diff --git a/domain/modelling/generators/heating_recommendation.py b/domain/modelling/generators/heating_recommendation.py
index cdadf50bf..7efbc05a4 100644
--- a/domain/modelling/generators/heating_recommendation.py
+++ b/domain/modelling/generators/heating_recommendation.py
@@ -283,10 +283,14 @@ _OIL_FUEL_CODES = frozenset({28, 4, 71, 73, 75, 76})
 _LPG_FUEL_CODES = frozenset({27, 2, 3, 5, 9})
 
 
-def ashp_cost_inputs(epc: EpcPropertyData) -> AshpCostInputs:
+def ashp_cost_inputs(
+    epc: EpcPropertyData, design_heat_loss_kw: Optional[float] = None
+) -> AshpCostInputs:
     """Read an `EpcPropertyData` into the typed inputs `Products.ashp_bundle_cost`
-    needs: the existing system, property-size band, design heat loss (floor-area
-    proxy), radiator count, and whether a wet system can be reused (ADR-0025)."""
+    needs: the existing system, property-size band, design heat loss, radiator
+    count, and whether a wet system can be reused (ADR-0025). ``design_heat_loss_kw``
+    (the calculator's value, ADR-0049) sizes the cost band; when omitted it falls
+    back to the floor-area proxy so cost and pump size share one figure."""
     system: AshpExistingSystem = _existing_system(epc)
     floor_area: float = epc.total_floor_area_m2
     return AshpCostInputs(
diff --git a/tests/domain/modelling/test_ashp_cost_inputs.py b/tests/domain/modelling/test_ashp_cost_inputs.py
index c75cf52fc..ce528f1bb 100644
--- a/tests/domain/modelling/test_ashp_cost_inputs.py
+++ b/tests/domain/modelling/test_ashp_cost_inputs.py
@@ -82,3 +82,19 @@ def test_oil_and_lpg_dwellings_are_reusable_wet_systems() -> None:
     assert ashp_cost_inputs(_with_fuel(28)).existing_system is AshpExistingSystem.OIL
     assert ashp_cost_inputs(_with_fuel(27)).existing_system is AshpExistingSystem.LPG
     assert ashp_cost_inputs(_with_fuel(28)).has_reusable_wet_system is True
+
+
+def test_threaded_design_heat_loss_drives_the_cost_band_over_the_proxy() -> None:
+    # When the caller threads the calculator's real design heat loss (ADR-0049),
+    # the ASHP cost sizes to it — not the floor-area proxy — so a leaky dwelling's
+    # bigger pump is priced on the right band.
+    # Arrange
+    epc = parse_recommendation_summary(
+        "ashp_from_system_boiler_with_cylinder_001431_before.pdf"
+    )
+
+    # Act
+    inputs: AshpCostInputs = ashp_cost_inputs(epc, design_heat_loss_kw=17.5)
+
+    # Assert — the threaded value, not the 4.5 kW proxy.
+    assert inputs.design_heat_loss_kw == 17.5