diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 8065084c..f30b97dc 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -162,6 +162,30 @@ def _pv_orientation(field: Any) -> Optional[int]:
     return int(_measurement_value(field))
 
 
+# Full-SAP `sap_openings` occasionally lodge width/height in MILLIMETRES, mixed
+# with metre rows in the same array (e.g. a 2025×2100 mm window beside a 3.06×1 m
+# one). No real window/door dimension approaches 50 m, so a value that large is a
+# millimetre lodgement and is divided by 1000. Without this a 2025×2100 mm window
+# becomes 4.25M m², the HTC blows up and SAP clamps to 1 (uprn_10093117227).
+_SAP_OPENING_MAX_PLAUSIBLE_METRES: Final[float] = 50.0
+
+
+def _sanitise_opening_dimension_m(value: float) -> float:
+    """Coerce a lodged opening dimension to metres, dividing by 1000 when it is
+    implausibly large (a millimetre lodgement)."""
+    if value > _SAP_OPENING_MAX_PLAUSIBLE_METRES:
+        return value / 1000.0
+    return value
+
+
+def _sap_opening_area_m2(width: Any, height: Any) -> float:
+    """Measured opening area in m², with mm-lodged width/height coerced to
+    metres via `_sanitise_opening_dimension_m`."""
+    return _sanitise_opening_dimension_m(float(width)) * _sanitise_opening_dimension_m(
+        float(height)
+    )
+
+
 def _map_schema_21_pv(
     es_pv_supply: Any,
 ) -> tuple[Optional[PhotovoltaicSupply], Optional[List[PhotovoltaicArray]]]:
@@ -783,8 +807,8 @@ class EpcPropertyDataMapper:
                         orientation=op.orientation if op.orientation is not None else 0,
                         window_type=ot.frame_type if ot.frame_type is not None else 0,
                         glazing_type=ot.glazing_type,
-                        window_width=float(op.width),
-                        window_height=float(op.height),
+                        window_width=_sanitise_opening_dimension_m(float(op.width)),
+                        window_height=_sanitise_opening_dimension_m(float(op.height)),
                         draught_proofed=False,
                         window_location=op.location or "",
                         window_wall_type=0,
@@ -821,7 +845,7 @@ class EpcPropertyDataMapper:
                     continue
                 roof_windows.append(
                     SapRoofWindow(
-                        area_m2=float(op.width) * float(op.height),
+                        area_m2=_sap_opening_area_m2(op.width, op.height),
                         u_value_raw=ot.u_value,
                         orientation=op.orientation if op.orientation is not None else 0,
                         # default 45° pitch when unlodged — matches the
@@ -2778,7 +2802,7 @@ def _sap_door_aggregates(schema: SapSchema17_1) -> Tuple[int, Optional[float]]:
             if ot is None or ot.type not in _SAP_OPENING_TYPE_DOORS:
                 continue
             count += 1
-            area = float(op.width) * float(op.height)
+            area = _sap_opening_area_m2(op.width, op.height)
             weighted_u_area += ot.u_value * area
             total_area += area
     u_value = weighted_u_area / total_area if total_area > 0 else None
diff --git a/datatypes/epc/domain/tests/test_from_sap_schema.py b/datatypes/epc/domain/tests/test_from_sap_schema.py
index ffd16e17..64b6ee5c 100644
--- a/datatypes/epc/domain/tests/test_from_sap_schema.py
+++ b/datatypes/epc/domain/tests/test_from_sap_schema.py
@@ -108,6 +108,58 @@ class TestFromSapSchema17_1FabricDescriptions:
         assert result.walls and result.walls[0].description
 
 
+class TestFromSapSchema17_1OpeningUnitSanitisation:
+    """Some full-SAP certs lodge sap_openings width/height in MILLIMETRES (mixed
+    with metre rows in the same array, e.g. a 2025×2100 mm window next to a
+    3.06×1 m one). The mapper must read them as metres — any dimension
+    implausibly large (>50 m) is mm and is divided by 1000 — else the mm window
+    becomes ~4.25M m², the HTC blows up and SAP clamps to 1. Regression for
+    uprn_10093117227 / 10090317693 / 10091636031."""
+
+    def test_mm_lodged_window_is_converted_to_metres(self) -> None:
+        # Arrange — cert lodges a 2025×2100 mm "Side Elevation" window mixed with
+        # metre-lodged windows in the same sap_openings array.
+        data = load("sap_17_1_mm_openings.json")
+
+        # Act
+        epc = EpcPropertyDataMapper.from_api_response(data)
+
+        # Assert — no window is physically impossible; the mm row is now ~2 m.
+        assert max(w.window_width for w in epc.sap_windows) < 50.0
+
+    def test_mm_lodged_roof_window_is_converted_to_metres(self) -> None:
+        # Arrange — inject a millimetre-lodged roof-window dimension (990 mm) into
+        # a fixture whose roof windows are otherwise lodged in metres.
+        data = load("sap_17_1_house.json")
+        for bp in data["sap_building_parts"]:
+            for op in bp["sap_openings"]:
+                if op["name"] == "Bed 1 Roof":
+                    op["width"] = 990  # mm (was 0.99 m)
+
+        # Act
+        epc = EpcPropertyDataMapper.from_api_response(data)
+
+        # Assert — the roof-window area stays physically plausible (not ~720 m²).
+        assert epc.sap_roof_windows is not None
+        assert max(rw.area_m2 for rw in epc.sap_roof_windows) < 50.0
+
+    def test_mm_lodged_door_does_not_dominate_area_weighted_u(self) -> None:
+        # Arrange — the flat fixture has 2 doors (U 1.4 over 1.89 m², U 1.8 over
+        # 2.12 m²) → area-weighted U 1.6115. Lodge the first door's width in mm;
+        # un-sanitised, its 1890 m² area swamps the weighting → U collapses to 1.4.
+        data = load("sap_17_1_flat.json")
+        for bp in data["sap_building_parts"]:
+            for op in bp["sap_openings"]:
+                if op["name"] == "Front":
+                    op["width"] = 1890  # mm (was 1.89 m)
+
+        # Act
+        epc = EpcPropertyDataMapper.from_api_response(data)
+
+        # Assert — the mm door is rescaled, so the area weighting is unchanged.
+        assert epc.insulated_door_u_value == pytest.approx(1.6114713, abs=1e-4)
+
+
 class TestFromSapSchema17_1Windows:
     """Slice 4a (D2): vertical-window openings (opening-type 4) collapse onto
     sap_windows with measured per-window geometry and U-value."""
diff --git a/datatypes/epc/schema/tests/fixtures/sap_17_1_mm_openings.json b/datatypes/epc/schema/tests/fixtures/sap_17_1_mm_openings.json
new file mode 100644
index 00000000..0670e791
--- /dev/null
+++ b/datatypes/epc/schema/tests/fixtures/sap_17_1_mm_openings.json
@@ -0,0 +1,376 @@
+{
+  "uprn": 10093117227,
+  "roofs": [
+    {
+      "description": "Average thermal transmittance 0.11 W/m\u00b2K",
+      "energy_efficiency_rating": 5,
+      "environmental_efficiency_rating": 5
+    }
+  ],
+  "walls": [
+    {
+      "description": "Average thermal transmittance 0.22 W/m\u00b2K",
+      "energy_efficiency_rating": 5,
+      "environmental_efficiency_rating": 5
+    }
+  ],
+  "floors": [
+    {
+      "description": "Average thermal transmittance 0.12 W/m\u00b2K",
+      "energy_efficiency_rating": 5,
+      "environmental_efficiency_rating": 5
+    }
+  ],
+  "status": "entered",
+  "tenure": "ND",
+  "windows": {
+    "description": "High performance glazing",
+    "energy_efficiency_rating": 5,
+    "environmental_efficiency_rating": 5
+  },
+  "lighting": {
+    "description": "Low energy lighting in all fixed outlets",
+    "energy_efficiency_rating": 5,
+    "environmental_efficiency_rating": 5
+  },
+  "postcode": "GU31 5BF",
+  "data_type": 2,
+  "hot_water": {
+    "description": "From main system",
+    "energy_efficiency_rating": 3,
+    "environmental_efficiency_rating": 2
+  },
+  "post_town": "PETERSFIELD",
+  "built_form": 1,
+  "created_at": "2018-04-12 07:48:54",
+  "living_area": 16.22,
+  "orientation": 6,
+  "region_code": 16,
+  "report_type": 3,
+  "sap_heating": {
+    "thermal_store": 1,
+    "water_fuel_type": 39,
+    "water_heating_code": 901,
+    "hot_water_store_size": 180,
+    "main_heating_details": [
+      {
+        "main_fuel_type": 39,
+        "heat_emitter_type": 1,
+        "main_heating_code": 191,
+        "emitter_temperature": "NA",
+        "main_heating_number": 1,
+        "main_heating_control": 2106,
+        "main_heating_category": 2,
+        "main_heating_fraction": 1,
+        "central_heating_pump_age": 2,
+        "main_heating_data_source": 3,
+        "has_separate_delayed_start": "false",
+        "load_or_weather_compensation": 0,
+        "is_central_heating_pump_in_heated_space": "true"
+      }
+    ],
+    "has_hot_water_cylinder": "true",
+    "immersion_heating_type": 1,
+    "has_cylinder_thermostat": "true",
+    "hot_water_store_heat_loss": 1.95,
+    "has_fixed_air_conditioning": "false",
+    "secondary_heating_category": 1,
+    "is_cylinder_in_heated_space": "true",
+    "hot_water_store_heat_loss_source": 2
+  },
+  "sap_version": 9.92,
+  "schema_type": "SAP-Schema-17.1",
+  "uprn_source": "Address Matched",
+  "country_code": "ENG",
+  "main_heating": [
+    {
+      "description": "Boiler and radiators, electric",
+      "energy_efficiency_rating": 1,
+      "environmental_efficiency_rating": 2
+    }
+  ],
+  "air_tightness": {
+    "description": "Air permeability 4.7 m\u00b3/h.m\u00b2 (as tested)",
+    "energy_efficiency_rating": 4,
+    "environmental_efficiency_rating": 4
+  },
+  "dwelling_type": "Detached bungalow",
+  "language_code": 1,
+  "property_type": 1,
+  "address_line_1": "1 Hilly Field Mews",
+  "address_line_2": "Parsonage Estate",
+  "address_line_3": "Rogate",
+  "assessment_date": "2018-04-12",
+  "assessment_type": "SAP",
+  "completion_date": "2018-04-12",
+  "inspection_date": "2018-04-12",
+  "sap_ventilation": {
+    "psv_count": 0,
+    "pressure_test": 1,
+    "air_permeability": 4.7,
+    "open_flues_count": 0,
+    "ventilation_type": 1,
+    "extract_fans_count": 2,
+    "open_fireplaces_count": 0,
+    "sheltered_sides_count": 2,
+    "flueless_gas_fires_count": 0
+  },
+  "design_water_use": 1,
+  "sap_data_version": 9.92,
+  "total_floor_area": 50,
+  "transaction_type": 6,
+  "conservatory_type": 1,
+  "registration_date": "2018-04-12",
+  "sap_energy_source": {
+    "pv_arrays": [
+      {
+        "pitch": 3,
+        "peak_power": 1.77,
+        "orientation": 4,
+        "overshading": 1,
+        "pv_connection": 2
+      }
+    ],
+    "electricity_tariff": 2,
+    "wind_turbines_count": 0,
+    "wind_turbine_terrain_type": 1,
+    "fixed_lighting_outlets_count": 10,
+    "low_energy_fixed_lighting_outlets_count": 10,
+    "low_energy_fixed_lighting_outlets_percentage": 100
+  },
+  "sap_opening_types": [
+    {
+      "name": "Front Door",
+      "type": 1,
+      "u_value": 1,
+      "data_source": 2,
+      "glazing_type": 1
+    },
+    {
+      "name": "Windows",
+      "type": 4,
+      "u_value": 1.4,
+      "data_source": 2,
+      "frame_factor": 0.7,
+      "glazing_type": 7,
+      "solar_transmittance": 0.63
+    }
+  ],
+  "secondary_heating": {
+    "description": "None",
+    "energy_efficiency_rating": 0,
+    "environmental_efficiency_rating": 0
+  },
+  "lzc_energy_sources": [
+    11
+  ],
+  "sap_building_parts": [
+    {
+      "sap_roofs": [
+        {
+          "name": "Roof 1",
+          "u_value": 0.11,
+          "roof_type": 2,
+          "description": "External Roof",
+          "kappa_value": 9,
+          "total_roof_area": 50.29
+        }
+      ],
+      "sap_walls": [
+        {
+          "name": "External Wall 1",
+          "u_value": 0.22,
+          "wall_type": 2,
+          "description": "External Wall",
+          "kappa_value": 60,
+          "total_wall_area": 86.33,
+          "is_curtain_walling": "false"
+        },
+        {
+          "name": "Internal Wall 0",
+          "u_value": 0,
+          "wall_type": 5,
+          "kappa_value": 9,
+          "total_wall_area": 66.46
+        }
+      ],
+      "identifier": "Main Dwelling",
+      "overshading": 2,
+      "sap_openings": [
+        {
+          "name": "Front Door",
+          "type": "Front Door",
+          "width": 1000,
+          "height": 2100,
+          "location": "External Wall 1",
+          "orientation": 0
+        },
+        {
+          "name": "Front Elevation",
+          "type": "Windows",
+          "width": 3.06,
+          "height": 1,
+          "location": "External Wall 1",
+          "orientation": 6
+        },
+        {
+          "name": "Side Elevation",
+          "type": "Windows",
+          "width": 2025,
+          "height": 2100,
+          "location": "External Wall 1",
+          "orientation": 4
+        },
+        {
+          "name": "Rear Elevation",
+          "type": "Windows",
+          "width": 3.6,
+          "height": 1,
+          "location": "External Wall 1",
+          "orientation": 2
+        }
+      ],
+      "construction_year": 2017,
+      "sap_thermal_bridges": {
+        "thermal_bridges": [
+          {
+            "length": 7.83,
+            "psi_value": 0.24,
+            "psi_value_source": 3,
+            "thermal_bridge_type": "E2"
+          },
+          {
+            "length": 6.83,
+            "psi_value": 0.04,
+            "psi_value_source": 2,
+            "thermal_bridge_type": "E3"
+          },
+          {
+            "length": 19.5,
+            "psi_value": 0.05,
+            "psi_value_source": 2,
+            "thermal_bridge_type": "E4"
+          },
+          {
+            "length": 33.46,
+            "psi_value": 0.16,
+            "psi_value_source": 2,
+            "thermal_bridge_type": "E5"
+          },
+          {
+            "length": 22.49,
+            "psi_value": 0.06,
+            "psi_value_source": 2,
+            "thermal_bridge_type": "E10"
+          },
+          {
+            "length": 10.97,
+            "psi_value": 0.24,
+            "psi_value_source": 2,
+            "thermal_bridge_type": "E12"
+          },
+          {
+            "length": 15.48,
+            "psi_value": 0.09,
+            "psi_value_source": 2,
+            "thermal_bridge_type": "E16"
+          },
+          {
+            "length": 5.16,
+            "psi_value": -0.09,
+            "psi_value_source": 2,
+            "thermal_bridge_type": "E17"
+          }
+        ],
+        "thermal_bridge_code": 5
+      },
+      "building_part_number": 1,
+      "sap_floor_dimensions": [
+        {
+          "storey": 0,
+          "u_value": 0.12,
+          "floor_type": 2,
+          "description": "Ground Floor",
+          "kappa_value": 75,
+          "storey_height": 2.58,
+          "heat_loss_area": 50.29,
+          "total_floor_area": 50.29
+        }
+      ]
+    }
+  ],
+  "heating_cost_current": {
+    "value": 403,
+    "currency": "GBP"
+  },
+  "co2_emissions_current": 1.3,
+  "energy_rating_average": 60,
+  "energy_rating_current": 80,
+  "lighting_cost_current": {
+    "value": 41,
+    "currency": "GBP"
+  },
+  "main_heating_controls": [
+    {
+      "description": "Programmer, room thermostat and TRVs",
+      "energy_efficiency_rating": 4,
+      "environmental_efficiency_rating": 4
+    }
+  ],
+  "has_hot_water_cylinder": "true",
+  "heating_cost_potential": {
+    "value": 410,
+    "currency": "GBP"
+  },
+  "hot_water_cost_current": {
+    "value": 124,
+    "currency": "GBP"
+  },
+  "suggested_improvements": [
+    {
+      "sequence": 1,
+      "typical_saving": {
+        "value": 50,
+        "currency": "GBP"
+      },
+      "indicative_cost": "\u00a34,000 - \u00a36,000",
+      "improvement_type": "N",
+      "improvement_details": {
+        "improvement_number": 19
+      },
+      "improvement_category": 5,
+      "energy_performance_rating": 82,
+      "environmental_impact_rating": 84
+    }
+  ],
+  "co2_emissions_potential": 0.8,
+  "energy_rating_potential": 82,
+  "lighting_cost_potential": {
+    "value": 41,
+    "currency": "GBP"
+  },
+  "schema_version_original": "LIG-17.0",
+  "hot_water_cost_potential": {
+    "value": 66,
+    "currency": "GBP"
+  },
+  "is_in_smoke_control_area": "unknown",
+  "renewable_heat_incentive": {
+    "rhi_new_dwelling": {
+      "space_heating": 2190,
+      "water_heating": 1600
+    }
+  },
+  "seller_commission_report": "Y",
+  "energy_consumption_current": 150,
+  "has_fixed_air_conditioning": "false",
+  "multiple_glazed_percentage": 100,
+  "calculation_software_version": "4.05r02",
+  "energy_consumption_potential": 99,
+  "environmental_impact_current": 78,
+  "current_energy_efficiency_band": "C",
+  "environmental_impact_potential": 84,
+  "has_heated_separate_conservatory": "false",
+  "potential_energy_efficiency_band": "B",
+  "co2_emissions_current_per_floor_area": 25
+}
\ No newline at end of file
diff --git a/scripts/hyde/autonomous_run_findings.md b/scripts/hyde/autonomous_run_findings.md
index da20c5d7..1efa7ab2 100644
--- a/scripts/hyde/autonomous_run_findings.md
+++ b/scripts/hyde/autonomous_run_findings.md
@@ -18,7 +18,14 @@ habitable_room_count, multiple_glazing_type). Fail loud at from_dict (correct):
 
 ## 🐞 BUGS FOUND (diagnosed; NOT fixed — for your review)
 
-### 1. Opening dimensions in millimetres read as metres — MAPPER bug (SEVERE)
+### 1. Opening dimensions in millimetres read as metres — MAPPER bug (SEVERE) ✅ FIXED
+**FIXED** (TDD, `_sanitise_opening_dimension_m` + `_sap_opening_area_m2`): any opening
+dimension > 50 m is mm → ÷1000. Applied to windows, roof windows, and the door
+area-weighting. The 3 broken certs now score 90 / 81 / 79 (were all 1). 3 RED→GREEN
+slices + refactor; fixture sap_17_1_mm_openings.json; 0 new pyright errors.
+Original report below.
+
+
 Full-SAP certs whose `sap_building_parts[].sap_openings[]` lodge width/height in
 **mm** (mixed with metre rows in the same array) → the 17.1 mapper treats all as
 metres → multi-million-m² windows → HTC in the millions → **SAP clamps to 1**.