diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index fd3d3190..0ef51687 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -4015,25 +4015,54 @@ def heat_transmission_section_from_cert(epc: EpcPropertyData) -> HeatTransmissio
     )
 
 
-def _corridor_door_count(epc: EpcPropertyData) -> int:
-    """RdSAP §3.7 + Table 26 — number of doors opening to an unheated
-    corridor/stairwell (each billed at U=1.4 on the sheltered wall).
+def _has_sheltered_corridor_wall(epc: EpcPropertyData) -> bool:
+    """Whether the dwelling is accessed via an unheated corridor/stairwell.
 
-    The presence of a SHELTERED alternative wall (`is_sheltered`, the
-    RdSAP §5.9 wall-to-unheated-corridor surface) is the evidence that the
-    dwelling is accessed via an unheated corridor, so its entrance door
-    opens to that corridor. RdSAP convention assumes one such access door
-    when the sheltered wall is present and the cert lodges at least one
-    door; the remainder are external. Returns 0 when no sheltered alt wall
-    is lodged (houses, exposed-gable flats) so the door channel is
-    unchanged for every non-corridor dwelling.
+    A SHELTERED alternative wall (`is_sheltered`, the RdSAP §5.9
+    wall-to-unheated-corridor surface) is the evidence that the dwelling's
+    entrance faces an unheated corridor or stairwell. False for houses and
+    exposed-gable flats (no sheltered alt wall lodged).
     """
-    has_sheltered_alt = any(
+    return any(
         (bp.sap_alternative_wall_1 is not None and bp.sap_alternative_wall_1.is_sheltered)
         or (bp.sap_alternative_wall_2 is not None and bp.sap_alternative_wall_2.is_sheltered)
         for bp in (epc.sap_building_parts or [])
     )
-    return 1 if has_sheltered_alt and epc.door_count > 0 else 0
+
+
+def _corridor_door_count(epc: EpcPropertyData) -> int:
+    """RdSAP §3.7 + Table 26 — number of doors opening to an unheated
+    corridor/stairwell (each billed at U=1.4 on the sheltered wall).
+
+    A sheltered alternative wall (`_has_sheltered_corridor_wall`) is the
+    evidence that the dwelling is accessed via an unheated corridor, so its
+    entrance door opens to that corridor. RdSAP convention assumes one such
+    access door when the sheltered wall is present and the cert lodges at
+    least one door; the remainder are external. Returns 0 when no sheltered
+    alt wall is lodged (houses, exposed-gable flats) so the door channel is
+    unchanged for every non-corridor dwelling.
+    """
+    return 1 if _has_sheltered_corridor_wall(epc) and epc.door_count > 0 else 0
+
+
+def _has_draught_lobby(epc: EpcPropertyData, sv: Optional[SapVentilation]) -> bool:
+    """RdSAP 10 §2 (13) — presence of a draught lobby.
+
+    Spec (RdSAP 10 Specification 10-06-2025, p.30, "Draught lobby"):
+    "add infiltration 0.05 if draught lobby is not present, or use 0.0 if
+    present. ... Flat or maisonette: Assume draught lobby if entrance door
+    is facing corridor (heated or unheated) or stairwell."
+
+    A sheltered corridor wall (`_has_sheltered_corridor_wall`) is exactly
+    that evidence: the flat's entrance faces an unheated corridor/stairwell,
+    so a draught lobby is assumed present regardless of the lodged value.
+    Otherwise fall back to the lodged value — which, when undetermined, is
+    the RdSAP "assume no draught lobby if cannot be determined" default for
+    houses.
+    """
+    if _has_sheltered_corridor_wall(epc):
+        return True
+    return bool(sv.has_draught_lobby) if sv is not None and sv.has_draught_lobby is not None else False
 
 
 def _rooflight_total_area_m2_from_cert(epc: EpcPropertyData) -> float:
@@ -4829,7 +4858,7 @@ def ventilation_from_cert(
         flueless_gas_fires=vc.flueless_gas_fires,
         has_suspended_timber_floor=eff_has_susp,
         suspended_timber_floor_sealed=eff_sealed,
-        has_draught_lobby=bool(sv.has_draught_lobby) if sv is not None and sv.has_draught_lobby is not None else False,
+        has_draught_lobby=_has_draught_lobby(epc, sv),
         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,
diff --git a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
index b5497ec8..8a055253 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -30,6 +30,7 @@ from datatypes.epc.domain.epc_property_data import (
     EpcPropertyData,
     MainHeatingDetail,
     PhotovoltaicArray,
+    SapAlternativeWall,
     SapFloorDimension,
     SapVentilation,
 )
@@ -1253,6 +1254,52 @@ def test_ventilation_from_cert_routes_mev_decentralised_to_extract_or_piv_outsid
         assert abs(w25 - expected) <= 1e-4
 
 
+def test_corridor_flat_assumes_draught_lobby_present_zeroing_line_13() -> None:
+    # Arrange — RdSAP 10 Specification (10-06-2025) p.30, "Draught lobby":
+    # "add infiltration 0.05 if draught lobby is not present, or use 0.0 if
+    # present. ... Flat or maisonette: Assume draught lobby if entrance door
+    # is facing corridor (heated or unheated) or stairwell." A SHELTERED
+    # alternative wall is the RdSAP §5.9 wall-to-unheated-corridor surface —
+    # the same evidence the corridor door rides on — so the flat's entrance
+    # faces a corridor and a draught lobby is assumed present, zeroing line
+    # (13). Simulated case 34 (cert 001431 storage flat): the cascade
+    # previously added the 0.05 no-lobby penalty, over-counting (16)/(18) by
+    # 0.05 ACH → +46 kWh/yr space demand → SAP −0.18.
+    from dataclasses import replace
+
+    corridor_part = replace(
+        make_building_part(construction_age_band="G"),
+        sap_alternative_wall_1=SapAlternativeWall(
+            wall_area=12.5, wall_dry_lined="N", wall_construction=4,
+            wall_insulation_type=4, wall_thickness_measured="Y",
+            wall_insulation_thickness="NI", is_sheltered=True,
+        ),
+    )
+    exposed_part = make_building_part(construction_age_band="G")
+    corridor_flat = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2, country_code="ENG",
+        sap_building_parts=[corridor_part],
+    )
+    exposed_flat = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2, country_code="ENG",
+        sap_building_parts=[exposed_part],
+    )
+
+    # Act
+    v_corridor = ventilation_from_cert(corridor_flat)
+    v_exposed = ventilation_from_cert(exposed_flat)
+
+    # Assert — the corridor flat zeroes (13); a flat with no sheltered
+    # corridor wall keeps the 0.05 no-lobby penalty (cannot be determined).
+    assert abs(v_corridor.draught_lobby_ach - 0.0) <= 1e-9
+    assert abs(v_exposed.draught_lobby_ach - 0.05) <= 1e-9
+    # The lobby removes 0.05 ACH from (16); shelter (21) drops proportionally.
+    assert v_corridor.infiltration_rate_ach < v_exposed.infiltration_rate_ach
+    assert abs(
+        (v_exposed.infiltration_rate_ach - v_corridor.infiltration_rate_ach) - 0.05
+    ) <= 1e-9
+
+
 def test_index_less_mev_uses_table_4g_default_sfp_for_fan_electricity() -> None:
     # Arrange — an MEV system with NO PCDB record (index absent / not in
     # Table 322). SAP 10.2 §2.6.3 / Table 4g note 1 prescribes a default