From f1c6825cae95bdd38b921ae0586602bfc724e19e Mon Sep 17 00:00:00 2001
From: Jun-te Kim <juntekim@googlemail.com>
Date: Fri, 19 Jun 2026 13:18:45 +0000
Subject: [PATCH] =?UTF-8?q?Resolve=20a=20landlord=20double-glazing=20overr?=
 =?UTF-8?q?ide=20to=20its=20glazing=20code=20=F0=9F=9F=A5?=
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Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 .../epc/property_overlays/glazing_overlay.py  | 23 +++++++++++++++++++
 domain/modelling/simulation.py                | 23 +++++++++++++++++++
 tests/domain/epc/test_glazing_overlay.py      | 20 ++++++++++++++++
 3 files changed, 66 insertions(+)
 create mode 100644 domain/epc/property_overlays/glazing_overlay.py
 create mode 100644 tests/domain/epc/test_glazing_overlay.py

diff --git a/domain/epc/property_overlays/glazing_overlay.py b/domain/epc/property_overlays/glazing_overlay.py
new file mode 100644
index 00000000..9da7582d
--- /dev/null
+++ b/domain/epc/property_overlays/glazing_overlay.py
@@ -0,0 +1,23 @@
+"""Map a Landlord-Override glazing value to a glazing Simulation Overlay.
+
+A glazing value is one canonical glazing description carrying type + era
+("Double glazing, 2002 or later", "Single glazing", "Triple glazing, 2002 or
+later"). The calculator derives each window's U-value from its SAP10
+`glazing_type` code via the RdSAP Table 24 cascade, so the overlay decomposes
+the value into that code and emits a whole-dwelling `GlazingOverlay` (a landlord
+describes the dwelling's glazing as a whole, with no per-window geometry, so
+`building_part` is ignored). `_fold_glazing` expands it across every window.
+Unresolvable values produce no overlay.
+"""
+
+from __future__ import annotations
+
+from typing import Optional
+
+from domain.modelling.simulation import EpcSimulation
+
+
+def glazing_overlay_for(
+    glazing_value: str, building_part: int
+) -> Optional[EpcSimulation]:
+    raise NotImplementedError
diff --git a/domain/modelling/simulation.py b/domain/modelling/simulation.py
index 624826f6..217d446b 100644
--- a/domain/modelling/simulation.py
+++ b/domain/modelling/simulation.py
@@ -73,6 +73,28 @@ class WindowOverlay:
     solar_transmittance: Optional[float] = None
 
 
+@dataclass(frozen=True)
+class GlazingOverlay:
+    """All-optional partial of the dwelling's whole-glazing state — the
+    correction a Landlord Override makes when the lodged glazing is wrong.
+
+    Unlike a per-window `WindowOverlay` (keyed by `sap_windows` index), this
+    targets no single window: a landlord describes the dwelling's glazing as a
+    whole ("Double glazing, 2002 or later") with no per-window geometry, so the
+    overlay builder (which never sees the baseline window list) emits one of
+    these and `_fold_glazing` expands it across every `sap_windows` entry.
+
+    `glazing_type` is the SAP10 glazing-type code (Table 24 / `u_window`
+    cascade: 1=single, 2=double 2002-2021, 3=double pre-2002, 9=triple 2002+,
+    …). The fold sets it on every window AND clears each window's lodged
+    transmission U-value, so the Table-24 cascade re-derives the corrected U
+    from the new type (the lodged U was for the OLD, mis-recorded glazing).
+    A `None` field means "leave the baseline value unchanged".
+    """
+
+    glazing_type: Optional[int] = None
+
+
 @dataclass(frozen=True)
 class LightingOverlay:
     """All-optional partial of the dwelling's fixed-lighting bulb counts — the
@@ -220,6 +242,7 @@ class EpcSimulation:
     windows: Mapping[int, WindowOverlay] = field(default_factory=_no_windows)
     ventilation: Optional[VentilationOverlay] = None
     lighting: Optional[LightingOverlay] = None
+    glazing: Optional[GlazingOverlay] = None
     heating: Optional[HeatingOverlay] = None
     secondary_heating: Optional[SecondaryHeatingOverlay] = None
     solar: Optional[SolarOverlay] = None
diff --git a/tests/domain/epc/test_glazing_overlay.py b/tests/domain/epc/test_glazing_overlay.py
new file mode 100644
index 00000000..b54e134e
--- /dev/null
+++ b/tests/domain/epc/test_glazing_overlay.py
@@ -0,0 +1,20 @@
+"""The Landlord-Override glazing → glazing Simulation Overlay mapping.
+
+A glazing value resolves to the SAP10 `glazing_type` code the calculator's
+Table-24 cascade reads; the overlay is whole-dwelling (expanded across every
+window by `_fold_glazing`).
+"""
+
+from __future__ import annotations
+
+from domain.epc.property_overlays.glazing_overlay import glazing_overlay_for
+
+
+def test_double_glazing_post_2002_overlays_its_glazing_code() -> None:
+    # Act
+    simulation = glazing_overlay_for("Double glazing, 2002 or later", 0)
+
+    # Assert — double glazing 2002-2021 is SAP10 glazing_type code 2.
+    assert simulation is not None
+    assert simulation.glazing is not None
+    assert simulation.glazing.glazing_type == 2