diff --git a/domain/modelling/generators/heating_recommendation.py b/domain/modelling/generators/heating_recommendation.py
index 7efbc05a4..44173de98 100644
--- a/domain/modelling/generators/heating_recommendation.py
+++ b/domain/modelling/generators/heating_recommendation.py
@@ -293,10 +293,15 @@ def ashp_cost_inputs(
     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
+    heat_loss_kw: float = (
+        design_heat_loss_kw
+        if design_heat_loss_kw is not None
+        else floor_area * _KW_PER_M2
+    )
     return AshpCostInputs(
         existing_system=system,
         is_small_property=floor_area <= _SMALL_PROPERTY_MAX_M2,
-        design_heat_loss_kw=floor_area * _KW_PER_M2,
+        design_heat_loss_kw=heat_loss_kw,
         radiator_count=_radiator_count(epc),
         has_reusable_wet_system=system
         in (AshpExistingSystem.GAS, AshpExistingSystem.OIL, AshpExistingSystem.LPG),
@@ -762,7 +767,9 @@ def _ashp_option(
     # single catalogue scalar; the catalogue row is read only for its id, so an
     # absent ASHP row must not suppress the bundle — it just carries no id.
     product: Optional[Product] = products.get_optional(_ASHP_MEASURE_TYPE)
-    cost: Cost = Products().ashp_bundle_cost(ashp_cost_inputs(epc))
+    cost: Cost = Products().ashp_bundle_cost(
+        ashp_cost_inputs(epc, design_heat_loss_kw=heat_loss_kw)
+    )
     return MeasureOption(
         measure_type=_ASHP_MEASURE_TYPE,
         description=(