diff --git a/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py b/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
index 7a473041..cffbd2d3 100644
--- a/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
+++ b/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
@@ -214,16 +214,18 @@ def _dwelling_exposure(dwelling_type: Optional[str]) -> DwellingExposure:
 
 
 def _region_index(region_code: Optional[str]) -> int:
-    """Coerce EpcPropertyData.region_code (str) to the integer Appendix U
-    region index. Out-of-range or unparseable → 0 (UK average)."""
-    if region_code is None:
-        return 0
-    try:
-        idx = int(region_code)
-    except (TypeError, ValueError):
-        return 0
-    if 0 <= idx <= 21:
-        return idx
+    """SAP rating must be computed with UK-average weather per Appendix U:
+    "Calculations for fabric energy efficiency (FEE), regulation compliance
+    (TER and DER, TPER and DPER) and for ratings (SAP rating and environmental
+    impact rating) are done with UK average weather. Other calculations (such
+    as for energy use and costs on EPCs) are done using local weather."
+
+    Since our calculator's primary output is the SAP rating, we always return
+    region 0 (UK average) regardless of the cert's actual region_code. A
+    future slice may add a `compute_local_weather` flag to also produce the
+    energy-use kWh totals at local weather.
+    """
+    _ = region_code
     return 0
 
 
diff --git a/packages/domain/src/domain/sap/rdsap/tests/test_cert_to_inputs.py b/packages/domain/src/domain/sap/rdsap/tests/test_cert_to_inputs.py
index 9bd27a9c..2915dde0 100644
--- a/packages/domain/src/domain/sap/rdsap/tests/test_cert_to_inputs.py
+++ b/packages/domain/src/domain/sap/rdsap/tests/test_cert_to_inputs.py
@@ -95,12 +95,15 @@ def test_minimal_cert_round_trips_through_calculator_and_returns_sap_result() ->
     assert result.total_fuel_cost_gbp > 0
 
 
-def test_region_code_string_is_translated_to_appendix_u_region_index() -> None:
-    # Arrange — `region_code` on EpcPropertyData is a string ("1"="Thames",
-    # "20"="Shetland"); the mapper must coerce it to the integer index
-    # Appendix U expects, falling back to 0 (UK average) when missing.
+def test_calculator_always_uses_uk_average_weather_for_rating() -> None:
+    # Arrange — SAP 10.2 Appendix U explicitly states: "Calculations for
+    # ratings (SAP rating and environmental impact rating) are done with
+    # UK average weather". The mapper must therefore always set region=0
+    # regardless of the cert's lodged region_code; a future slice may
+    # add a separate local-weather path for the energy-use display
+    # numbers that EPC software shows alongside the rating.
     base = _typical_semi_detached_epc()
-    thames = base
+    thames = base  # region_code="1"
     no_region = make_minimal_sap10_epc(
         total_floor_area_m2=_TYPICAL_TFA_M2,
         habitable_rooms_count=4,
@@ -113,8 +116,8 @@ def test_region_code_string_is_translated_to_appendix_u_region_index() -> None:
     inputs_thames = cert_to_inputs(thames)
     inputs_default = cert_to_inputs(no_region)
 
-    # Assert
-    assert inputs_thames.region == 1
+    # Assert — both collapse to UK average (region 0) for rating purposes.
+    assert inputs_thames.region == 0
     assert inputs_default.region == 0