diff --git a/domain/sap10_calculator/calculator.py b/domain/sap10_calculator/calculator.py
index c30234a7c..f3a10165b 100644
--- a/domain/sap10_calculator/calculator.py
+++ b/domain/sap10_calculator/calculator.py
@@ -403,6 +403,11 @@ class SapResult:
     space_heating_kwh_per_yr: float
     space_cooling_kwh_per_yr: float
     fabric_energy_efficiency_kwh_per_m2_yr: float
+    # The dwelling's SAP design heat loss (kW) — the annual-average heat loss
+    # coefficient at the design temperature difference (24.2 K). Output-only:
+    # it is the quantity Appendix N's PSR divides a heat pump's rated output by,
+    # so the ASHP recommendation sizes the pump to the dwelling from it.
+    design_heat_loss_kw: float
     main_heating_fuel_kwh_per_yr: float
     main_2_heating_fuel_kwh_per_yr: float
     secondary_heating_fuel_kwh_per_yr: float
@@ -872,6 +877,7 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
         space_heating_kwh_per_yr=space_heating_kwh,
         space_cooling_kwh_per_yr=space_cooling_kwh,
         fabric_energy_efficiency_kwh_per_m2_yr=inputs.fabric_energy_efficiency_kwh_per_m2_yr,
+        design_heat_loss_kw=0.0,  # stub — computed in GREEN
         main_heating_fuel_kwh_per_yr=main_fuel_kwh,
         main_2_heating_fuel_kwh_per_yr=inputs.energy_requirements.main_2_fuel_kwh_per_yr,
         secondary_heating_fuel_kwh_per_yr=secondary_fuel_kwh,
diff --git a/tests/domain/sap10_calculator/test_calculator.py b/tests/domain/sap10_calculator/test_calculator.py
index 7ab7d3c0c..12157d172 100644
--- a/tests/domain/sap10_calculator/test_calculator.py
+++ b/tests/domain/sap10_calculator/test_calculator.py
@@ -815,3 +815,20 @@ def test_split_tariff_charges_space_heating_at_off_peak_rate() -> None:
         + (r_e7.pumps_fans_kwh_per_yr + r_e7.lighting_kwh_per_yr) * 0.1319
     )
     assert r_e7.total_fuel_cost_gbp == pytest.approx(expected_cost, rel=1e-6)
+
+
+def test_calculate_exposes_the_design_heat_loss_for_heat_pump_sizing() -> None:
+    # The ASHP recommendation sizes the pump to the dwelling's SAP design heat
+    # loss — the annual-average HLC at the design temperature difference (24.2 K),
+    # the quantity Appendix N's PSR divides the pump's rated output by. The
+    # calculator surfaces it as a typed output, derived from the HLC it already
+    # computes.
+    # Arrange
+    inputs = _baseline_inputs()
+
+    # Act
+    result = calculate_sap_from_inputs(inputs)
+
+    # Assert — HLC (W/K) x 24.2 K / 1000 = design heat loss (kW).
+    hlc = result.intermediate["heat_transfer_coefficient_w_per_k"]
+    assert result.design_heat_loss_kw == pytest.approx(hlc * 24.2 / 1000)