diff --git a/domain/modelling/generators/heating_recommendation.py b/domain/modelling/generators/heating_recommendation.py
index f4f7375dc..4fc0b0736 100644
--- a/domain/modelling/generators/heating_recommendation.py
+++ b/domain/modelling/generators/heating_recommendation.py
@@ -335,17 +335,23 @@ def recommend_heating(
     products: ProductRepository,
     restrictions: PlanningRestrictions = PlanningRestrictions(),
     considered_measures: Optional[frozenset[MeasureType]] = None,
+    *,
+    design_heat_loss_kw: Optional[float] = None,
 ) -> Optional[Recommendation]:
     """Return a "Heating & Hot Water" Recommendation of competing whole-system
     bundles for the dwelling, else None when no bundle is eligible. ASHP is
-    additionally gated by the Property's planning protections (ADR-0024)."""
+    additionally gated by the Property's planning protections (ADR-0024).
+
+    ``design_heat_loss_kw`` (the calculator's SAP design heat loss) sizes the
+    ASHP to the dwelling so SAP's Appendix-N PSR efficiency reads near its peak
+    (ADR-0049); when omitted, sizing falls back to the floor-area proxy."""
     options: list[MeasureOption] = []
 
     hhr_option = _hhr_storage_option(epc, products)
     if hhr_option is not None:
         options.append(hhr_option)
 
-    ashp_option = _ashp_option(epc, products, restrictions)
+    ashp_option = _ashp_option(epc, products, restrictions, design_heat_loss_kw)
     if ashp_option is not None:
         options.append(ashp_option)
 
@@ -730,9 +736,11 @@ def _ashp_option(
     epc: EpcPropertyData,
     products: ProductRepository,
     restrictions: PlanningRestrictions,
+    design_heat_loss_kw: Optional[float] = None,
 ) -> Optional[MeasureOption]:
     """The air-source heat-pump bundle, offered for any non-flat house/bungalow
-    that is not listed/heritage and not already a heat pump."""
+    that is not listed/heritage and not already a heat pump. The pump is sized to
+    ``design_heat_loss_kw`` (ADR-0049), falling back to the floor-area proxy."""
     if not _ashp_eligible(epc, restrictions):
         return None
     # Cost is composed per-dwelling from the rate sheet (ADR-0025), not the
diff --git a/tests/domain/modelling/test_heating_recommendation.py b/tests/domain/modelling/test_heating_recommendation.py
index 51d41f035..e01a55c17 100644
--- a/tests/domain/modelling/test_heating_recommendation.py
+++ b/tests/domain/modelling/test_heating_recommendation.py
@@ -791,3 +791,24 @@ def test_ashp_sizing_caps_at_the_largest_pump_for_a_high_heat_loss_dwelling() ->
 
     # Assert — capped at the 12 kW rung, not left undersized at 5 kW.
     assert pcdb_id == 110281
+
+
+def test_ashp_bundle_sizes_the_pump_to_the_design_heat_loss() -> None:
+    # A high-heat-loss house (17.5 kW) must get a pump sized to it — the capped
+    # 12 kW aroTHERM plus (PCDB 110281) — so SAP's Appendix-N PSR efficiency reads
+    # near its peak, not the fixed 5 kW unit (110257) whose PSR collapses.
+    # Arrange
+    baseline: EpcPropertyData = _gas_boiler_house()
+
+    # Act
+    recommendation: Recommendation | None = recommend_heating(
+        baseline, _StubProducts(), design_heat_loss_kw=17.5
+    )
+
+    # Assert — the ASHP overlay carries the sized 12 kW record, not the fixed 5 kW.
+    assert recommendation is not None
+    ashp = next(
+        o for o in recommendation.options if o.measure_type == "air_source_heat_pump"
+    )
+    assert ashp.overlay.heating is not None
+    assert ashp.overlay.heating.main_heating_index_number == 110281