diff --git a/docs/adr/0049-ashp-sized-to-the-dwelling-design-heat-loss.md b/docs/adr/0049-ashp-sized-to-the-dwelling-design-heat-loss.md
new file mode 100644
index 000000000..ce53044a7
--- /dev/null
+++ b/docs/adr/0049-ashp-sized-to-the-dwelling-design-heat-loss.md
@@ -0,0 +1,98 @@
+# The ASHP recommendation sizes the heat pump to the dwelling's design heat loss
+
+## Status
+
+accepted
+
+## Context
+
+The ASHP measure (ADR-0024) installed **one fixed heat pump** — a 4.37 kW Vaillant
+aroTHERM plus (PCDB Table 362 record 110257) — on every eligible dwelling, as a
+"representative, contractor-installable" end-state. SAP 10.2 Appendix N reads a
+heat pump's efficiency at the dwelling's **PSR** (Power-to-Space-heating-Ratio =
+rated output / design heat loss): efficiency peaks around PSR 0.8 and, per
+footnote 44, **collapses toward 100% below the record's smallest PSR**.
+
+A portfolio-796 audit found the fixed pump scoring the ASHP **below the baseline**
+on high-heat-loss dwellings. Property 712794 (uprn 10002468116): a leaky dwelling
+with a **17.5 kW design heat loss**, so a 4.37 kW pump lands at **PSR 0.25** — the
+efficiency collapse zone. The calculator correctly applied ~195% SPF; combined
+with pricing electricity at the standard tariff against an off-peak storage
+baseline, the ASHP scored **SAP 10.33 vs the baseline 11.96 (−1.63)** and the
+Optimiser never selected it. Every electric-heated dwelling too leaky for a 4.37 kW
+pump was capped below the goal band by this — a modelling defect, not a calculator
+one (SAP's PSR curve is correct: a grossly undersized heat pump *is* inefficient).
+
+The fix needs the dwelling's design heat loss, which the calculator computes
+internally (annual-average HLC × the SAP design temperature difference, 24.2 K)
+but did not surface.
+
+Sizing raised a second question the accredited data answered. MCS capacity sizing
+targets **PSR ≈ 1.0** (output ≥ design heat loss). But the **relodged Elmhurst ASHP
+cert** — a real dwelling our calculator gives a 6.40 kW design heat loss — was
+fitted with the **5 kW** Vaillant (110257, 4.37 kW output), i.e. **PSR 0.68**, and
+scores SAP 72.08. A PSR-1.0 rule picks the 7 kW pump for that dwelling → SAP 73.86,
+**oversizing by a rung and inflating SAP by ~1.8**, and no longer reproduces the
+accredited cert. SAP efficiency at PSR 0.68 and 1.0 is near-identical (~400%), so
+the real installer's smaller, cheaper choice is the realistic one.
+
+## Decision
+
+Size the heat pump to the dwelling and expose what sizing needs:
+
+1. **`SapResult.design_heat_loss_kw`** — the calculator surfaces the design heat
+   loss (annual-average HLC × 24.2 K / 1000), the quantity Appendix N's PSR
+   divides the pump output by.
+
+2. **A curated aroTHERM plus ladder** — one representative PCDB record per nominal
+   size across the domestic range (`_ASHP_SIZING_LADDER`): 3.5 kW (110249), 5 kW
+   (110257, the prior anchor), 7 kW (110265), 10 kW (110273), 12 kW (110281). All
+   the same "& AI VIH RW" variant, so the ladder is a consistent product line.
+
+3. **Size to the Appendix-N efficiency peak, not MCS capacity.**
+   `select_ashp_pcdb_id` picks the rung whose rated output is nearest **0.8 ×
+   design heat loss** (PSR ≈ 0.8, the efficiency peak). This reproduces the pump a
+   real installer fits — validated against the relodged Elmhurst cert at delta 0 —
+   and keeps the pump clear of the low-PSR collapse. A load beyond the 12 kW range
+   naturally selects the largest rung (an honest, capped undersize; those
+   dwellings need fabric-first).
+
+4. **Cost and efficiency share one figure.** `ashp_cost_inputs` takes the same
+   design heat loss (over the floor-area proxy), so the rate-sheet install band
+   (ADR-0025) and the pump's efficiency record are sized from one number. The
+   orchestrator reads `design_heat_loss_kw` off a baseline score and threads it
+   into the heating generator; the harness computes it via a calculator pass.
+
+## Consequences
+
+- Property 712794's ASHP goes from **−1.63 SAP (never selected)** to **+44.2 SAP
+  (selected)**; its plan lifts **G (12.0) → B (89.1)**. Electric-heated dwellings
+  previously capped below goal C by the undersized pump can now reach it — the
+  largest HIGH-severity mover on the portfolio-796 `plan-stops-short-of-goal` and
+  `plan-stuck-in-low-band` groups.
+- The accredited relodged Elmhurst ASHP cert still reproduces at **delta 0** (the
+  6.40 kW dwelling sizes to the 5 kW Vaillant it was actually fitted with). Two
+  self-snapshot cascade pins re-pin to their correctly-sized larger pumps (both
+  dwellings were undersized by the fixed unit, so their end-state SAP rises).
+- Adding `design_heat_loss_kw` to `SapResult` is a required field; manual
+  constructions (test stubs) carry it.
+- Extends [ADR-0024](0024-ashp-bundle-absolute-end-state.md) (the ASHP end-state)
+  and [ADR-0025](0025-ashp-cost-composed-from-the-rate-sheet.md) (size-banded
+  cost); the cost bands already scaled — they now receive the real heat loss.
+
+### Alternatives rejected
+
+- **MCS capacity sizing (PSR ≈ 1.0, output ≥ design heat loss).** Rejected: it
+  oversizes relative to real installs, inflating SAP ~1.8 per dwelling and
+  breaking the accredited-cert reproduction. SAP efficiency is equivalent at
+  PSR 0.68–1.0, so the realistic (installer) size is preferred.
+- **Keep the fixed 4.37 kW pump.** Rejected: the collapse on high-heat-loss
+  dwellings is the defect under repair.
+- **Fabric-first sequencing (score the ASHP only after insulation).** A real
+  concern — the Optimiser scores each measure independently against the
+  un-insulated baseline (ADR-0016) — but a separate change to measure scoring/
+  sequencing, not pump sizing. Sizing the pump fixes the standalone score
+  regardless; noted for follow-up.
+- **Nearest record from all ~10,000 Table 362 heat pumps.** Rejected: an opaque,
+  unauditable selection; a curated single-product ladder keeps ADR-0024's
+  "representative, contractor-installable" intent.
diff --git a/domain/modelling/generators/heating_recommendation.py b/domain/modelling/generators/heating_recommendation.py
index 70c33142c..ebb7d1f5e 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
@@ -137,6 +138,42 @@ _ASHP_OVERLAY = HeatingOverlay(
 )
 
 
+# ASHP sizing ladder (ADR-0049): the Vaillant aroTHERM plus "& AI VIH RW" family,
+# one representative PCDB Table 362 record per nominal size, ascending by rated
+# max output (kW). The pump is sized to the dwelling's design heat loss so SAP
+# 10.2 Appendix N reads the heat-pump efficiency near its PSR peak (~1.0), rather
+# than a single fixed unit that is grossly undersized on high-heat-loss dwellings
+# — where the PSR collapses and the ASHP scores at/below the resistance baseline.
+# 110257 (the 5 kW rung) is the previously-fixed anchor, validated against the
+# relodged after-cert.
+_ASHP_SIZING_LADDER: tuple[tuple[float, int], ...] = (
+    (3.76, 110249),   # aroTHERM plus 3.5 kW
+    (4.37, 110257),   # aroTHERM plus 5 kW
+    (6.40, 110265),   # aroTHERM plus 7 kW
+    (7.93, 110273),   # aroTHERM plus 10 kW
+    (11.48, 110281),  # aroTHERM plus 12 kW
+)
+# SAP 10.2 Appendix-N heat-pump efficiency peaks around PSR 0.8 (output / design
+# heat loss); sizing to it reproduces real installer choices (accredited-cert
+# validated) rather than the MCS capacity target (PSR 1.0), which oversizes.
+_EFFICIENCY_PEAK_PSR: float = 0.8
+
+
+def select_ashp_pcdb_id(design_heat_loss_kw: float) -> int:
+    """The PCDB heat-pump record to install for a dwelling with
+    `design_heat_loss_kw` (ADR-0049): the ladder rung whose rated output sits
+    nearest the SAP 10.2 Appendix-N efficiency peak — PSR ~= 0.8, i.e. a rated
+    output of ``0.8 x design_heat_loss`` — which reproduces the pump a real
+    installer fits (validated against the relodged Elmhurst ASHP cert) and keeps
+    the heat pump clear of the low-PSR efficiency collapse. A load beyond the
+    domestic aroTHERM plus range naturally selects the largest rung."""
+    target_output_kw: float = _EFFICIENCY_PEAK_PSR * design_heat_loss_kw
+    return min(
+        _ASHP_SIZING_LADDER,
+        key=lambda rung: abs(rung[0] - target_output_kw),
+    )[1]
+
+
 # --- Gas boiler upgrade (Heating/HW expansion): replace an existing wet boiler
 # with a modern gas condensing boiler. Validated against Elmhurst before/after
 # re-lodgements (cert 001431): the upgrade always targets mains gas — gas->gas
@@ -254,16 +291,25 @@ _OIL_FUEL_CODES = frozenset({28, 4, 71, 73, 75, 76})
 _LPG_FUEL_CODES = frozenset({27, 2, 3, 5, 9})
 
 
-def ashp_cost_inputs(epc: EpcPropertyData) -> AshpCostInputs:
+def ashp_cost_inputs(
+    epc: EpcPropertyData, design_heat_loss_kw: Optional[float] = None
+) -> AshpCostInputs:
     """Read an `EpcPropertyData` into the typed inputs `Products.ashp_bundle_cost`
-    needs: the existing system, property-size band, design heat loss (floor-area
-    proxy), radiator count, and whether a wet system can be reused (ADR-0025)."""
+    needs: the existing system, property-size band, design heat loss, radiator
+    count, and whether a wet system can be reused (ADR-0025). ``design_heat_loss_kw``
+    (the calculator's value, ADR-0049) sizes the cost band; when omitted it falls
+    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),
@@ -307,17 +353,23 @@ 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)
 
@@ -702,23 +754,37 @@ 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
+    # 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.
     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=(
             "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/domain/sap10_calculator/calculator.py b/domain/sap10_calculator/calculator.py
index c30234a7c..34f0f1ce7 100644
--- a/domain/sap10_calculator/calculator.py
+++ b/domain/sap10_calculator/calculator.py
@@ -65,6 +65,11 @@ from domain.sap10_calculator.worksheet.rating import (
 
 _AIR_HEAT_CAPACITY_WH_PER_M3_K: Final[float] = 0.33
 _TIME_CONSTANT_DIVISOR_KJ_TO_WH: Final[float] = 3.6
+# SAP design temperature difference (21 C internal - -3.2 C external): the delta
+# the design heat loss and Appendix N's PSR are evaluated at (matches
+# `_SAP_DESIGN_HEAT_LOSS_DELTA_T_K` in rdsap.cert_to_inputs).
+_DESIGN_HEAT_LOSS_DELTA_T_K: Final[float] = 24.2
+_WATTS_PER_KW: Final[float] = 1000.0
 
 # §9a default — used as `CalculatorInputs.energy_requirements` default for
 # synthetic constructions that bypass cert_to_inputs. All-zero fuel; the
@@ -403,6 +408,11 @@ 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 +882,9 @@ 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=(sum(monthly_hlc) / 12.0)
+        * _DESIGN_HEAT_LOSS_DELTA_T_K
+        / _WATTS_PER_KW,
         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/harness/console.py b/harness/console.py
index b60528832..a9aa222b5 100644
--- a/harness/console.py
+++ b/harness/console.py
@@ -273,10 +273,13 @@ def candidate_recommendations(
         if solar_insights is not None and "solarPotential" in solar_insights
         else None
     )
+    # The SAP design heat loss sizes the ASHP to the dwelling (ADR-0049).
+    design_heat_loss_kw: float = Sap10Calculator().calculate(epc).design_heat_loss_kw
     return _candidate_recommendations(
         epc,
         products or ProductJsonRepository(catalogue_path),
         planning_restrictions,
         solar_potential,
         considered_measures,
+        design_heat_loss_kw,
     )
diff --git a/orchestration/modelling_orchestrator.py b/orchestration/modelling_orchestrator.py
index e3180a4d2..7a30d7e82 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
@@ -331,7 +332,13 @@ def _candidate_recommendations(
                 MeasureType.SYSTEM_TUNE_UP,
                 MeasureType.SYSTEM_TUNE_UP_ZONED,
             ),
-            lambda: recommend_heating(effective_epc, products, planning_restrictions, considered_measures),
+            lambda: recommend_heating(
+                effective_epc,
+                products,
+                planning_restrictions,
+                considered_measures,
+                design_heat_loss_kw=design_heat_loss_kw,
+            ),
         ),
         (
             admitted(MeasureType.SECONDARY_HEATING_REMOVAL),
@@ -384,9 +391,20 @@ def _scored_candidate_groups(
 ) -> list[list[ScoredOption]]:
     """One group per Recommendation: each Option scored independently against
     the baseline (role-1 warm-start signal, ADR-0016)."""
+    # The SAP design heat loss sizes the ASHP to the dwelling (ADR-0049); read it
+    # off a baseline score, which the group scoring computes anyway.
+    baseline_result = scorer.score(effective_epc, []).sap_result
+    design_heat_loss_kw: Optional[float] = (
+        baseline_result.design_heat_loss_kw if baseline_result is not None else None
+    )
     groups: list[list[ScoredOption]] = []
     for recommendation in _candidate_recommendations(
-        effective_epc, products, planning_restrictions, solar_potential, considered_measures
+        effective_epc,
+        products,
+        planning_restrictions,
+        solar_potential,
+        considered_measures,
+        design_heat_loss_kw,
     ):
         options = list(recommendation.options)
         impacts: list[MeasureImpact] = independent_option_impacts(
diff --git a/tests/domain/billing/test_energy_breakdown.py b/tests/domain/billing/test_energy_breakdown.py
index f754bc662..630c314b1 100644
--- a/tests/domain/billing/test_energy_breakdown.py
+++ b/tests/domain/billing/test_energy_breakdown.py
@@ -40,6 +40,7 @@ def _sap_result(
         space_heating_kwh_per_yr=0.0,
         space_cooling_kwh_per_yr=0.0,
         fabric_energy_efficiency_kwh_per_m2_yr=0.0,
+        design_heat_loss_kw=0.0,
         main_heating_fuel_kwh_per_yr=main_heating_fuel_kwh_per_yr,
         main_2_heating_fuel_kwh_per_yr=main_2_heating_fuel_kwh_per_yr,
         secondary_heating_fuel_kwh_per_yr=secondary_heating_fuel_kwh_per_yr,
diff --git a/tests/domain/modelling/test_ashp_cost_inputs.py b/tests/domain/modelling/test_ashp_cost_inputs.py
index c75cf52fc..ce528f1bb 100644
--- a/tests/domain/modelling/test_ashp_cost_inputs.py
+++ b/tests/domain/modelling/test_ashp_cost_inputs.py
@@ -82,3 +82,19 @@ def test_oil_and_lpg_dwellings_are_reusable_wet_systems() -> None:
     assert ashp_cost_inputs(_with_fuel(28)).existing_system is AshpExistingSystem.OIL
     assert ashp_cost_inputs(_with_fuel(27)).existing_system is AshpExistingSystem.LPG
     assert ashp_cost_inputs(_with_fuel(28)).has_reusable_wet_system is True
+
+
+def test_threaded_design_heat_loss_drives_the_cost_band_over_the_proxy() -> None:
+    # When the caller threads the calculator's real design heat loss (ADR-0049),
+    # the ASHP cost sizes to it — not the floor-area proxy — so a leaky dwelling's
+    # bigger pump is priced on the right band.
+    # Arrange
+    epc = parse_recommendation_summary(
+        "ashp_from_system_boiler_with_cylinder_001431_before.pdf"
+    )
+
+    # Act
+    inputs: AshpCostInputs = ashp_cost_inputs(epc, design_heat_loss_kw=17.5)
+
+    # Assert — the threaded value, not the 4.5 kW proxy.
+    assert inputs.design_heat_loss_kw == 17.5
diff --git a/tests/domain/modelling/test_elmhurst_cascade_pins.py b/tests/domain/modelling/test_elmhurst_cascade_pins.py
index c99f7b6f0..a52cf9787 100644
--- a/tests/domain/modelling/test_elmhurst_cascade_pins.py
+++ b/tests/domain/modelling/test_elmhurst_cascade_pins.py
@@ -664,7 +664,11 @@ def test_ashp_overlay_scores_the_vaillant_end_state_from_a_gas_boiler() -> None:
     before: EpcPropertyData = parse_recommendation_summary(
         "ashp_from_gas_boiler_001431_before.pdf"
     )
-    recommendation: Recommendation | None = recommend_heating(before, _AnyProduct())
+    recommendation: Recommendation | None = recommend_heating(
+        before,
+        _AnyProduct(),
+        design_heat_loss_kw=Sap10Calculator().calculate(before).design_heat_loss_kw,
+    )
     assert recommendation is not None
     option = next(
         o for o in recommendation.options if o.measure_type == "air_source_heat_pump"
@@ -678,12 +682,15 @@ def test_ashp_overlay_scores_the_vaillant_end_state_from_a_gas_boiler() -> None:
     # CO2/PE are the postcode DEMAND cascade now that `Sap10Calculator.
     # calculate` computes EPC emissions/PE on local weather (SAP 10.2
     # Appendix U p.124); SAP is unchanged (UK-average rating cascade).
+    # Re-pinned after ADR-0049 sizes the pump to this dwelling's 9.69 kW design
+    # heat loss (the 10 kW aroTHERM plus, 110273) instead of the fixed 5 kW unit,
+    # which was undersized here — the correctly-sized pump raises the end-state SAP.
     _assert_overlay_scores(
         before,
         option.overlay,
-        sap=51.99820176096402,
-        co2=1065.7593506066496,
-        pe=10995.781557709413,
+        sap=69.6498827017577,
+        co2=698.896585317197,
+        pe=7271.044741124457,
     )
 
 
@@ -697,7 +704,11 @@ def test_ashp_overlay_scores_the_vaillant_end_state_from_a_gas_boiler_instant_hw
     before: EpcPropertyData = parse_recommendation_summary(
         "ashp_from_gas_boiler_instant_hw_001431_before.pdf"
     )
-    recommendation: Recommendation | None = recommend_heating(before, _AnyProduct())
+    recommendation: Recommendation | None = recommend_heating(
+        before,
+        _AnyProduct(),
+        design_heat_loss_kw=Sap10Calculator().calculate(before).design_heat_loss_kw,
+    )
     assert recommendation is not None
     option = next(
         o for o in recommendation.options if o.measure_type == "air_source_heat_pump"
@@ -709,12 +720,15 @@ def test_ashp_overlay_scores_the_vaillant_end_state_from_a_gas_boiler_instant_hw
     # see `test_gas_boiler_instant_hw_before_baselines`.
     # CO2/PE are the postcode DEMAND cascade now (see the boiler-1 pin above);
     # SAP is unchanged (UK-average rating cascade).
+    # Re-pinned after ADR-0049 sizes the pump to this dwelling's 12.87 kW design
+    # heat loss (the capped 12 kW aroTHERM plus, 110281) instead of the fixed 5 kW
+    # unit, which was badly undersized here — the sized pump raises the SAP.
     _assert_overlay_scores(
         before,
         option.overlay,
-        sap=39.00740809309464,
-        co2=1845.8588018295509,
-        pe=18944.42568846759,
+        sap=72.22779402266684,
+        co2=870.0854694592007,
+        pe=9025.363011043328,
     )
 
 
@@ -734,7 +748,11 @@ def test_ashp_overlay_reproduces_the_relodged_after_from_a_system_boiler_with_cy
     after: EpcPropertyData = parse_recommendation_summary(
         "ashp_from_system_boiler_with_cylinder_001431_after.pdf"
     )
-    recommendation: Recommendation | None = recommend_heating(before, _AnyProduct())
+    recommendation: Recommendation | None = recommend_heating(
+        before,
+        _AnyProduct(),
+        design_heat_loss_kw=Sap10Calculator().calculate(before).design_heat_loss_kw,
+    )
     assert recommendation is not None
     option = next(
         o for o in recommendation.options if o.measure_type == "air_source_heat_pump"
diff --git a/tests/domain/modelling/test_heating_recommendation.py b/tests/domain/modelling/test_heating_recommendation.py
index 366c51869..470ff51aa 100644
--- a/tests/domain/modelling/test_heating_recommendation.py
+++ b/tests/domain/modelling/test_heating_recommendation.py
@@ -7,7 +7,10 @@ later slices. Detection + pricing only; impact is produced by scoring (ADR-0016)
 
 from datatypes.epc.domain.epc_property_data import EpcPropertyData
 from domain.geospatial.planning_restrictions import PlanningRestrictions
-from domain.modelling.generators.heating_recommendation import recommend_heating
+from domain.modelling.generators.heating_recommendation import (
+    recommend_heating,
+    select_ashp_pcdb_id,
+)
 from domain.modelling.measure_type import MeasureType
 from domain.modelling.product import Product
 from domain.modelling.recommendation import Recommendation
@@ -200,8 +203,11 @@ 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 5.5 kW design heat loss sizes to the 5 kW aroTHERM plus (110257):
+    # PSR-0.8 target ~4.4 kW, nearest the 4.37 kW rung.
+    recommendation: Recommendation | None = recommend_heating(
+        baseline, _StubProducts(), design_heat_loss_kw=5.5
+    )
 
     # Assert — the ASHP bundle carries the absolute heat-pump end-state.
     assert recommendation is not None
@@ -764,3 +770,48 @@ def test_boiler_upgrade_leaves_adequate_controls_unchanged() -> None:
     ).overlay.heating
     assert overlay is not None
     assert overlay.main_heating_control is None
+
+
+def test_ashp_sizing_targets_the_appendix_n_efficiency_peak() -> None:
+    # A 6 kW design heat loss sizes to the SAP Appendix-N efficiency peak
+    # (PSR ~= 0.8, so ~4.8 kW rated output): the 5 kW aroTHERM plus (PCDB 110257,
+    # ~4.37 kW) — what a real installer fits — not the oversized 7 kW unit, and
+    # well clear of the low-PSR efficiency collapse.
+    # Act
+    pcdb_id = select_ashp_pcdb_id(design_heat_loss_kw=6.0)
+
+    # Assert — the 5 kW aroTHERM plus rung.
+    assert pcdb_id == 110257
+
+
+def test_ashp_sizing_caps_at_the_largest_pump_for_a_high_heat_loss_dwelling() -> None:
+    # A very leaky dwelling (17.5 kW design heat loss, e.g. UPRN 10002468116)
+    # exceeds the domestic aroTHERM plus range, so sizing caps at the largest
+    # rung — the 12 kW aroTHERM plus (PCDB 110281, ~11.48 kW) — an honest, still
+    # far better than the fixed 5 kW pump whose PSR collapsed to ~0.25.
+    # Act
+    pcdb_id = select_ashp_pcdb_id(design_heat_loss_kw=17.5)
+
+    # 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
diff --git a/tests/domain/property_baseline/test_calculator_rebaseliner.py b/tests/domain/property_baseline/test_calculator_rebaseliner.py
index f6002bd48..49a2364dc 100644
--- a/tests/domain/property_baseline/test_calculator_rebaseliner.py
+++ b/tests/domain/property_baseline/test_calculator_rebaseliner.py
@@ -40,6 +40,7 @@ def _sap_result(
         space_heating_kwh_per_yr=0.0,
         space_cooling_kwh_per_yr=0.0,
         fabric_energy_efficiency_kwh_per_m2_yr=0.0,
+        design_heat_loss_kw=0.0,
         main_heating_fuel_kwh_per_yr=0.0,
         main_2_heating_fuel_kwh_per_yr=0.0,
         secondary_heating_fuel_kwh_per_yr=0.0,
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)
diff --git a/tests/harness/test_report.py b/tests/harness/test_report.py
index 7aa820a64..84330584e 100644
--- a/tests/harness/test_report.py
+++ b/tests/harness/test_report.py
@@ -30,9 +30,13 @@ _GOLDEN = (
 _WITHIN_TOLERANCE = "0036-6325-1100-0063-1226"
 _DIVERGENT = "0240-0200-5706-2365-8010"
 
-# 0330 fires all three trigger kinds: an uninsulated cavity wall (cavity fill),
-# its dependent mechanical ventilation, and an uninsulated solid floor.
-_THREE_MEASURES = "0330-2249-8150-2326-4121"
+# 0390 fires three measures — an uninsulated solid floor, low-energy lighting,
+# and the ASHP bundle — so every fired measure's trigger attributes are
+# exercised together. (0330, the previous fixture, now reaches band C on the
+# correctly-sized ASHP alone: ADR-0049 sizes the pump to the dwelling's design
+# heat loss, so the undersized-pump-era companion solid_floor_insulation is no
+# longer needed there.)
+_THREE_MEASURES = "0390-2254-6420-2126-5561"
 
 
 def _triggers_by_measure(report: PropertyReport) -> dict[str, MeasureTrigger]:
@@ -80,17 +84,12 @@ def test_each_fired_measure_carries_the_attributes_that_triggered_it() -> None:
     # Assert — the Plan ran and every fired measure names its trigger fields.
     assert report.plan is not None
     assert report.plan_error is None
-    # The gain-maximising package: the efficient representative ASHP (ADR-0025)
-    # plus solid-floor insulation. The cavity wall + its forced mechanical
-    # ventilation (ADR-0016) are NOT selected — the wall earns +SAP alone but
-    # the forced-ventilation penalty makes the pair net-negative, so the
-    # Optimiser correctly leaves them out (see test_measure_dependency /
-    # test_optimiser for the forced-edge unit coverage; cavity_wall +
-    # mechanical_ventilation trigger fields are exercised in
-    # test_cavity_wall_recommendation / the ventilation generator tests).
+    # The selected package: the dwelling-sized ASHP (ADR-0025/ADR-0049) plus
+    # solid-floor insulation and low-energy lighting.
     triggers: dict[str, MeasureTrigger] = _triggers_by_measure(report)
     assert set(triggers) == {
         "solid_floor_insulation",
+        "low_energy_lighting",
         "air_source_heat_pump",
     }
     # Solid-floor insulation fired off an uninsulated solid ground floor.
@@ -98,6 +97,12 @@ def test_each_fired_measure_carries_the_attributes_that_triggered_it() -> None:
         "floor_insulation_thickness": None,
         "floor_construction_type": "Solid",
     }
+    # Low-energy lighting fired off the dwelling's lodged bulb counts.
+    assert triggers["low_energy_lighting"].triggers == {
+        "incandescent_fixed_lighting_bulbs_count": 0,
+        "cfl_fixed_lighting_bulbs_count": 0,
+        "low_energy_fixed_lighting_bulbs_count": 9,
+    }
     # The ASHP bundle fired off the gas-dwelling main it replaces.
     assert triggers["air_source_heat_pump"].triggers == {
         "property_type": "0",
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
diff --git a/tests/orchestration/test_property_baseline_orchestrator.py b/tests/orchestration/test_property_baseline_orchestrator.py
index 912dc9085..b4125059d 100644
--- a/tests/orchestration/test_property_baseline_orchestrator.py
+++ b/tests/orchestration/test_property_baseline_orchestrator.py
@@ -128,6 +128,7 @@ def _sap_result_with_lighting() -> SapResult:
         space_heating_kwh_per_yr=0.0,
         space_cooling_kwh_per_yr=0.0,
         fabric_energy_efficiency_kwh_per_m2_yr=0.0,
+        design_heat_loss_kw=0.0,
         main_heating_fuel_kwh_per_yr=0.0,
         main_2_heating_fuel_kwh_per_yr=0.0,
         secondary_heating_fuel_kwh_per_yr=0.0,