Slice 95: API mapper TFA from per-bp dims + window area 2dp rounding — cert 001479 to 1e-4

The end-to-end production cascade `from_api_response → cert_to_inputs →
calculate_sap_from_inputs` now hits cert 001479's worksheet continuous
SAP 69.0094 at abs < 1e-4 (was +0.000584). Two fixes:

1. API mapper: `from_rdsap_schema_21_0_{0,1}` computes `total_floor_
   area_m2` as Σ per-bp `sap_floor_dimensions[*].total_floor_area.value`
   (cert 001479: 30.45+30.77+5.37+1.92 = 68.51), not the lodged scalar
   (rounded integer 69). `water_heating_from_cert` reads `epc.total_
   floor_area_m2` directly for occupancy N (Appendix J), which propagates
   to HW kWh (+6.31 → ~0), Appendix L lighting (+0.98 → 0), and internal
   gains (+25.72 W·months → 0).

2. Cascade window area rounding per RdSAP 10 §15 "Rounding of data"
   (p.66): "All element areas (gross) including window areas: 2 d.p."
   `solar_gains.py` and `internal_gains.py` now round `w * h` to 2 d.p.
   to match the existing `heat_transmission.py` pattern (line 344).
   Closes the residual solar gains delta (+1.50 W·months → 0) that
   became dominant once TFA was fixed.

Re-pinned 5 golden cert residuals where TFA + area rounding shifted
output: 0240 (SAP -14→-15, PE +14.6650→+17.8450, CO2 +0.8060→+1.0097),
6035 (PE +48.2971→+49.5139, CO2 +1.1016→+1.1423), 8135 (PE -2.4194→
-2.4072, CO2 -0.0198→-0.0195), 2130 (PE -38.1521→-38.1666), 0390
(PE +1.6837→+1.6962, CO2 +0.0637→+0.0639).

New test: `test_api_001479_full_chain_sap_matches_worksheet_pdf_
exactly` formalises Layer 4 of the validation stack as a 1e-4 gate.

Pyright net-zero (mapper.py 33).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/documents_parser/tests/test_summary_pdf_mapper_chain.py

@@ -29,6 +29,7 @@ Textract directly.
 from __future__ import annotations
 
 import dataclasses
+import json
 import re
 import subprocess
 from pathlib import Path
@@ -56,6 +57,16 @@ _SUMMARY_000490_PDF = _FIXTURES / "Summary_000490.pdf"
 _SUMMARY_000516_PDF = _FIXTURES / "Summary_000516.pdf"
 _SUMMARY_001479_PDF = _FIXTURES / "Summary_001479.pdf"
 
+# GOV.UK EPB API JSON for cert 001479 — the API-path counterpart of the
+# Summary_001479.pdf fixture. Together they drive the API ≡ Summary
+# parity workstream; Layer 4 of the validation stack is "API cascade SAP
+# matches worksheet continuous SAP at 1e-4".
+_API_001479_JSON = (
+    Path(__file__).parents[3]
+    / "packages/domain/src/domain/sap/rdsap/tests/fixtures/golden"
+    / "0535-9020-6509-0821-6222.json"
+)
+
 
 def _summary_pdf_to_textract_style_pages(pdf_path: Path) -> list[str]:
     """Convert a Summary PDF into the per-page text format the existing
@@ -390,6 +401,30 @@ def test_summary_001479_full_chain_sap_matches_worksheet_pdf_exactly() -> None:
     assert abs(result.sap_score_continuous - worksheet_unrounded_sap) < 1e-4
 
 
+def test_api_001479_full_chain_sap_matches_worksheet_pdf_exactly() -> None:
+    # Arrange — cert 001479 has both an Elmhurst Summary PDF and a GOV.UK
+    # EPB API JSON (ref 0535-9020-6509-0821-6222). The Summary cascade
+    # already pins at worksheet's 69.0094 ± 1e-4 above; this test is the
+    # Layer 4 production-path gate: API JSON → from_api_response →
+    # cert_to_inputs → calculate_sap_from_inputs must also hit 69.0094
+    # at 1e-4. Identical inputs must produce identical outputs; the
+    # calculator is deterministic, so any drift is a mapper coverage gap.
+    doc = json.loads(_API_001479_JSON.read_text())
+    epc = EpcPropertyDataMapper.from_api_response(doc)
+
+    # Act
+    result = calculate_sap_from_inputs(
+        cert_to_inputs(epc, prices=SAP_10_2_SPEC_PRICES)
+    )
+
+    # Assert — 1e-4 pin against the worksheet's continuous SAP. ±0.5 is
+    # the API-only fallback (project memory `feedback_api_tolerance_1e_
+    # minus_4`); when the worksheet is available, identical-inputs-must-
+    # produce-identical-outputs is the bar.
+    worksheet_unrounded_sap = 69.0094
+    assert abs(result.sap_score_continuous - worksheet_unrounded_sap) < 1e-4
+
+
 # ============================================================================
 # Mapper-vs-hand-built EpcPropertyData diff tests
 # ============================================================================

datatypes/epc/domain/mapper.py

@@ -1146,6 +1146,7 @@ class EpcPropertyDataMapper:
     def from_rdsap_schema_21_0_0(schema: RdSapSchema21_0_0) -> EpcPropertyData:
         es = schema.sap_energy_source
         pv_supply, pv_arrays = _map_schema_21_pv(es.photovoltaic_supply)
+        _per_bp_tfa = _total_floor_area_from_building_parts(schema.sap_building_parts)
         return EpcPropertyData(
             uprn=schema.uprn,
             assessment_type=schema.assessment_type,
@@ -1163,7 +1164,9 @@ class EpcPropertyDataMapper:
             inspection_date=date.fromisoformat(schema.inspection_date),
             completion_date=date.fromisoformat(schema.completion_date),
             registration_date=date.fromisoformat(schema.registration_date),
-            total_floor_area_m2=float(schema.total_floor_area),
+            total_floor_area_m2=(
+                _per_bp_tfa if _per_bp_tfa is not None else float(schema.total_floor_area)
+            ),
             solar_water_heating=schema.solar_water_heating == "Y",
             has_hot_water_cylinder=schema.has_hot_water_cylinder == "true",
             has_fixed_air_conditioning=schema.has_fixed_air_conditioning == "true",
@@ -1418,6 +1421,7 @@ class EpcPropertyDataMapper:
     def from_rdsap_schema_21_0_1(schema: RdSapSchema21_0_1) -> EpcPropertyData:
         es = schema.sap_energy_source
         pv_supply, pv_arrays = _map_schema_21_pv(es.photovoltaic_supply)
+        _per_bp_tfa = _total_floor_area_from_building_parts(schema.sap_building_parts)
         return EpcPropertyData(
             # General
             uprn=schema.uprn,
@@ -1436,7 +1440,9 @@ class EpcPropertyDataMapper:
             inspection_date=date.fromisoformat(schema.inspection_date),
             completion_date=date.fromisoformat(schema.completion_date),
             registration_date=date.fromisoformat(schema.registration_date),
-            total_floor_area_m2=float(schema.total_floor_area),
+            total_floor_area_m2=(
+                _per_bp_tfa if _per_bp_tfa is not None else float(schema.total_floor_area)
+            ),
             # Property flags
             solar_water_heating=schema.solar_water_heating == "Y",
             has_hot_water_cylinder=schema.has_hot_water_cylinder == "true",
@@ -1887,6 +1893,28 @@ def _measurement_value(field: Any) -> float:
     return float(field)
 
 
+def _total_floor_area_from_building_parts(building_parts: Any) -> Optional[float]:
+    """Sum per-bp `sap_floor_dimensions[*].total_floor_area` to recover the
+    precise TFA. The GOV.UK EPB API JSON's top-level `total_floor_area`
+    is rounded to the integer (cert 001479: 30.45+30.77+5.37+1.92 = 68.51
+    → lodged 69), but the worksheet computes continuous SAP from the
+    unrounded geometry. `epc.total_floor_area_m2` is read directly by
+    `water_heating_from_cert` to derive occupancy N (Appendix J), which
+    drives HW, lighting (Appendix L), and internal-gains kWh — so the
+    rounded scalar shifts SAP by ~+0.0006 on cert 001479. Returns None
+    when no per-bp dims are lodged so callers fall back to the scalar."""
+    if not building_parts:
+        return None
+    total = 0.0
+    found = False
+    for bp in building_parts:
+        floor_dims: Any = bp.sap_floor_dimensions or []
+        for fd in floor_dims:
+            total += _measurement_value(fd.total_floor_area)
+            found = True
+    return total if found else None
+
+
 def _first_pv_battery(
     schema_pv_batteries: Any,
 ) -> Optional[PvBatteries]:

packages/domain/src/domain/sap/rdsap/tests/test_golden_fixtures.py

@@ -74,9 +74,9 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
     _GoldenExpectation(
         cert_number="0240-0200-5706-2365-8010",
         actual_sap=73,
-        expected_sap_resid=-14,
+        expected_sap_resid=-15,
-        expected_pe_resid_kwh_per_m2=+14.6650,
+        expected_pe_resid_kwh_per_m2=+17.8450,
-        expected_co2_resid_tonnes_per_yr=+0.8060,
+        expected_co2_resid_tonnes_per_yr=+1.0097,
         notes=(
             "Detached house, TFA 202, age J, oil boiler, Table 4b code 130. "
             "API response lodges sap_room_in_roof.room_in_roof_type_1 with "
@@ -119,8 +119,8 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
         cert_number="6035-7729-2309-0879-2296",
         actual_sap=70,
         expected_sap_resid=-6,
-        expected_pe_resid_kwh_per_m2=+48.2971,
+        expected_pe_resid_kwh_per_m2=+49.5139,
-        expected_co2_resid_tonnes_per_yr=+1.1016,
+        expected_co2_resid_tonnes_per_yr=+1.1423,
         notes=(
             "Mid-terrace, TFA 128, age A, gas combi Table 4b code 104. "
             "Slice 59 per-bp window apportionment tightens all 3 "
@@ -147,8 +147,8 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
         cert_number="8135-1728-8500-0511-3296",
         actual_sap=72,
         expected_sap_resid=+0,
-        expected_pe_resid_kwh_per_m2=-2.4194,
+        expected_pe_resid_kwh_per_m2=-2.4072,
-        expected_co2_resid_tonnes_per_yr=-0.0198,
+        expected_co2_resid_tonnes_per_yr=-0.0195,
         notes=(
             "Semi-detached, TFA 102, age C, gas PCDB-listed. Cert lodges "
             "blocked_chimneys_count=1. Slice 59 per-bp window apportionment "
@@ -160,7 +160,7 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
         cert_number="2130-1033-4050-5007-8395",
         actual_sap=82,
         expected_sap_resid=+1,
-        expected_pe_resid_kwh_per_m2=-38.1521,
+        expected_pe_resid_kwh_per_m2=-38.1666,
         expected_co2_resid_tonnes_per_yr=+0.3047,
         notes=(
             "End-terrace + 1 extension, TFA 64, gas combi PCDB index 17505, "
@@ -180,8 +180,8 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
         cert_number="0390-2254-6420-2126-5561",
         actual_sap=65,
         expected_sap_resid=+0,
-        expected_pe_resid_kwh_per_m2=+1.6837,
+        expected_pe_resid_kwh_per_m2=+1.6962,
-        expected_co2_resid_tonnes_per_yr=+0.0637,
+        expected_co2_resid_tonnes_per_yr=+0.0639,
         notes=(
             "End-terrace + 1 extension, TFA 80, gas combi PCDB index 18119, "
             "no PV, no secondary, postcode LN12 (PCDB Table 172 match). "

packages/domain/src/domain/sap/worksheet/internal_gains.py

@@ -25,11 +25,19 @@ from __future__ import annotations
 
 from dataclasses import dataclass
 from enum import Enum
-from math import cos, exp, pi
+from math import cos, exp, floor, pi
 from typing import Final, Optional
 
 from datatypes.epc.domain.epc_property_data import EpcPropertyData, SapWindow
 
+
+def _round_area_2dp(value: float) -> float:
+    """Half-away-from-zero rounding to 2 d.p. matching heat_transmission.
+    RdSAP 10 §15 "Rounding of data" (p.66): "All element areas (gross)
+    including window areas: 2 d.p." Inlined rather than imported so this
+    module doesn't reach into heat_transmission's private helpers."""
+    return floor(value * 100.0 + 0.5) / 100.0
+
 _DAYS_PER_YEAR: Final[float] = 365.0
 _APPLIANCES_E_A_COEFF: Final[float] = 207.8
 _APPLIANCES_E_A_EXPONENT: Final[float] = 0.4714
@@ -571,8 +579,11 @@ def _daylight_factor_from_cert(
     if tfa <= 0.0 or (not epc.sap_windows and rooflight_total_area_m2 <= 0.0):
         return 1.433
     z_l = _Z_L_BY_OVERSHADING[overshading]
+    # RdSAP 10 §15 "Rounding of data" (p.66): "All element areas (gross)
+    # including window areas: 2 d.p." — mirrors solar_gains and heat_
+    # transmission so G_L sees the same area as the fabric cascade.
     wall_g_l_numerator = sum(
-        float(w.window_width) * float(w.window_height)
+        _round_area_2dp(float(w.window_width) * float(w.window_height))
         * _g_light(w) * _frame_factor(w) * z_l
         for w in epc.sap_windows
     )

packages/domain/src/domain/sap/worksheet/solar_gains.py

@@ -31,7 +31,7 @@ from __future__ import annotations
 
 from dataclasses import dataclass
 from enum import Enum
-from math import cos, radians, sin
+from math import cos, floor, radians, sin
 from typing import Final
 
 from datatypes.epc.domain.epc_property_data import EpcPropertyData, SapWindow
@@ -43,6 +43,14 @@ from domain.sap.climate.appendix_u import (
 from domain.sap.worksheet.internal_gains import OvershadingCategory
 
 
+def _round_area_2dp(value: float) -> float:
+    """Half-away-from-zero rounding to 2 d.p. matching heat_transmission.
+    RdSAP 10 §15 "Rounding of data" (p.66): "All element areas (gross)
+    including window areas: 2 d.p." Inlined rather than imported so this
+    module doesn't reach into heat_transmission's private helpers."""
+    return floor(value * 100.0 + 0.5) / 100.0
+
+
 # Table 6d first column — winter solar access factor Z for heating gains.
 # Distinct from the lighting Z_L (third column, §5) and cooling Z (second
 # column, out of scope). SAP 10.2 spec p178.
@@ -304,7 +312,10 @@ def _vertical_window_gain_monthly_w(
 ) -> tuple[float, ...]:
     """Compute the 12-tuple of monthly solar gain (W) for one vertical wall
     window. Pitch = 90° always; Table 6b/6c lookups derive g⊥ and FF."""
-    area = float(w.window_width) * float(w.window_height)
+    # RdSAP 10 §15 "Rounding of data" (p.66): "All element areas (gross)
+    # including window areas: 2 d.p." — matches heat_transmission's per-
+    # window area rounding so solar gains and conduction agree on area.
+    area = _round_area_2dp(float(w.window_width) * float(w.window_height))
     g_perp = _g_perpendicular(w)
     ff = _frame_factor(w)
     return tuple(