Slice S0380.34: round living area in Decimal arithmetic per RdSAP10 §15 — closes cert 2536 +0.0007 SAP residual

RdSAP10 §15 p.66 (Rounding of data):
    "All internal floor areas and living area: 2 d.p."

Cert 2536 (3 habitable rooms → Table 27 fraction 0.30,
TFA 45.65 m^2) sits ON the HALF_UP rounding boundary:
    0.30 (exact) * 45.65 = 13.6950
    HALF_UP 2 d.p.        = 13.70
                            (worksheet fLA = 13.70 / 45.65 = 0.3001)

Float arithmetic drops the spec product BELOW the boundary:
    0.30 (binary) ~= 0.2999999...
    product ~= 13.69499...
    HALF_UP 2 d.p. = 13.69
                     (cascade fLA = 13.69 / 45.65 = 0.29989)

The 0.00021 fLA shortfall feeds straight into the worksheet
(91) -> (92) MIT blend, undershoots MIT by ~0.001 C, and
shaves 0.29 kWh off (98c) useful space heating — a +0.0007
SAP residual via the (211) main heating fuel x p/kWh.

Compute the product in Decimal so HALF_UP lands on the exact
.005 decimal boundary the spec defines. Certs that sit off the
boundary (e.g. 2800/4800: 0.30 x 46.87 = 14.0610 -> 14.06 in
both Decimal and float) are unaffected.

Cohort-2 distribution after S0380.31..S0380.34:
    36 exact + 2 <=0.07 (was 35 exact + 3 <=0.07).
Cert 2536: +0.000715 -> -9.2e-8.

The remaining 2800 / 4800 +0.0007 residuals come from a
different cause (off the HALF_UP boundary) — defer to a
separate slice.

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

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -51,6 +51,7 @@ from __future__ import annotations
 
 import math
 from dataclasses import dataclass
+from decimal import ROUND_HALF_UP, Decimal
 from typing import Callable, Final, Literal, Optional
 
 from datatypes.epc.domain.epc_property_data import (
@@ -120,7 +121,6 @@ from domain.sap10_calculator.worksheet.solar_gains import (
 from domain.sap10_calculator.worksheet.heat_transmission import (
     DwellingExposure,
     HeatTransmission,
-    _AREA_ROUND_DP,
     _round_half_up,
     heat_transmission_from_cert,
 )
@@ -509,11 +509,23 @@ def _living_area_fraction(
     2 d.p. half-up, then divided back by TFA to yield the LINE_91 that
     feeds the §7 zone blend. This roundtrip is why fixtures lodge
     e.g. 0.3001 (= 17.04/56.79) rather than the raw 0.30 Table 27 entry.
+
+    The multiplication runs in Decimal arithmetic so HALF_UP rounding
+    lands on the exact decimal boundary the spec defines. Float Table 27
+    fractions (e.g. 0.30 → 0.2999999...) otherwise drop products that
+    sit on the .005 boundary below the round-up threshold, e.g. cert
+    2536 (3 rooms, TFA 45.65): exact 0.30 × 45.65 = 13.6950 → 13.70;
+    float gives 13.69499... → 13.69, propagating a −0.0007 SAP residual
+    via the §7 MIT blend.
     """
     fraction = _living_area_fraction_default(habitable_rooms_count)
     if total_floor_area_m2 <= 0.0:
         return fraction
-    living_area_m2 = _round_half_up(fraction * total_floor_area_m2, _AREA_ROUND_DP)
+    living_area_m2 = float(
+        (Decimal(str(fraction)) * Decimal(str(total_floor_area_m2))).quantize(
+            Decimal("0.01"), rounding=ROUND_HALF_UP
+        )
+    )
     return living_area_m2 / total_floor_area_m2
 
 

domain/sap10_calculator/rdsap/tests/test_cert_to_inputs.py

@@ -633,6 +633,35 @@ def test_living_area_fraction_uses_rdsap_table_27_by_habitable_rooms() -> None:
     assert inputs_four.living_area_fraction == 0.25
 
 
+def test_living_area_rounds_half_up_at_2_dp_decimal_boundary_per_rdsap_15() -> None:
+    # Arrange — RdSAP10 §15 p.66 ("Rounding of data") requires "All
+    # internal floor areas and living area: 2 d.p." Cert 2536 lodges
+    # 3 habitable rooms (Table 27 fraction 0.30) and TFA 45.65 m². The
+    # exact-decimal product 0.30 × 45.65 = 13.6950 sits ON the HALF_UP
+    # rounding boundary and must round to 13.70 (away from zero). Float
+    # representation drops 0.30 to 0.299999... and the product to
+    # 13.69499..., taking the boundary below 13.6950 — without Decimal
+    # arithmetic the cascade gets 13.69 instead and lodges fLA = 0.29989
+    # instead of the worksheet's 0.30011, leaving a +0.0007 SAP residual
+    # via the §7 MIT blend.
+    from domain.sap10_calculator.rdsap.cert_to_inputs import (
+        _living_area_fraction,  # pyright: ignore[reportPrivateUsage]
+    )
+
+    # Act
+    fla_boundary = _living_area_fraction(habitable_rooms_count=3, total_floor_area_m2=45.65)
+    fla_off_boundary = _living_area_fraction(habitable_rooms_count=3, total_floor_area_m2=46.87)
+
+    # Assert — worksheet cert 2536 dr87-0001-000889 line (91) = 0.3001:
+    # 0.30 × 45.65 = 13.6950 lands ON the half-up boundary and the spec-
+    # faithful living area is 13.70 → fLA = 13.70 / 45.65 = 0.30011.
+    assert abs(fla_boundary - (13.70 / 45.65)) <= 1e-12
+    # Cert 2800 / 4800 (TFA 46.87) sit OFF the boundary: 0.30 × 46.87
+    # = 14.0610 rounds down to 14.06 under HALF_UP, so the Decimal path
+    # matches the float path and fLA = 14.06 / 46.87.
+    assert abs(fla_off_boundary - (14.06 / 46.87)) <= 1e-12
+
+
 def test_main_heating_efficiency_reads_sap_main_heating_code() -> None:
     # Arrange — Direction check: a gas combi (Table 4b code 102, 84% eff)
     # vs a non-condensing gas boiler (code 105, 70% eff) must show through