Fix Scottish additional dwelling supplement rate

changelog.d/1795.fixed.md

@@ -0,0 +1 @@
+- Updated the Scottish Additional Dwelling Supplement to 8% for annual model years from 2025, retaining its statutory 5 December 2024 parameter date.

policyengine_uk/parameters/gov/revenue_scotland/lbtt/residential/README.md

@@ -1 +1,20 @@
 # Residential
+
+## Additional Dwelling Supplement
+
+[SSI 2024/367, articles 1–3](https://www.legislation.gov.uk/ssi/2024/367/made)
+raises the supplement from 6% to 8% for transactions with an effective date
+on or after 5 December 2024. Transactions under contracts entered into on or
+before 4 December 2024 retain the previous 6% rate, even if their effective
+date is later. [Revenue Scotland's guidance](https://revenue.scot/taxes/land-buildings-transaction-tax/additional-dwelling-supplement-ads)
+confirms the rate and transition.
+
+The parameter retains the statutory 5 December 2024 change date. The model's
+existing annual transactional-tax convention samples parameters on 30 April,
+so annual 2024 calculations use 6% and annual 2025 onward calculations use
+8%. The annual household purchase inputs have no contract or effective-date
+field: they cannot select protected contracts or distinguish purchases before
+and after the December change within 2024. These annual calculations do not
+represent every actual transaction in those years. The rate update preserves
+that convention and does not introduce a blended annual rate or new date
+inputs.

policyengine_uk/parameters/gov/revenue_scotland/lbtt/residential/additional_residence_surcharge.yaml

@@ -3,6 +3,17 @@ values:
   2016-04-01: 0.03
   2019-01-25: 0.04
   2022-12-16: 0.06
+  # The annual model samples 30 April; the raw statutory date remains here.
+  # Pre-5 December contracts protected by article 3 cannot be selected
+  # individually without contract/effective-date inputs (see README.md).
+  2024-12-05:
+    value: 0.08
+    metadata:
+      reference:
+        - title: The Land and Buildings Transaction Tax (additional amount) (Scotland) Amendment Order 2024, arts. 1–3
+          href: https://www.legislation.gov.uk/ssi/2024/367/made
+        - title: Revenue Scotland, ADS rates and transitional provision
+          href: https://revenue.scot/taxes/land-buildings-transaction-tax/additional-dwelling-supplement-ads
 metadata:
   unit: /1
   label: Lbtt fixed rate increase for secondary residences

policyengine_uk/tests/policy/baseline/gov/revenue_scotland/lbtt/additional_dwelling_supplement.yaml

@@ -0,0 +1,90 @@
+# SSI 2024/367 arts1–3:8% from5December2024 for unprotected contracts.
+# https://www.legislation.gov.uk/ssi/2024/367/made
+# Existing annual transactional-tax convention samples30April, so2024
+# retains6% and2025onward uses8%. These cases do not simulate actual
+# late2024transaction dates or the protected pre5December contract rule.
+# A £300,000 purchase's unchanged ordinary LBTT is£4,600:
+# £105,000*2%+£50,000*5% (SSI2015/126 TableA).
+# ADS adds£18,000 at6%, or£24,000 at8%. Purchase/rent inputs are explicit;
+# no liability flag, intermediate tax or property stock is forced.
+
+- name: Annual 2024 additional purchase retains the April sample at six percent
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    age: 45
+    region: SCOTLAND
+    main_residential_property_purchased: 0
+    additional_residential_property_purchased: 300_000
+    non_residential_property_purchased: 0
+    rent: 0
+    cumulative_residential_rent: 0
+    non_residential_rent: 0
+    cumulative_non_residential_rent: 0
+  output:
+    land_and_buildings_transaction_tax: 22_600
+
+- name: Annual 2025 additional purchase pays eight percent ADS
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    age: 45
+    region: SCOTLAND
+    main_residential_property_purchased: 0
+    additional_residential_property_purchased: 300_000
+    non_residential_property_purchased: 0
+    rent: 0
+    cumulative_residential_rent: 0
+    non_residential_rent: 0
+    cumulative_non_residential_rent: 0
+  output:
+    land_and_buildings_transaction_tax: 28_600
+
+- name: Annual 2026 additional purchase retains eight percent ADS
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    age: 45
+    region: SCOTLAND
+    main_residential_property_purchased: 0
+    additional_residential_property_purchased: 300_000
+    non_residential_property_purchased: 0
+    rent: 0
+    cumulative_residential_rent: 0
+    non_residential_rent: 0
+    cumulative_non_residential_rent: 0
+  output:
+    land_and_buildings_transaction_tax: 28_600
+
+- name: Ordinary main-home purchase receives no ADS
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    age: 45
+    region: SCOTLAND
+    main_residential_property_purchased: 300_000
+    main_residential_property_purchased_is_first_home: false
+    additional_residential_property_purchased: 0
+    non_residential_property_purchased: 0
+    rent: 0
+    cumulative_residential_rent: 0
+    non_residential_rent: 0
+    cumulative_non_residential_rent: 0
+  output:
+    land_and_buildings_transaction_tax: 4_600
+
+- name: English purchase has no Scottish LBTT liability
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    age: 45
+    region: LONDON
+    main_residential_property_purchased: 0
+    additional_residential_property_purchased: 300_000
+    non_residential_property_purchased: 0
+    rent: 0
+    cumulative_residential_rent: 0
+    non_residential_rent: 0
+    cumulative_non_residential_rent: 0
+  output:
+    land_and_buildings_transaction_tax: 0

policyengine_uk/tests/test_lbtt_additional_dwelling_rate.py

@@ -0,0 +1,48 @@
+"""ADS statutory date and the existing annual transactional-tax convention.
+
+SSI 2024/367 arts1–3 changes6% to8% from5December2024, with protection
+for contracts entered on/before4December. Annual purchase inputs have no
+contract/effective date; processed parameters sample30April, so these
+annual readings cannot select protected contracts or late2024 purchases.
+"""
+
+from pathlib import Path
+
+import pytest
+import yaml
+from policyengine_core.parameters import Parameter
+
+import policyengine_uk
+from policyengine_uk import CountryTaxBenefitSystem
+
+PATH = "gov.revenue_scotland.lbtt.residential.additional_residence_surcharge"
+FILE = (
+    Path(policyengine_uk.__file__).parent
+    / "parameters/gov/revenue_scotland/lbtt/residential/additional_residence_surcharge.yaml"
+)
+
+
+def test_raw_ads_rate_keeps_history_and_changes_on_5_december_2024():
+    data = yaml.safe_load(FILE.read_text())
+    data["values"] = {str(date): value for date, value in data["values"].items()}
+    raw = Parameter(PATH, data=data, file_path=str(FILE))
+    for date, expected in [
+        ("2016-04-01", 0.03),
+        ("2019-01-25", 0.04),
+        ("2022-12-16", 0.06),
+        ("2024-12-04", 0.06),
+        ("2024-12-05", 0.08),
+    ]:
+        assert raw(date) == expected, date
+    assert not raw.metadata.get("fiscal_year_blend", False)
+    assert not raw.metadata.get("preserve_calendar_dates", False)
+
+
+@pytest.fixture(scope="module")
+def system():
+    return CountryTaxBenefitSystem()
+
+
+@pytest.mark.parametrize("year,expected", [(2024, 0.06), (2025, 0.08), (2026, 0.08)])
+def test_annual_ads_rate_preserves_april30_sampling(system, year, expected):
+    assert system.parameters.get_child(PATH)(f"{year}-01-01") == expected