diff --git a/changelog.d/ca-pre-tcja-mortgage-debt-limits.fixed.md b/changelog.d/ca-pre-tcja-mortgage-debt-limits.fixed.md
new file mode 100644
index 00000000000..1a5966850c3
--- /dev/null
+++ b/changelog.d/ca-pre-tcja-mortgage-debt-limits.fixed.md
@@ -0,0 +1 @@
+Apply the pre-TCJA $1,000,000 home acquisition and $100,000 home equity debt limits to California mortgage interest (R&TC 17225(b)).
diff --git a/policyengine_us/parameters/gov/states/ca/tax/income/deductions/itemized/interest/home_acquisition_debt_limit.yaml b/policyengine_us/parameters/gov/states/ca/tax/income/deductions/itemized/interest/home_acquisition_debt_limit.yaml
new file mode 100644
index 00000000000..4d25d159b9d
--- /dev/null
+++ b/policyengine_us/parameters/gov/states/ca/tax/income/deductions/itemized/interest/home_acquisition_debt_limit.yaml
@@ -0,0 +1,29 @@
+description: California limits the home acquisition debt on which mortgage interest is deductible to this amount, based on filing status.
+SINGLE:
+  2018-01-01: 1_000_000
+JOINT:
+  2018-01-01: 1_000_000
+HEAD_OF_HOUSEHOLD:
+  2018-01-01: 1_000_000
+SEPARATE:
+  2018-01-01: 500_000
+SURVIVING_SPOUSE:
+  2018-01-01: 1_000_000
+metadata:
+  propagate_metadata_to_children: true
+  breakdown:
+    - filing_status
+  unit: currency-USD
+  period: year
+  label: California home acquisition debt limit
+  reference:
+    # California does not apply the TCJA limit in IRC 163(h)(3)(F), so the
+    # pre-TCJA limit in IRC 163(h)(3)(B)(ii) applies whenever the debt arose.
+    - title: California Revenue and Taxation Code Section 17225(b)
+      href: https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=RTC&sectionNum=17225
+    - title: SB 711 (2025) conformity chart, RTC 17225(b)
+      href: https://srev.senate.ca.gov/media/71#page=14
+    - title: 2025 Schedule CA (540) Instructions, Part II line 8
+      href: https://www.ftb.ca.gov/forms/2025/2025-540-ca-instructions.html
+    - title: 2017 IRS Publication 936, Home acquisition debt limit
+      href: https://www.irs.gov/pub/irs-prior/p936--2017.pdf#page=9
diff --git a/policyengine_us/parameters/gov/states/ca/tax/income/deductions/itemized/interest/home_equity_debt_limit.yaml b/policyengine_us/parameters/gov/states/ca/tax/income/deductions/itemized/interest/home_equity_debt_limit.yaml
new file mode 100644
index 00000000000..8f165369091
--- /dev/null
+++ b/policyengine_us/parameters/gov/states/ca/tax/income/deductions/itemized/interest/home_equity_debt_limit.yaml
@@ -0,0 +1,29 @@
+description: California 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
+HEAD_OF_HOUSEHOLD:
+  2018-01-01: 100_000
+SEPARATE:
+  2018-01-01: 50_000
+SURVIVING_SPOUSE:
+  2018-01-01: 100_000
+metadata:
+  propagate_metadata_to_children: true
+  breakdown:
+    - filing_status
+  unit: currency-USD
+  period: year
+  label: California home equity debt limit
+  reference:
+    # California does not apply the TCJA suspension of home equity interest in
+    # IRC 163(h)(3)(F), so the limit in IRC 163(h)(3)(C)(ii) applies.
+    - title: California Revenue and Taxation Code Section 17225(b)
+      href: https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=RTC&sectionNum=17225
+    - title: SB 711 (2025) conformity chart, RTC 17225(b)
+      href: https://srev.senate.ca.gov/media/71#page=14
+    - title: 2025 Schedule CA (540) Instructions, Part II line 8
+      href: https://www.ftb.ca.gov/forms/2025/2025-540-ca-instructions.html
+    - title: 2017 IRS Publication 936, Home equity debt limit
+      href: https://www.irs.gov/pub/irs-prior/p936--2017.pdf#page=10
diff --git a/policyengine_us/tests/policy/baseline/gov/states/ca/tax/income/deductions/itemized/ca_mortgage_interest_deduction.yaml b/policyengine_us/tests/policy/baseline/gov/states/ca/tax/income/deductions/itemized/ca_mortgage_interest_deduction.yaml
new file mode 100644
index 00000000000..e01785b9b6d
--- /dev/null
+++ b/policyengine_us/tests/policy/baseline/gov/states/ca/tax/income/deductions/itemized/ca_mortgage_interest_deduction.yaml
@@ -0,0 +1,246 @@
+- name: Case 1, a post-TCJA mortgage under $750,000 is fully deductible in both.
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 45
+        home_mortgage_interest: 30_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+        filing_status: SINGLE
+        first_home_mortgage_balance: 600_000
+        first_home_mortgage_origination_year: 2020
+    households:
+      household:
+        members: [person1]
+        state_code: CA
+  output:
+    deductible_mortgage_interest_tax_unit: 30_000
+    ca_mortgage_interest_deduction: 30_000
+
+- name: Case 2, a post-TCJA mortgage between $750,000 and $1 million is fully deductible in California.
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 45
+        home_mortgage_interest: 45_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+        filing_status: SINGLE
+        first_home_mortgage_balance: 900_000
+        first_home_mortgage_origination_year: 2020
+    households:
+      household:
+        members: [person1]
+        state_code: CA
+  output:
+    # Federal: $750,000 / $900,000 * $45,000 = $37,500.
+    deductible_mortgage_interest_tax_unit: 37_500
+    ca_mortgage_interest_deduction: 45_000
+
+- name: Case 3, debt between $1 million and $1.1 million is fully deductible as acquisition plus home equity debt.
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 45
+        home_mortgage_interest: 52_500
+    tax_units:
+      tax_unit:
+        members: [person1]
+        filing_status: SINGLE
+        first_home_mortgage_balance: 1_050_000
+        first_home_mortgage_origination_year: 2024
+    households:
+      household:
+        members: [person1]
+        state_code: CA
+  output:
+    # Federal: $750,000 / $1,050,000 * $52,500 = $37,500.
+    deductible_mortgage_interest_tax_unit: 37_500
+    # 2017 Pub. 936 Table 1: line 6 $1,000,000, line 7 $50,000, line 8 $1,050,000.
+    ca_mortgage_interest_deduction: 52_500
+
+- name: Case 4, a post-TCJA mortgage over $1.1 million uses the $1.1 million qualified loan limit.
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 45
+        home_mortgage_interest: 60_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+        filing_status: SINGLE
+        first_home_mortgage_balance: 1_500_000
+        first_home_mortgage_origination_year: 2020
+    households:
+      household:
+        members: [person1]
+        state_code: CA
+  output:
+    # Federal: $750,000 / $1,500,000 * $60,000 = $30,000.
+    deductible_mortgage_interest_tax_unit: 30_000
+    # Line 8: $1,000,000 + $100,000. Line 11: round($1,100,000 / $1,500,000, 3) = 0.733.
+    # 0.733 * $60,000 = $43,980.
+    ca_mortgage_interest_deduction: 43_980
+
+- name: Case 5, a pre-TCJA mortgage over $1.1 million also gets California's home equity allowance.
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 45
+        home_mortgage_interest: 60_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+        filing_status: SINGLE
+        first_home_mortgage_balance: 1_500_000
+        first_home_mortgage_origination_year: 2015
+    households:
+      household:
+        members: [person1]
+        state_code: CA
+  output:
+    # Federal: $1,000,000 / $1,500,000 * $60,000 = $40,000.
+    deductible_mortgage_interest_tax_unit: 40_000
+    ca_mortgage_interest_deduction: 43_980
+
+- name: Case 6, married filing separately uses the $500,000 and $50,000 limits.
+  period: 2022
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 45
+        home_mortgage_interest: 30_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+        filing_status: SEPARATE
+        first_home_mortgage_balance: 600_000
+        first_home_mortgage_origination_year: 2019
+    households:
+      household:
+        members: [person1]
+        state_code: CA
+  output:
+    # Federal: $375,000 / $600,000 * $30,000 = $18,750.
+    deductible_mortgage_interest_tax_unit: 18_750
+    # round($550,000 / $600,000, 3) = 0.917. 0.917 * $30,000 = $27,510.
+    ca_mortgage_interest_deduction: 27_510
+
+- name: Case 7, joint filers with two mortgages apply the limits to the combined balance.
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 45
+        home_mortgage_interest: 40_000
+      person2:
+        age: 45
+        home_mortgage_interest: 25_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+        filing_status: JOINT
+        first_home_mortgage_balance: 800_000
+        first_home_mortgage_origination_year: 2017
+        second_home_mortgage_balance: 500_000
+        second_home_mortgage_origination_year: 2020
+    households:
+      household:
+        members: [person1, person2]
+        state_code: CA
+  output:
+    # Federal: $800,000 / $1,300,000 * $65,000 = $40,000.
+    deductible_mortgage_interest_tax_unit: 40_000
+    # round($1,100,000 / $1,300,000, 3) = 0.846. 0.846 * $65,000 = $54,990.
+    ca_mortgage_interest_deduction: 54_990
+
+- name: Case 8, grandfathered debt over the limits stays fully deductible.
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 70
+        home_mortgage_interest: 60_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+        filing_status: SINGLE
+        first_home_mortgage_balance: 1_500_000
+        first_home_mortgage_origination_year: 1986
+    households:
+      household:
+        members: [person1]
+        state_code: CA
+  output:
+    deductible_mortgage_interest_tax_unit: 60_000
+    # Lines 4-6: the larger of $1,500,000 and $1,000,000, capped at $1,500,000.
+    ca_mortgage_interest_deduction: 60_000
+
+- name: Case 9, interest with no reported balance is fully deductible.
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 45
+        home_mortgage_interest: 20_000
+    households:
+      household:
+        members: [person1]
+        state_code: CA
+  output:
+    ca_mortgage_interest_deduction: 20_000
+
+- name: Case 10, a directly supplied federal deduction carries over when no gross interest is reported.
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 45
+        deductible_mortgage_interest: 12_000
+    households:
+      household:
+        members: [person1]
+        state_code: CA
+  output:
+    ca_mortgage_interest_deduction: 12_000
+
+- name: Case 11, California itemized deductions replace the federal mortgage interest.
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 45
+        home_mortgage_interest: 60_000
+        real_estate_taxes: 10_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+        filing_status: SINGLE
+        first_home_mortgage_balance: 1_500_000
+        first_home_mortgage_origination_year: 2020
+    households:
+      household:
+        members: [person1]
+        state_code: CA
+  output:
+    interest_deduction: 30_000
+    # $43,980 mortgage interest + $10,000 real estate taxes.
+    ca_itemized_deductions_pre_limitation: 53_980
diff --git a/policyengine_us/variables/gov/states/ca/tax/income/deductions/itemized/ca_itemized_deductions_pre_limitation.py b/policyengine_us/variables/gov/states/ca/tax/income/deductions/itemized/ca_itemized_deductions_pre_limitation.py
index 091b5150824..8dd4c2bd763 100644
--- a/policyengine_us/variables/gov/states/ca/tax/income/deductions/itemized/ca_itemized_deductions_pre_limitation.py
+++ b/policyengine_us/variables/gov/states/ca/tax/income/deductions/itemized/ca_itemized_deductions_pre_limitation.py
@@ -38,12 +38,16 @@ def formula(tax_unit, period, parameters):
         ]
         deductions += [
             "ca_investment_interest_expense_deduction",
+            "ca_mortgage_interest_deduction",
             "real_estate_taxes",
             "ca_charitable_deduction",
             "ca_misc_deduction",
             "ca_casualty_loss_deduction",
         ]
-        # Federal investment interest is nested in interest_deduction.
+        # Federal investment and mortgage interest are nested in
+        # interest_deduction; California replaces both (Schedule CA lines 8-9).
         return add(tax_unit, period, deductions) - add(
-            tax_unit, period, ["investment_interest_expense"]
+            tax_unit,
+            period,
+            ["investment_interest_expense", "deductible_mortgage_interest"],
         )
diff --git a/policyengine_us/variables/gov/states/ca/tax/income/deductions/itemized/ca_mortgage_interest_deduction.py b/policyengine_us/variables/gov/states/ca/tax/income/deductions/itemized/ca_mortgage_interest_deduction.py
new file mode 100644
index 00000000000..429462ef224
--- /dev/null
+++ b/policyengine_us/variables/gov/states/ca/tax/income/deductions/itemized/ca_mortgage_interest_deduction.py
@@ -0,0 +1,75 @@
+from policyengine_us.model_api import *
+from policyengine_us.variables.household.expense.tax_unit.mortgage_interest_structure import (
+    _debt_by_vintage,
+)
+
+
+class ca_mortgage_interest_deduction(Variable):
+    value_type = float
+    entity = TaxUnit
+    label = "California home mortgage interest deduction"
+    unit = USD
+    documentation = (
+        "Home mortgage interest California allows on Schedule CA (540), Part "
+        "II line 8. California does not apply IRC 163(h)(3)(F), so the "
+        "pre-TCJA home acquisition and home equity debt limits apply "
+        "regardless of when the debt was incurred."
+    )
+    reference = (
+        "https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=RTC&sectionNum=17225",
+        "https://www.ftb.ca.gov/forms/2025/2025-540-ca-instructions.html",
+        # PDF pages 9-11
+        "https://www.irs.gov/pub/irs-prior/p936--2017.pdf#page=9",
+    )
+    definition_period = YEAR
+    defined_for = StateCode.CA
+
+    def formula(tax_unit, period, parameters):
+        p = parameters(period).gov.states.ca.tax.income.deductions.itemized.interest
+        p_federal = parameters(period).gov.irs.deductions.itemized.interest.mortgage
+        filing_status = tax_unit("filing_status", period)
+        # 2017 Publication 936, Table 1. Only grandfathered debt (line 1) is
+        # treated differently by vintage; all later debt is line 2.
+        grandfathered_debt, pre_tcja_debt, post_tcja_debt = _debt_by_vintage(
+            [
+                tax_unit("first_home_mortgage_balance", period),
+                tax_unit("second_home_mortgage_balance", period),
+            ],
+            [
+                tax_unit("first_home_mortgage_origination_year", period),
+                tax_unit("second_home_mortgage_origination_year", period),
+            ],
+            p_federal.grandfathered_origination_year,
+            p_federal.pre_tcja_origination_year,
+        )
+        # Line 9.
+        debt = grandfathered_debt + pre_tcja_debt + post_tcja_debt
+        # Lines 3-6. Grandfathered debt has no dollar limit but reduces the
+        # home acquisition debt limit.
+        acquisition_debt = min_(
+            max_(grandfathered_debt, p.home_acquisition_debt_limit[filing_status]),
+            debt,
+        )
+        # Line 7. Debt over the home acquisition debt limit may qualify as home
+        # equity debt. The fair market value limit on home equity debt is not
+        # modeled.
+        home_equity_debt = min_(
+            debt - acquisition_debt, p.home_equity_debt_limit[filing_status]
+        )
+        # Lines 8 and 11. Without a reported balance, the interest is treated
+        # as within the limits, as in the federal calculation.
+        deductible_share = np.divide(
+            acquisition_debt + home_equity_debt,
+            debt,
+            out=np.ones_like(debt),
+            where=debt > 0,
+        )
+        deductible_share = np.round(deductible_share, 3)
+        # Line 10 uses total interest paid, before the federal debt limit.
+        gross_interest = tax_unit("home_mortgage_interest_tax_unit", period)
+        # A directly supplied federal deduction has already been limited, so
+        # keep it when no gross interest is reported.
+        federal_deduction = add(tax_unit, period, ["deductible_mortgage_interest"])
+        return where(
+            gross_interest > 0, gross_interest * deductible_share, federal_deduction
+        )