diff --git a/backend/documents_parser/tests/fixtures/Summary_001431_case45.pdf b/backend/documents_parser/tests/fixtures/Summary_001431_case45.pdf
new file mode 100644
index 00000000..48a5273e
Binary files /dev/null and b/backend/documents_parser/tests/fixtures/Summary_001431_case45.pdf differ
diff --git a/domain/sap10_calculator/calculator.py b/domain/sap10_calculator/calculator.py
index f7099f18..11cecf73 100644
--- a/domain/sap10_calculator/calculator.py
+++ b/domain/sap10_calculator/calculator.py
@@ -42,7 +42,7 @@ Appendix L + U. RdSAP10 Table 32 (p.95) for fuel prices/CO2/PE factors.
 from __future__ import annotations
 
 from abc import ABC, abstractmethod
-from dataclasses import dataclass, field
+from dataclasses import dataclass, field, replace
 from typing import Final, Optional, TYPE_CHECKING
 
 from domain.sap10_calculator.climate.appendix_u import external_temperature_c
@@ -863,6 +863,25 @@ class Sap10Calculator(SapCalculator):
     """
 
     def calculate(self, epc: "EpcPropertyData") -> SapResult:
-        from domain.sap10_calculator.rdsap.cert_to_inputs import cert_to_inputs
+        # SAP 10.2 Appendix U paragraph 1 (p.124): the SAP and EI ratings are
+        # computed on UK-average climate (so ratings are nationally
+        # comparable), but "other calculations (such as for energy use and
+        # costs on EPCs) are done using local weather" — the EPC-displayed
+        # CO2 emissions and primary energy use postcode-district weather from
+        # the PCDB. So we run two climate cascades and graft the demand
+        # cascade's CO2/PE onto the rating cascade's SAP result. (Worked
+        # example: simulated case 45 — rating SAP 60.53/CO2 692.13 on
+        # UK-average; demand CO2 626.78/PE 6581.59 on the W6 postcode.)
+        from domain.sap10_calculator.rdsap.cert_to_inputs import (
+            cert_to_demand_inputs,
+            cert_to_inputs,
+        )
 
-        return calculate_sap_from_inputs(cert_to_inputs(epc))
+        rating = calculate_sap_from_inputs(cert_to_inputs(epc))
+        demand = calculate_sap_from_inputs(cert_to_demand_inputs(epc))
+        return replace(
+            rating,
+            co2_kg_per_yr=demand.co2_kg_per_yr,
+            primary_energy_kwh_per_yr=demand.primary_energy_kwh_per_yr,
+            primary_energy_kwh_per_m2=demand.primary_energy_kwh_per_m2,
+        )
diff --git a/tests/domain/modelling/test_elmhurst_cascade_pins.py b/tests/domain/modelling/test_elmhurst_cascade_pins.py
index 0e6832b7..19186eae 100644
--- a/tests/domain/modelling/test_elmhurst_cascade_pins.py
+++ b/tests/domain/modelling/test_elmhurst_cascade_pins.py
@@ -686,12 +686,15 @@ def test_ashp_overlay_scores_the_vaillant_end_state_from_a_gas_boiler() -> None:
     # dwelling's baseline fabric and so the ASHP end-state SAP. Still a snapshot
     # of the Vaillant overlay's own output, validated transitively by the
     # system-boiler pin below (which reproduces a real Vaillant cert at delta 0).
+    # 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).
     _assert_overlay_scores(
         before,
         option.overlay,
         sap=51.99820176096402,
-        co2=1268.4645083243888,
-        pe=13080.20756425629,
+        co2=1065.7593506066496,
+        pe=10995.781557709413,
     )
 
 
@@ -715,12 +718,14 @@ def test_ashp_overlay_scores_the_vaillant_end_state_from_a_gas_boiler_instant_hw
     # boiler-1 pin above); the same merge also resolved this cert's main-fuel
     # mapper gap (§14.2 mains-gas derivation), so its raw before now baselines —
     # 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).
     _assert_overlay_scores(
         before,
         option.overlay,
         sap=39.00740809309464,
-        co2=2248.6089062232704,
-        pe=23094.10189037302,
+        co2=1845.8588018295509,
+        pe=18944.42568846759,
     )
 
 
diff --git a/tests/domain/sap10_calculator/worksheet/_elmhurst_worksheet_001431_case45.py b/tests/domain/sap10_calculator/worksheet/_elmhurst_worksheet_001431_case45.py
new file mode 100644
index 00000000..7d9ab95f
--- /dev/null
+++ b/tests/domain/sap10_calculator/worksheet/_elmhurst_worksheet_001431_case45.py
@@ -0,0 +1,107 @@
+"""Mapper-driven cascade pin against the Elmhurst P960-0001-001431
+"simulated case 45" worksheet — a ~47 m² GROUND-FLOOR FLAT heated by an
+air-source HEAT PUMP (PCDB 100053 ECODAN, radiators, MCS=No) with a
+WHC-903 electric-immersion DHW and a 110 L cylinder, postcode W6 9BF
+(SAP Region "Thames Valley").
+
+Case 45 is the 1e-4 oracle for the SAP 10.2 Appendix U (PDF p.124) TWO-
+CLIMATE-CASCADE split. The P960 prints the current dwelling TWICE:
+
+  * Block 1 — "11a. SAP rating / 12a. CO2" — computed on UK-AVERAGE
+    weather (Appendix U Tables U1-U3 region 0). Drives the SAP/EI rating.
+    Space-heat demand (98c) = 7333.79; SAP value (258) = 60.5318 (-> 61);
+    total CO2 (272) = 692.13.
+  * Block 2 — "CALCULATION OF EPC COSTS, EMISSIONS AND PRIMARY ENERGY" —
+    computed on POSTCODE-DISTRICT weather (PCDB Table 172, W6). Drives the
+    EPC-displayed figures. Space-heat demand (98c) = 5921.05; total CO2
+    (272) = 626.78; total primary energy (286) = 6581.59.
+
+Per Appendix U paragraph 1: "Other calculations (such as for energy use
+and costs on EPCs) are done using local weather." `Sap10Calculator.
+calculate` therefore runs both cascades and grafts the demand cascade's
+CO2/PE onto the rating cascade's SAP — this fixture pins BOTH.
+
+Like the other `_elmhurst_worksheet_001431_case*` fixtures it does NOT
+hand-build the EpcPropertyData: it routes the Summary PDF through
+ElmhurstSiteNotesExtractor + from_elmhurst_site_notes so the pin exercises
+the WHOLE extractor + mapper + calculator pipeline.
+
+Source: user-simulated PDFs at `sap worksheets/golden fixture debugging/
+simulated case 45/`. The Summary is mirrored into the tracked
+`backend/documents_parser/tests/fixtures/Summary_001431_case45.pdf` so the
+test runs without depending on the unstaged workspace.
+
+Per [[feedback-zero-error-strict]]: pins are abs <= 1e-4 against the PDF.
+"""
+
+from __future__ import annotations
+
+import re
+import subprocess
+from pathlib import Path
+from typing import Final
+
+from backend.documents_parser.elmhurst_extractor import ElmhurstSiteNotesExtractor
+from datatypes.epc.domain.epc_property_data import EpcPropertyData
+from datatypes.epc.domain.mapper import EpcPropertyDataMapper
+
+# parents[0]=worksheet/, [1]=sap10_calculator/, [2]=domain/, [3]=tests/,
+# [4]=repo root.
+_SUMMARY_PDF: Final[Path] = (
+    Path(__file__).resolve().parents[4]
+    / "backend" / "documents_parser" / "tests" / "fixtures"
+    / "Summary_001431_case45.pdf"
+)
+
+# Block 1 — UK-average RATING cascade (`cert_to_inputs`).
+RATING_SPACE_HEATING_KWH: Final[float] = 7333.7892    # (98c)
+RATING_SAP_CONTINUOUS: Final[float] = 60.5318         # (258) un-rounded
+RATING_SAP_INTEGER: Final[int] = 61                   # (258)
+RATING_CO2_KG_PER_YR: Final[float] = 692.1287         # (272)
+
+# Block 2 — POSTCODE-district DEMAND cascade (`cert_to_demand_inputs`).
+DEMAND_SPACE_HEATING_KWH: Final[float] = 5921.0486    # (98c)
+DEMAND_CO2_KG_PER_YR: Final[float] = 626.7797         # (272)
+DEMAND_PRIMARY_ENERGY_KWH: Final[float] = 6581.5936   # (286)
+
+
+def _summary_pdf_to_textract_style_pages(pdf_path: Path) -> list[str]:
+    """Convert a Summary PDF into the per-page text format the
+    ElmhurstSiteNotesExtractor expects (label/value token sequences).
+    Mirror of the helper in the other `_elmhurst_worksheet_*` fixtures.
+    """
+    info = subprocess.run(
+        ["pdfinfo", str(pdf_path)], capture_output=True, text=True, check=True,
+    ).stdout
+    m = re.search(r"Pages:\s+(\d+)", info)
+    if m is None:
+        raise RuntimeError(f"Could not parse page count from {pdf_path}")
+    page_count = int(m.group(1))
+    pages: list[str] = []
+    for i in range(1, page_count + 1):
+        layout = subprocess.run(
+            [
+                "pdftotext", "-layout", "-f", str(i), "-l", str(i),
+                str(pdf_path), "-",
+            ],
+            capture_output=True, text=True, check=True,
+        ).stdout
+        tokens: list[str] = []
+        for line in layout.splitlines():
+            if not line.strip():
+                tokens.append("")
+                continue
+            parts = [p for p in re.split(r"\s{2,}", line.strip()) if p]
+            tokens.extend(parts)
+        pages.append("\n".join(tokens))
+    return pages
+
+
+def build_epc() -> EpcPropertyData:
+    """Route the simulated case-45 Summary through extractor + mapper.
+    No hand-built EpcPropertyData — the extractor and mapper are part of
+    the test target. This module is a pin PROVIDER (build_epc + constants);
+    the collected assertions live in `test_section_cascade_pins`."""
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_PDF)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+    return EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
diff --git a/tests/domain/sap10_calculator/worksheet/test_e2e_elmhurst_sap_score.py b/tests/domain/sap10_calculator/worksheet/test_e2e_elmhurst_sap_score.py
index 1637f281..1625bcbc 100644
--- a/tests/domain/sap10_calculator/worksheet/test_e2e_elmhurst_sap_score.py
+++ b/tests/domain/sap10_calculator/worksheet/test_e2e_elmhurst_sap_score.py
@@ -24,7 +24,10 @@ from typing import Final
 
 import pytest
 
-from domain.sap10_calculator.calculator import Sap10Calculator
+from domain.sap10_calculator.calculator import (
+    Sap10Calculator,
+    calculate_sap_from_inputs,
+)
 from domain.sap10_calculator.rdsap.cert_to_inputs import (
     cert_to_inputs,
     water_heating_section_from_cert,
@@ -338,8 +341,13 @@ def test_sap_result_pin(fixture_name: str, field_name: str) -> None:
     epc = _FIXTURE_MODULES[fixture_name].build_epc()
     expected = getattr(pin, field_name)
 
-    # Act
-    result = Sap10Calculator().calculate(epc)
+    # Act — these pins are the worksheet's Block-1 (energy-rating) line refs,
+    # i.e. the UK-average RATING cascade. `Sap10Calculator.calculate` now
+    # grafts the postcode DEMAND cascade's CO2/PE onto the result (SAP 10.2
+    # Appendix U p.124), so the rating-cascade fields are pinned via
+    # `cert_to_inputs` directly; the demand cascade is pinned separately
+    # (corpus gauge + simulated case 45 Block-2 pins).
+    result = calculate_sap_from_inputs(cert_to_inputs(epc))
     actual = getattr(result, field_name)
 
     # Assert
diff --git a/tests/domain/sap10_calculator/worksheet/test_section_cascade_pins.py b/tests/domain/sap10_calculator/worksheet/test_section_cascade_pins.py
index 6547a585..ce93e21c 100644
--- a/tests/domain/sap10_calculator/worksheet/test_section_cascade_pins.py
+++ b/tests/domain/sap10_calculator/worksheet/test_section_cascade_pins.py
@@ -46,6 +46,7 @@ from tests.domain.sap10_calculator.worksheet import (
     _elmhurst_worksheet_001431_case21 as _w001431_case21,
     _elmhurst_worksheet_001431_case43 as _w001431_case43,
     _elmhurst_worksheet_001431_case44 as _w001431_case44,
+    _elmhurst_worksheet_001431_case45 as _w001431_case45,
 )
 
 
@@ -491,6 +492,67 @@ def test_case44_blower_door_pressure_test_matches_pdf() -> None:
     _pin(vent.effective_monthly_ach[0], 0.5812, "§2 (25) Jan case44")
 
 
+def test_case45_heat_pump_two_climate_cascade_matches_pdf() -> None:
+    """Simulated case 45 (heat-pump ground-floor flat, postcode W6) is the
+    1e-4 oracle for the SAP 10.2 Appendix U (p.124) two-climate-cascade
+    split. The P960 prints the current dwelling twice:
+
+      * Block 1 ("11a SAP rating / 12a CO2") on UK-AVERAGE weather (region
+        0): space heat (98c) 7333.79, SAP (258) 60.5318, CO2 (272) 692.13.
+      * Block 2 ("EPC COSTS, EMISSIONS AND PRIMARY ENERGY") on POSTCODE
+        weather (PCDB Table 172, W6): space heat (98c) 5921.05, CO2 (272)
+        626.78, primary energy (286) 6581.59.
+
+    The SAP/EI rating reads the rating cascade; the EPC-displayed CO2/PE
+    read the demand cascade. Pins both ends at abs=1e-4."""
+    # Arrange
+    from domain.sap10_calculator.calculator import calculate_sap_from_inputs
+    from domain.sap10_calculator.rdsap.cert_to_inputs import cert_to_demand_inputs
+
+    epc = _w001431_case45.build_epc()
+    # The split only exists because the postcode resolves to local weather.
+    assert local_climate_for_cert(epc) is not None
+
+    # Act — both climate cascades from the one cert.
+    rating = calculate_sap_from_inputs(cert_to_inputs(epc))
+    demand = calculate_sap_from_inputs(cert_to_demand_inputs(epc))
+
+    # Assert — Block 1 (UK-average rating cascade).
+    _pin(
+        rating.space_heating_kwh_per_yr,
+        _w001431_case45.RATING_SPACE_HEATING_KWH,
+        "(98c) rating case45",
+    )
+    _pin(
+        rating.sap_score_continuous,
+        _w001431_case45.RATING_SAP_CONTINUOUS,
+        "(258) rating case45",
+    )
+    assert rating.sap_score == _w001431_case45.RATING_SAP_INTEGER
+    _pin(
+        rating.co2_kg_per_yr,
+        _w001431_case45.RATING_CO2_KG_PER_YR,
+        "(272) rating case45",
+    )
+
+    # Assert — Block 2 (postcode demand cascade).
+    _pin(
+        demand.space_heating_kwh_per_yr,
+        _w001431_case45.DEMAND_SPACE_HEATING_KWH,
+        "(98c) demand case45",
+    )
+    _pin(
+        demand.co2_kg_per_yr,
+        _w001431_case45.DEMAND_CO2_KG_PER_YR,
+        "(272) demand case45",
+    )
+    _pin(
+        demand.primary_energy_kwh_per_yr,
+        _w001431_case45.DEMAND_PRIMARY_ENERGY_KWH,
+        "(286) demand case45",
+    )
+
+
 def test_case6_main_2_emitter_and_control_extracted() -> None:
     """Simulated case 6's §14.1 Main Heating2 lodges its OWN emitter
     ("Underfloor Heating") and control ("SAP code 2110, ...") — the two
diff --git a/tests/infrastructure/epc_client/test_sap_accuracy_corpus.py b/tests/infrastructure/epc_client/test_sap_accuracy_corpus.py
index 7b510ab3..ca4681e4 100644
--- a/tests/infrastructure/epc_client/test_sap_accuracy_corpus.py
+++ b/tests/infrastructure/epc_client/test_sap_accuracy_corpus.py
@@ -30,11 +30,7 @@ from typing import Any
 import pytest
 
 from datatypes.epc.domain.mapper import EpcPropertyDataMapper
-from domain.sap10_calculator.calculator import calculate_sap_from_inputs
-from domain.sap10_calculator.rdsap.cert_to_inputs import (
-    SAP_10_2_SPEC_PRICES,
-    cert_to_inputs,
-)
+from domain.sap10_calculator.calculator import Sap10Calculator
 
 _CORPUS = Path(
     "backend/epc_api/json_samples/RdSAP-Schema-21.0.1/corpus.jsonl"
@@ -127,10 +123,25 @@ _CORPUS = Path(
 # "ground floor" floor_type as exposed (worksheet-validated to 1e-4 on simulated
 # case 45: floor (28a) 0 -> 25.38 W/K, fabric (33) 75.6 -> 101.01) -> 69.5% ->
 # 69.7% (MAE 0.859 -> 0.854). Pinned in test_heat_transmission.
+# POSTCODE DEMAND CASCADE (SAP 10.2 Appendix U paragraph 1, p.124): the
+# CO2/PE over-estimate diagnosed above as "per-cert mapper/demand fidelity"
+# was largely a CLIMATE-cascade bug. The SAP/EI rating is computed on
+# UK-average weather (Tables U1-U3 region 0), but EPC-displayed energy use,
+# CO2 emissions and primary energy use POSTCODE-DISTRICT weather from PCDB
+# Table 172 — "other calculations (such as for energy use and costs on EPCs)
+# are done using local weather". We were feeding the UK-average demand to all
+# three outputs, so warm-region certs (most of England, warmer than the
+# UK-average) over-counted heating demand → CO2/PE high. `Sap10Calculator.
+# calculate` now grafts the demand cascade's CO2/PE onto the rating cascade's
+# SAP. Across the corpus this moved CO2 MAE 0.26 -> 0.12 t/yr (bias +0.18 ->
+# +0.04) and PE MAE 13.6 -> 3.8 kWh/m2/yr (bias +9.0 -> +0.24); SAP unchanged
+# (rating cascade). Worksheet-validated to 1e-4 on simulated case 45 (rating
+# CO2 692.13; demand CO2 626.78, PE 6581.59). The residual PE/CO2 spread is
+# now the genuine per-cert mapper-fidelity tail.
 _MIN_WITHIN_HALF_SAP = 0.695
 _MAX_SAP_MAE = 0.86
-_MAX_CO2_MAE_TONNES = 0.30      # t CO2 / yr vs co2_emissions_current
-_MAX_PE_PER_M2_MAE = 14.0       # kWh / m2 / yr vs energy_consumption_current
+_MAX_CO2_MAE_TONNES = 0.13      # t CO2 / yr vs co2_emissions_current
+_MAX_PE_PER_M2_MAE = 4.2        # kWh / m2 / yr vs energy_consumption_current
 
 
 def _load_corpus() -> list[dict[str, Any]]:
@@ -155,8 +166,12 @@ def test_api_path_sap_accuracy_on_rdsap_21_0_1_corpus(
     co2_signed_errs_t: list[float] = []      # our − lodged, tonnes/yr
     pe_signed_errs: list[float] = []          # our − lodged, kWh/m²/yr
     skipped = 0
+    _calculator = Sap10Calculator()
 
     # Act — run the API → EpcPropertyData → calculator pipeline per cert.
+    # `Sap10Calculator.calculate` runs both climate cascades (SAP 10.2
+    # Appendix U p.124): the SAP rating on UK-average weather, CO2/PE on
+    # postcode-district weather — exactly the two figures the EPC lodges.
     for doc in corpus:
         lodged_sap = doc.get("energy_rating_current")
         if lodged_sap is None:
@@ -164,9 +179,7 @@ def test_api_path_sap_accuracy_on_rdsap_21_0_1_corpus(
             continue
         try:
             epc = EpcPropertyDataMapper.from_api_response(doc)
-            result = calculate_sap_from_inputs(
-                cert_to_inputs(epc, prices=SAP_10_2_SPEC_PRICES)
-            )
+            result = _calculator.calculate(epc)
         except Exception:
             # A mapper / calculator raise is a coverage gap tracked elsewhere
             # (eval_api_sap_accuracy.py); here we gauge the certs that compute.