfathered debt, and 0 means unknown, not grandfathered.
+        grandfathered_debts = [
+            where(
+                (year > 0) & (year <= p_irs.grandfathered_origination_year),
+                balance,
+                0,
+            )
+            for balance, year in zip(balances, origination_years)
+        ]
+        other_debts = [
+            balance - grandfathered
+            for balance, grandfathered in zip(balances, grandfathered_debts)
+        ]
+        filing_status = tax_unit("filing_status", period)
+        p = parameters(period).gov.states.ny.tax.income.deductions.itemized.interest
+        qualified_loan_limit = _ny_qualified_loan_limit(
+            grandfathered_debts,
+            other_debts,
+            non_acquisition_debts,
+            fair_market_values,
+            p.home_acquisition_debt_limit[filing_status],
+            p.home_equity_debt_limit[filing_status],
+        )
+        non_acquisition_debt = sum(non_acquisition_debts)
+        # Line 9: total average balances.
+        total_debt = sum(balances) + non_acquisition_debt
+        deductible_share = _ny_deductible_share(qualified_loan_limit, total_debt)
+        return _ny_allowed_interest(
+            add(tax_unit, period, ["deductible_mortgage_interest"]),
+            add(tax_unit, period, ["non_deductible_mortgage_interest"]),
+            non_acquisition_debt,
+            deductible_share,
+        )
diff --git a/policyengine_us/variables/gov/states/ny/tax/income/taxable_income/deductions/itemized/ny_mortgage_interest_inputs.py b/policyengine_us/variables/gov/states/ny/tax/income/taxable_income/deductions/itemized/ny_mortgage_interest_inputs.py
new file mode 100644
index 00000000000..6334a21a858
--- /dev/null
+++ b/policyengine_us/variables/gov/states/ny/tax/income/taxable_income/deductions/itemized/ny_mortgage_interest_inputs.py
@@ -0,0 +1,83 @@
+from policyengine_us.model_api import *
+
+_FAIR_MARKET_VALUE_DEFAULT = (
+    "Leave at 0 if unknown. PolicyEngine then counts the home as worth at least "
+    "the debt on it, so only the $100,000 ($50,000 separate) home equity debt "
+    "limit applies, as in its Hawaii calculation."
+)
+
+_NON_ACQUISITION_DEBT_DEFAULT = (
+    "Include the interest on this debt in the home mortgage interest inputs. "
+    "PolicyEngine's federal calculation reads only the acquisition mortgage "
+    "balances, so after 2017 it treats that interest like interest on those "
+    "balances: fully deductible when they fit the federal limit, and deductible "
+    "at the same share when they don't. Defaults to 0, which leaves New York's "
+    "calculation to the acquisition mortgage balances."
+)
+
+
+class ny_first_home_fair_market_value(Variable):
+    value_type = float
+    entity = TaxUnit
+    label = "New York first home fair market value"
+    unit = USD
+    definition_period = YEAR
+    default_value = 0
+    defined_for = StateCode.NY
+    documentation = (
+        "Fair market value of the main home, which limits the home equity "
+        "debt New York allows on it under pre-TCJA IRC 163(h)(3)(C)(i). "
+        + _FAIR_MARKET_VALUE_DEFAULT
+    )
+    reference = "https://www.nysenate.gov/legislation/laws/TAX/615"
+
+
+class ny_second_home_fair_market_value(Variable):
+    value_type = float
+    entity = TaxUnit
+    label = "New York second home fair market value"
+    unit = USD
+    definition_period = YEAR
+    default_value = 0
+    defined_for = StateCode.NY
+    documentation = (
+        "Fair market value of the second home, which limits the home equity "
+        "debt New York allows on it under pre-TCJA IRC 163(h)(3)(C)(i). "
+        + _FAIR_MARKET_VALUE_DEFAULT
+    )
+    reference = "https://www.nysenate.gov/legislation/laws/TAX/615"
+
+
+class ny_first_home_non_acquisition_debt(Variable):
+    value_type = float
+    entity = TaxUnit
+    label = "New York first home non-acquisition debt"
+    unit = USD
+    definition_period = YEAR
+    default_value = 0
+    defined_for = StateCode.NY
+    documentation = (
+        "Average balance of debt secured by the main home that is not home "
+        "acquisition debt (not used to buy, build or substantially improve "
+        "it), such as a home equity loan spent on other things. Pre-TCJA IRC "
+        "163(h)(3)(C) treats it as home equity debt for New York. "
+        + _NON_ACQUISITION_DEBT_DEFAULT
+    )
+    reference = "https://www.nysenate.gov/legislation/laws/TAX/615"
+
+
+class ny_second_home_non_acquisition_debt(Variable):
+    value_type = float
+    entity = TaxUnit
+    label = "New York second home non-acquisition debt"
+    unit = USD
+    definition_period = YEAR
+    default_value = 0
+    defined_for = StateCode.NY
+    documentation = (
+        "Average balance of debt secured by the second home that is not home "
+        "acquisition debt (not used to buy, build or substantially improve "
+        "it). Pre-TCJA IRC 163(h)(3)(C) treats it as home equity debt for "
+        "New York. " + _NON_ACQUISITION_DEBT_DEFAULT
+    )
+    reference = "https://www.nysenate.gov/legislation/laws/TAX/615"
diff --git a/policyengine_us/variables/gov/states/ny/tax/income/taxable_income/deductions/itemized/phase_out/ny_itemized_deductions_phase_out.py b/policyengine_us/variables/gov/states/ny/tax/income/taxable_income/deductions/itemized/phase_out/ny_itemized_deductions_phase_out.py
index db906ce1a2f..3a311801b19 100644
--- a/policyengine_us/variables/gov/states/ny/tax/income/taxable_income/deductions/itemized/phase_out/ny_itemized_deductions_phase_out.py
+++ b/policyengine_us/variables/gov/states/ny/tax/income/taxable_income/deductions/itemized/phase_out/ny_itemized_deductions_phase_out.py
@@ -45,7 +45,7 @@ class ny_itemized_deductions_phase_out(Variable):
     L3 = L1 - L2 (worksheet line 3) is therefore the mortgage interest, real
     estate taxes, charitable gifts, and 2%-floor miscellaneous deductions -
     exactly the 68(c) base subject to the limitation. Because investment
-    interest appears in both L1 (inside interest_deduction) and L2, it cancels
+    interest appears in both L1 (inside ny_interest_deduction) and L2, it cancels
     out of L3, as intended.
     """
 
diff --git a/source_conflicts/candidates.yaml b/source_conflicts/candidates.yaml
index 025bb5dd42b..74d0591305a 100644
--- a/source_conflicts/candidates.yaml
+++ b/source_conflicts/candidates.yaml
@@ -10464,15 +10464,18 @@ candidates:
     - policyengine_us/variables/gov/states/al/tax/income/deductions/itemized/al_investment_interest.py
     - policyengine_us/variables/gov/states/nc/tax/income/deductions/nc_mortgage_and_property_tax_before_limitation.py
     - policyengine_us/variables/gov/states/hi/tax/income/deductions/itemized/hi_mortgage_interest_deduction.py
+    - policyengine_us/variables/gov/states/ny/tax/income/taxable_income/deductions/itemized/ny_mortgage_interest_deduction.py
     prs:
     - https://github.com/PolicyEngine/policyengine-us/pull/10274
+    - https://github.com/PolicyEngine/policyengine-us/pull/10596
   state: unreviewed
-  note: 'PE-US follows two conventions, so follows stays unknown until one is picked. The federal deduction
+  note: 'PE-US follows three conventions, so follows stays unknown until one is picked. The federal deduction
     (deductible_mortgage_interest_tax_unit) uses the exact ratio, and Alabama and North Carolina read that federal
     amount. Hawaii (hi_mortgage_interest_deduction) follows the 2017 worksheet as a gap-fill, rounding line 11 to
     three places, and floors the result at the federal deduction, a PE safeguard that binds only when the rounded
     share falls below the exact federal share. Settling the federal reading (rulespec-us#1413) and using it for
-    both would make that floor unnecessary.'
+    Hawaii too would make that floor unnecessary. New York (ny_mortgage_interest_deduction) allows the larger of
+    the three-place share and the exact ratio.'
 - id: us:niit-excess-business-loss-addback
   program: income-tax
   years: