Cohort residual slice 9: u_rr_default_all_elements — RdSAP10 Table 18 col (4)

Adds the "Room-in-roof, all elements" U-value lookup keyed by age band,
with Scotland override for age K per Table 18 footnote (2). This is the
fallback U-value for the §3.9 Simplified RR cascade when no detailed
per-surface lodgement is available (the "as built / unknown" path per
footnote (1)).

Tests cover the spec table verbatim:
  - A-D 2.30, E 1.50, F 0.80, G 0.50, H 0.35, I 0.35, J 0.30,
  - K 0.25 (England) / 0.20 (Scotland), L 0.18, M 0.15.
Mid-range fallback 0.50 (matching age G) when neither age band nor
country lodged — robustness contract identical to u_roof.

Reference: RdSAP 10 (10-06-2025) Table 18 page 45.

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

packages/domain/src/domain/ml/rdsap_uvalues.py

@@ -390,6 +390,19 @@ _ROOF_BY_AGE: Final[dict[str, float]] = {
     "K": 0.16, "L": 0.16, "M": 0.15,
 }
 
+# Table 18 column (4): "Room-in-roof, all elements" default U by age band
+# when no detailed RR lodgement is available. Footnote (1) on each entry
+# confirms "value from the table applies for unknown and as built".
+# Scotland override per footnote (2): age band K → 0.20 W/m²K (other bands
+# unchanged).
+_RR_ALL_ELEMENTS_BY_AGE: Final[dict[str, float]] = {
+    "A": 2.30, "B": 2.30, "C": 2.30, "D": 2.30,
+    "E": 1.50, "F": 0.80, "G": 0.50, "H": 0.35,
+    "I": 0.35, "J": 0.30, "K": 0.25, "L": 0.18, "M": 0.15,
+}
+_RR_ALL_ELEMENTS_SCOTLAND_OVERRIDES: Final[dict[str, float]] = {"K": 0.20}
+_RR_ALL_ELEMENTS_MID_RANGE_FALLBACK: Final[float] = 0.50
+
 
 _ROOF_NO_INSULATION_MARKERS: Final[tuple[str, ...]] = (
     "no insulation",
@@ -449,6 +462,30 @@ def u_roof(
     return _ROOF_BY_AGE.get(age_band.upper(), 0.4)
 
 
+def u_rr_default_all_elements(
+    country: Optional[Country],
+    age_band: Optional[str],
+) -> float:
+    """RdSAP10 Table 18 column (4) — "Room-in-roof, all elements" default
+    U-value when no detailed RR lodgement is available.
+
+    Used as the catch-all fallback for the §3.9 Simplified RR cascade
+    when assessor didn't measure individual surfaces. The spec footnote
+    (1) on the table confirms: "value from the table applies for unknown
+    and as built". Footnote (2) overrides age K to 0.20 W/m²K in Scotland.
+
+    The mid-range fallback (0.50, matching age G) fires when the cert
+    lodges neither age band nor country — same robustness contract as
+    `u_roof` and the other cascade helpers (never raise on missing input).
+    """
+    if age_band is None:
+        return _RR_ALL_ELEMENTS_MID_RANGE_FALLBACK
+    band = age_band.upper()
+    if country is Country.SCT and band in _RR_ALL_ELEMENTS_SCOTLAND_OVERRIDES:
+        return _RR_ALL_ELEMENTS_SCOTLAND_OVERRIDES[band]
+    return _RR_ALL_ELEMENTS_BY_AGE.get(band, _RR_ALL_ELEMENTS_MID_RANGE_FALLBACK)
+
+
 # ---------------------------------------------------------------------------
 # Floor U-value  (BS EN ISO 13370 + Table 19 defaults)
 # ---------------------------------------------------------------------------

packages/domain/src/domain/ml/tests/test_rdsap_uvalues.py

@@ -32,6 +32,7 @@ from domain.ml.rdsap_uvalues import (
     u_floor,
     u_party_wall,
     u_roof,
+    u_rr_default_all_elements,
     u_wall,
     u_window,
 )
@@ -977,3 +978,56 @@ def test_country_from_code_recognises_known_codes() -> None:
     assert Country.from_code("SCT") is Country.SCT
     assert Country.from_code("NIR") is Country.NIR
     assert Country.from_code("EAW") is Country.ENG  # England-and-Wales aggregate maps to ENG
+
+
+def test_u_rr_default_all_elements_age_band_b_returns_table18_col4_value() -> None:
+    """RdSAP10 §5.11.4 + Table 18 column (4) — "Room-in-roof, all elements"
+    as-built / unknown default. Age band B (1900-1929) → 2.30 W/m²K (the
+    uninsulated row carries footnote (1): "value from the table applies
+    for unknown and as built")."""
+    # Arrange / Act
+    result = u_rr_default_all_elements(country=Country.ENG, age_band="B")
+
+    # Assert
+    assert result == pytest.approx(2.30, abs=0.001)
+
+
+def test_u_rr_default_all_elements_table18_col4_matches_spec_across_age_bands() -> None:
+    """Table 18 column (4) per RdSAP10 spec page 45:
+        A-D 2.30, E 1.50, F 0.80, G 0.50, H 0.35, I 0.35, J 0.30,
+        K 0.25, L 0.18, M 0.15.
+    """
+    # Arrange — expected RR-all-elements U-values for England.
+    expected = {
+        "A": 2.30, "B": 2.30, "C": 2.30, "D": 2.30,
+        "E": 1.50, "F": 0.80, "G": 0.50, "H": 0.35,
+        "I": 0.35, "J": 0.30, "K": 0.25, "L": 0.18, "M": 0.15,
+    }
+
+    # Act / Assert
+    for age_band, want in expected.items():
+        got = u_rr_default_all_elements(country=Country.ENG, age_band=age_band)
+        assert got == pytest.approx(want, abs=0.001), (
+            f"age={age_band}: got {got}, want {want}"
+        )
+
+
+def test_u_rr_default_all_elements_scotland_age_band_k_returns_0_20_per_footnote() -> None:
+    """Table 18 footnote (2): "0.20 W/m²K in Scotland" applies to the
+    age band K row of column (4). Other age bands unchanged."""
+    # Arrange / Act
+    result = u_rr_default_all_elements(country=Country.SCT, age_band="K")
+
+    # Assert
+    assert result == pytest.approx(0.20, abs=0.001)
+
+
+def test_u_rr_default_all_elements_unknown_age_band_falls_back_to_mid_range() -> None:
+    """Robustness: no age band → return the mid-range default rather than
+    raising. Picks the column (4) value at age G (0.50) as a sensible
+    middle estimate, matching the cascade convention used by `u_roof`."""
+    # Arrange / Act
+    result = u_rr_default_all_elements(country=None, age_band=None)
+
+    # Assert
+    assert result == pytest.approx(0.50, abs=0.001)