diff --git a/domain/building_geometry.py b/domain/building_geometry.py
new file mode 100644
index 00000000..a76e0468
--- /dev/null
+++ b/domain/building_geometry.py
@@ -0,0 +1,33 @@
+"""Building geometry derived purely from an EpcPropertyData.
+
+Reusable outside the SAP calculator (e.g. for Modelling cost quantities).
+Today this re-derives the heat-loss wall area; the calculator computes the
+same quantity inline (`heat_transmission._part_geometry`). A later, calculator-
+branch-coordinated refactor should DRY the two onto this module so there is a
+single source of truth. See the project memory on calculator geometry.
+"""
+
+from datatypes.epc.domain.epc_property_data import (
+    BuildingPartIdentifier,
+    EpcPropertyData,
+)
+
+
+def gross_heat_loss_wall_area(
+    epc: EpcPropertyData, identifier: BuildingPartIdentifier
+) -> float:
+    """Gross external wall area of one building part, in m^2: the sum over its
+    storeys of heat-loss perimeter x room height. This is the heat-loss area
+    (party walls are excluded — they are not on the heat-loss perimeter); it is
+    not netted of window/door openings.
+    """
+    part = next(
+        candidate
+        for candidate in epc.sap_building_parts
+        if candidate.identifier is identifier
+    )
+    area = sum(
+        fd.heat_loss_perimeter_m * fd.room_height_m
+        for fd in part.sap_floor_dimensions
+    )
+    return round(area, 2)
diff --git a/tests/domain/test_building_geometry.py b/tests/domain/test_building_geometry.py
new file mode 100644
index 00000000..816e334d
--- /dev/null
+++ b/tests/domain/test_building_geometry.py
@@ -0,0 +1,22 @@
+"""Behaviour of shared building geometry derived from EpcPropertyData —
+reusable outside the SAP calculator (e.g. for Modelling cost quantities)."""
+
+from datatypes.epc.domain.epc_property_data import BuildingPartIdentifier
+from domain.building_geometry import gross_heat_loss_wall_area
+from tests.domain.sap10_calculator.worksheet._elmhurst_worksheet_000490 import (
+    build_epc,
+)
+
+
+def test_gross_heat_loss_wall_area_sums_perimeter_times_height_per_storey() -> None:
+    # Arrange
+    # 000490 MAIN: floor 0 (perimeter 7.42 m x height 2.95 m) + floor 1
+    # (7.42 m x 3.24 m) = 21.889 + 24.0408 = 45.93 m^2. Party walls are
+    # excluded by construction (heat-loss perimeter, not total perimeter).
+    epc = build_epc()
+
+    # Act
+    area: float = gross_heat_loss_wall_area(epc, BuildingPartIdentifier.MAIN)
+
+    # Assert
+    assert abs(area - 45.93) <= 0.01