diff --git a/orchestration/modelling_orchestrator.py b/orchestration/modelling_orchestrator.py
index e3180a4d2..ae43ca85c 100644
--- a/orchestration/modelling_orchestrator.py
+++ b/orchestration/modelling_orchestrator.py
@@ -264,6 +264,7 @@ def _candidate_recommendations(
     planning_restrictions: PlanningRestrictions,
     solar_potential: Optional[SolarPotential],
     considered_measures: Optional[frozenset[MeasureType]],
+    design_heat_loss_kw: Optional[float] = None,
 ) -> list[Recommendation]:
     """Run the applicable Recommendation Generators; keep the ones that apply.
     Solid-wall insulation, glazing, heating and solar are additionally gated by
diff --git a/tests/orchestration/test_modelling_heat_pump_sizing_threading.py b/tests/orchestration/test_modelling_heat_pump_sizing_threading.py
new file mode 100644
index 000000000..0b46bb8a6
--- /dev/null
+++ b/tests/orchestration/test_modelling_heat_pump_sizing_threading.py
@@ -0,0 +1,60 @@
+"""The ModellingOrchestrator threads the dwelling's SAP design heat loss into
+the heating Recommendation Generator so the ASHP is sized to the dwelling
+(ADR-0049), mirroring how planning_restrictions and the solar potential are
+threaded. Tests the candidate-wiring seam directly; the end-to-end run-through-
+repos path is covered by the DB integration tests.
+"""
+
+from datatypes.epc.domain.epc_property_data import EpcPropertyData
+from domain.geospatial.planning_restrictions import PlanningRestrictions
+from domain.modelling.measure_type import MeasureType
+from domain.modelling.product import Product
+from domain.modelling.recommendation import Recommendation
+from orchestration.modelling_orchestrator import (
+    _candidate_recommendations,  # pyright: ignore[reportPrivateUsage]
+)
+from repositories.product.product_repository import ProductRepository
+from tests.domain.modelling._elmhurst_recommendation import (
+    parse_recommendation_summary,
+)
+
+
+class _StubProducts(ProductRepository):
+    def get(self, measure_type: str) -> Product:
+        return Product(
+            measure_type=measure_type,
+            unit_cost_per_m2=0.0,
+            contingency_rate=0.15,
+            id=909,
+        )
+
+
+def _ashp_eligible_house() -> EpcPropertyData:
+    epc = parse_recommendation_summary(
+        "ashp_from_system_boiler_with_cylinder_001431_before.pdf"
+    )
+    epc.property_type = "House"
+    return epc
+
+
+def test_candidate_recommendations_sizes_the_ashp_to_the_threaded_design_heat_loss() -> (
+    None
+):
+    # Arrange — an ASHP-eligible house and a high (17.5 kW) design heat loss.
+    epc = _ashp_eligible_house()
+
+    # Act
+    recommendations: list[Recommendation] = _candidate_recommendations(
+        epc, _StubProducts(), PlanningRestrictions(), None, None, design_heat_loss_kw=17.5
+    )
+
+    # Assert — the ASHP is sized to the load (capped 12 kW aroTHERM plus 110281),
+    # not the fixed 5 kW / floor-area-proxy unit.
+    heating = next(r for r in recommendations if r.surface == "Heating & Hot Water")
+    ashp = next(
+        o
+        for o in heating.options
+        if o.measure_type == MeasureType.AIR_SOURCE_HEAT_PUMP
+    )
+    assert ashp.overlay.heating is not None
+    assert ashp.overlay.heating.main_heating_index_number == 110281