diff --git a/domain/modelling/generators/heating_recommendation.py b/domain/modelling/generators/heating_recommendation.py
index 4fc0b0736..cdadf50bf 100644
--- a/domain/modelling/generators/heating_recommendation.py
+++ b/domain/modelling/generators/heating_recommendation.py
@@ -12,6 +12,7 @@ boiler bundles land in later slices. Detection + pricing only — impact is
 produced by scoring (ADR-0016).
 """
 
+from dataclasses import replace
 from typing import Optional
 
 from datatypes.epc.domain.epc_property_data import EpcPropertyData, MainHeatingDetail
@@ -743,6 +744,16 @@ def _ashp_option(
     ``design_heat_loss_kw`` (ADR-0049), falling back to the floor-area proxy."""
     if not _ashp_eligible(epc, restrictions):
         return None
+    # Size the pump to the dwelling: the calculator's design heat loss when the
+    # caller threaded it, else the floor-area proxy (ADR-0049).
+    heat_loss_kw: float = (
+        design_heat_loss_kw
+        if design_heat_loss_kw is not None
+        else epc.total_floor_area_m2 * _KW_PER_M2
+    )
+    sized_overlay: HeatingOverlay = replace(
+        _ASHP_OVERLAY, main_heating_index_number=select_ashp_pcdb_id(heat_loss_kw)
+    )
     # Cost is composed per-dwelling from the rate sheet (ADR-0025), not the
     # 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.
@@ -754,7 +765,7 @@ def _ashp_option(
             "Replace the heating with an air-source heat pump, time-and-"
             "temperature-zone controls and a heat-pump hot-water cylinder"
         ),
-        overlay=EpcSimulation(heating=_ASHP_OVERLAY),
+        overlay=EpcSimulation(heating=sized_overlay),
         cost=cost,
         material_id=product.id if product is not None else None,
     )
diff --git a/tests/domain/modelling/test_heating_recommendation.py b/tests/domain/modelling/test_heating_recommendation.py
index e01a55c17..2477150ef 100644
--- a/tests/domain/modelling/test_heating_recommendation.py
+++ b/tests/domain/modelling/test_heating_recommendation.py
@@ -203,8 +203,10 @@ def test_gas_boiler_house_yields_an_ashp_bundle() -> None:
     # bungalow regardless of current system or efficiency (ADR-0024).
     baseline: EpcPropertyData = _gas_boiler_house()
 
-    # Act
-    recommendation: Recommendation | None = recommend_heating(baseline, _StubProducts())
+    # Act — a ~4 kW design heat loss sizes to the 5 kW aroTHERM plus (110257).
+    recommendation: Recommendation | None = recommend_heating(
+        baseline, _StubProducts(), design_heat_loss_kw=4.0
+    )
 
     # Assert — the ASHP bundle carries the absolute heat-pump end-state.
     assert recommendation is not None