diff --git a/backend/documents_parser/elmhurst_extractor.py b/backend/documents_parser/elmhurst_extractor.py
index 6f7e4936..196291b4 100644
--- a/backend/documents_parser/elmhurst_extractor.py
+++ b/backend/documents_parser/elmhurst_extractor.py
@@ -1352,6 +1352,10 @@ class ElmhurstSiteNotesExtractor:
                 air_permeability_ap4_m3_h_m2 = float(ap4_raw.split()[0])
             except (ValueError, IndexError):
                 air_permeability_ap4_m3_h_m2 = None
+        # SAP 10.2 §2 (17) "Measured/design AP50" from a Blower Door test.
+        # Routes the cascade's (18) via `AP50 / 20 + (8)` (preferred over
+        # AP4). Absent when the test method is "Not available".
+        ap50_raw = self._local_float(pressure_lines, "Pressure Test Result (AP50)")
         # Summary §12.1 "Mechanical Ventilation Type" — scoped to §12.1
         # body so the global "Type" labels in §14 / §15 can't shadow it.
         mv_lines = self._section_lines(
@@ -1400,6 +1404,7 @@ class ElmhurstSiteNotesExtractor:
             mechanical_ventilation=self._bool_val("Mechanical Ventilation"),
             pressure_test_method=self._str_val("Test Method"),
             air_permeability_ap4_m3_h_m2=air_permeability_ap4_m3_h_m2,
+            air_permeability_ap50_m3_h_m2=ap50_raw,
             mechanical_ventilation_type=mechanical_ventilation_type,
             mechanical_ventilation_pcdf_reference=mev_pcdf_reference,
             wet_rooms_count=wet_rooms_count,
diff --git a/datatypes/epc/domain/epc_property_data.py b/datatypes/epc/domain/epc_property_data.py
index c12d87f0..f69ede90 100644
--- a/datatypes/epc/domain/epc_property_data.py
+++ b/datatypes/epc/domain/epc_property_data.py
@@ -202,6 +202,10 @@ class SapVentilation:
     # Pulse pressure test, m³/h per m² of envelope area. When present the
     # cascade routes (18) via the AP4 formula `0.263 × AP4^0.924 + (8)`.
     air_permeability_ap4_m3_h_m2: Optional[float] = None
+    # SAP 10.2 §2 (17) — air permeability at 50 Pa from a Blower Door test,
+    # m³/h per m² of envelope area. When present the cascade routes (18)
+    # via `AP50 / 20 + (8)` (preferred over AP4).
+    air_permeability_ap50_m3_h_m2: Optional[float] = None
     # SAP 10.2 §2 (23a)/(24a..d) — Elmhurst "Mechanical Ventilation Type"
     # string mapped to the `MechanicalVentilationKind` enum name (e.g.
     # "EXTRACT_OR_PIV_OUTSIDE" for MEV decentralised). The cascade uses
diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index acd24efd..514803ec 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -6918,5 +6918,6 @@ def _map_elmhurst_ventilation(
             else (False if has_suspended_timber_floor else None)
         ),
         air_permeability_ap4_m3_h_m2=v.air_permeability_ap4_m3_h_m2,
+        air_permeability_ap50_m3_h_m2=v.air_permeability_ap50_m3_h_m2,
         mechanical_ventilation_kind=_elmhurst_mv_kind(v.mechanical_ventilation_type),
     )
diff --git a/datatypes/epc/surveys/elmhurst_site_notes.py b/datatypes/epc/surveys/elmhurst_site_notes.py
index 02f27849..3b179253 100644
--- a/datatypes/epc/surveys/elmhurst_site_notes.py
+++ b/datatypes/epc/surveys/elmhurst_site_notes.py
@@ -202,6 +202,9 @@ class VentilationAndCooling:
     # SAP 10.2 §2 (17a) AP4 reading from §12.2 "Pressure Test Result
     # (AP4)" — only present when `pressure_test_method == "Pulse"`.
     air_permeability_ap4_m3_h_m2: Optional[float] = None
+    # SAP 10.2 §2 (17) AP50 reading from §12.2 "Pressure Test Result
+    # (AP50)" — present for a Blower Door test. Routes (18) via AP50/20.
+    air_permeability_ap50_m3_h_m2: Optional[float] = None
     # Summary §12.1 "Mechanical Ventilation Type" — e.g. "Mechanical
     # extract, decentralised (MEV dc)". None when `mechanical_ventilation
     # is False` (no MV system).
diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index f4a86295..c8a59fcf 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -4986,6 +4986,9 @@ def ventilation_from_cert(
     # cascade's `(18) = 0.263 × AP4^0.924 + (8)` formula; absent value
     # falls through to the components-based (16) ach.
     ap4 = sv.air_permeability_ap4_m3_h_m2 if sv is not None else None
+    # SAP 10.2 §2 (17) — AP50 Blower Door reading routes (18) via
+    # `AP50 / 20 + (8)`, preferred over AP4 when both are lodged.
+    ap50 = sv.air_permeability_ap50_m3_h_m2 if sv is not None else None
     # SAP 10.2 §2 (23a)/(24a..d) — MV kind dispatch chooses the (25)m
     # effective-ach formula. The Elmhurst mapper translates the lodged
     # "Mechanical Ventilation Type" string to an enum *name*; resolve
@@ -5023,6 +5026,7 @@ def ventilation_from_cert(
         window_pct_draught_proofed=float(epc.percent_draughtproofed or 0),
         sheltered_sides=int(sv.sheltered_sides) if sv is not None and sv.sheltered_sides is not None else 2,
         air_permeability_ap4=ap4,
+        air_permeability_ap50=ap50,
         mv_kind=mv_kind,
         mv_system_ach=mv_system_ach,
         **wind_kwargs,
diff --git a/tests/domain/sap10_calculator/worksheet/test_section_cascade_pins.py b/tests/domain/sap10_calculator/worksheet/test_section_cascade_pins.py
index d4725df9..19b9d7dc 100644
--- a/tests/domain/sap10_calculator/worksheet/test_section_cascade_pins.py
+++ b/tests/domain/sap10_calculator/worksheet/test_section_cascade_pins.py
@@ -427,6 +427,26 @@ def test_case44_non_separated_conservatory_fabric_matches_pdf() -> None:
     )
 
 
+def test_case44_blower_door_pressure_test_matches_pdf() -> None:
+    """Simulated case 44 lodges a Blower Door air-pressure test
+    (§12.2 "Pressure Test Result (AP50) 4.50"). SAP 10.2 §2 (17)-(18):
+    the AP50 reading routes infiltration via `(18) = AP50/20 + (8)` =
+    4.5/20 + 0.1167 = 0.3417, in preference to the components-based (16)
+    estimate. The extractor previously read only the AP4 (Pulse) column,
+    so a Blower Door result fell through to the structural-infiltration
+    default (effective ach 0.81 vs the worksheet's 0.58 → ventilation
+    heat loss over-counted by ~38%)."""
+    # Arrange
+    epc = _w001431_case44.build_epc()
+
+    # Act
+    vent = ventilation_from_cert(epc)
+
+    # Assert — (18) infiltration + (25) Jan effective ach, at abs=1e-4.
+    _pin(vent.pressure_test_ach, 0.3417, "§2 (18) case44")
+    _pin(vent.effective_monthly_ach[0], 0.5812, "§2 (25) Jan case44")
+
+
 def test_case6_main_2_emitter_and_control_extracted() -> None:
     """Simulated case 6's §14.1 Main Heating2 lodges its OWN emitter
     ("Underfloor Heating") and control ("SAP code 2110, ...") — the two