Window Fire Escape Calculator

Under Approved Document B, an egress window must have a clear openable area of at least 0.33 m² with no dimension less than 450mm. This means the clear opening must be at least 450mm wide AND 450mm high, with the total area at least 0.33 m². For example, 450mm x 734mm just meets the area requirement.

Per Approved Document B Vol 1 paragraph 2.10, the bottom of the unobstructed openable area must be no more than 1100mm above the floor. There is no Part B minimum sill height — a low sill is acceptable for escape. Approved Document K (protection from falling) sets the fall-prevention requirements: where a window sill is below 800mm and the external drop exceeds 600mm, restriction or guarding is needed (any restrictor must still be removable for escape). For loft or roof windows (e.g. a roof-pitch Velux-style escape window), the 1100mm floor-sill rule is harder to meet on a sloping roof — the centre of the openable area should instead be within reach of the floor (commonly taken as roughly 600–1100mm from the eaves), so always check the specific manufacturer and Building Control guidance for roof escape windows.

Under Approved Document B Vol 1 paragraph 2.2, on storeys no more than 4.5m above ground level served by a single stair, every habitable room (other than a kitchen) must have either an emergency escape window/external door OR direct access to a protected stairway. A ground-floor habitable room needs an opening onto a hall leading to a final exit or an escape window/door (paragraph 2.1). Critically, on floors more than 4.5m above ground a window is NOT an acceptable means of escape — see the next answer.

No. Approved Document B Vol 1 only accepts an emergency escape window as a means of escape on storeys a maximum of 4.5m above ground level (paragraph 2.2 — typically up to first-floor level). For a dwellinghouse with a storey more than 4.5m above ground (e.g. most loft conversions and second floors), paragraphs 2.4–2.6 require a protected stairway (fire-resisting construction, minimum REI 30) extending to a final exit, OR an alternative escape route — not a window. Where there is a storey more than 7.5m above ground, an alternative escape route must be provided from each such storey. AD B does not rely on fire-service ladder rescue as a means of escape for dwellings, so a compliant-sized window on a second floor still does not satisfy means of escape on its own.

Yes, but the restrictor must be easily removable without tools to allow full opening for escape. Child safety restrictors that limit opening to 100mm are acceptable provided they can be overridden by an adult in an emergency. The window must still achieve the required clear opening when the restrictor is released.

Not necessarily. If the bedroom has access to a protected hallway/landing that leads to a final exit, it may not need an egress window. In a two-storey house, first-floor bedrooms (up to 4.5m above ground) accessed only via an open-plan ground floor — inner rooms — do require an escape window. A loft bedroom whose floor is more than 4.5m above ground (the usual case) cannot rely on a window: AD B Vol 1 paragraphs 2.5–2.6 require a protected stairway to a final exit (or an alternative escape route), so the escape window alone is not sufficient there.