diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 902b05a4..5c54b9fc 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -43,7 +43,10 @@ from datatypes.epc.schema.rdsap_schema_17_1 import (
     RdSapSchema17_1,
     EnergyElement as EnergyElement_17_1,
 )
-from datatypes.epc.schema.sap_schema_17_1 import SapSchema17_1
+from datatypes.epc.schema.sap_schema_17_1 import (
+    SapSchema17_1,
+    EnergyElement as EnergyElement_SAP_17_1,
+)
 from datatypes.epc.schema.rdsap_schema_18_0 import (
     RdSapSchema18_0,
     EnergyElement as EnergyElement_18_0,
@@ -654,9 +657,14 @@ class EpcPropertyDataMapper:
             cfl_fixed_lighting_bulbs_count=0,
             led_fixed_lighting_bulbs_count=0,
             incandescent_fixed_lighting_bulbs_count=0,
-            roofs=[],
-            walls=[],
-            floors=[],
+            # D4: full SAP lodges the measured U as text in the element
+            # description ("Average thermal transmittance X W/m²K"); carry it
+            # through so u_wall/u_floor/u_roof parse it instead of re-deriving
+            # from a fabricated age band. "(other premises above/below)"
+            # sentinels survive untouched (bordering dwelling → no heat loss).
+            roofs=EpcPropertyDataMapper._map_energy_elements(schema.roofs),
+            walls=EpcPropertyDataMapper._map_energy_elements(schema.walls),
+            floors=EpcPropertyDataMapper._map_energy_elements(schema.floors),
             main_heating=[],
             sap_windows=[],
             sap_building_parts=[],
@@ -2219,6 +2227,7 @@ class EpcPropertyDataMapper:
             EnergyElement_20_0,
             EnergyElement_21_0,
             EnergyElement_21_0_1,
+            EnergyElement_SAP_17_1,
         ],
     ) -> EnergyElement:
         description = (
@@ -2243,6 +2252,7 @@ class EpcPropertyDataMapper:
                 EnergyElement_20_0,
                 EnergyElement_21_0,
                 EnergyElement_21_0_1,
+                EnergyElement_SAP_17_1,
             ]
         ],
     ) -> List[EnergyElement]:
diff --git a/datatypes/epc/domain/tests/test_from_sap_schema.py b/datatypes/epc/domain/tests/test_from_sap_schema.py
index 715040c1..fe31b8e3 100644
--- a/datatypes/epc/domain/tests/test_from_sap_schema.py
+++ b/datatypes/epc/domain/tests/test_from_sap_schema.py
@@ -75,3 +75,34 @@ class TestFromSapSchema17_1Tracer:
         # Assert
         assert result.uprn == 10092973954
         assert result.total_floor_area_m2 == 68.0
+
+
+class TestFromSapSchema17_1FabricDescriptions:
+    """Slice 3 (D4): the measured-U fabric descriptions flow through to
+    epc.walls/floors/roofs so the engine's u_wall/u_floor/u_roof can parse
+    'Average thermal transmittance X W/m²K' instead of re-deriving from a band."""
+
+    @pytest.fixture
+    def sample(self) -> EpcPropertyData:
+        schema = from_dict(SapSchema17_1, load("sap_17_1.json"))
+        return EpcPropertyDataMapper.from_sap_schema_17_1(schema)
+
+    def test_wall_description_carries_measured_u(self, sample: EpcPropertyData) -> None:
+        assert sample.walls[0].description == "Average thermal transmittance 0.17 W/m²K"
+
+    def test_floor_description_carries_measured_u(self, sample: EpcPropertyData) -> None:
+        assert sample.floors[0].description == "Average thermal transmittance 0.13 W/m²K"
+
+    def test_roof_description_carries_other_premises(
+        self, sample: EpcPropertyData
+    ) -> None:
+        # A ground-floor flat with premises above → no roof heat loss; the
+        # "(other premises above)" sentinel must survive so the engine treats
+        # it as internal rather than parsing a measured U.
+        assert sample.roofs[0].description == "(other premises above)"
+
+    @pytest.mark.parametrize("fixture", _ALL_FIXTURES)
+    def test_every_fixture_has_wall_descriptions(self, fixture: str) -> None:
+        schema = from_dict(SapSchema17_1, load(fixture))
+        result = EpcPropertyDataMapper.from_sap_schema_17_1(schema)
+        assert result.walls and result.walls[0].description
diff --git a/datatypes/epc/schema/sap_schema_17_1.py b/datatypes/epc/schema/sap_schema_17_1.py
index 3b8995cf..bc332a78 100644
--- a/datatypes/epc/schema/sap_schema_17_1.py
+++ b/datatypes/epc/schema/sap_schema_17_1.py
@@ -10,11 +10,24 @@ raises only on a missing *required* field.
 """
 
 from dataclasses import dataclass
-from typing import Union
+from typing import List, Union
 
 from .common import DescriptionV1
 
 
+@dataclass
+class EnergyElement:
+    """A fabric/system element with its lodged description. On full SAP the
+    `description` carries the measured U-value as text, e.g. "Average thermal
+    transmittance 0.17 W/m²K" (parsed downstream by u_wall/u_floor/u_roof),
+    or "(other premises above/below)" for an element bordering another
+    dwelling (no heat loss)."""
+
+    description: Union[str, DescriptionV1]
+    energy_efficiency_rating: int
+    environmental_efficiency_rating: int
+
+
 @dataclass
 class SapSchema17_1:
     uprn: int
@@ -30,3 +43,6 @@ class SapSchema17_1:
     inspection_date: str
     has_hot_water_cylinder: str
     has_fixed_air_conditioning: str
+    roofs: List[EnergyElement]
+    walls: List[EnergyElement]
+    floors: List[EnergyElement]