Fire Escape Window Calculator — Part B Emergency Egress

Under Approved Document B Volume 1, an escape window must have a clear opening of at least 450mm wide and 450mm high, with a minimum clear area of 0.33 square metres. Both minimum dimensions and the area requirement must be met simultaneously. For example, a 450mm x 450mm opening only gives 0.2 square metres, which fails the area test. A compliant opening could be 450mm x 734mm (0.33 square metres) or 550mm x 600mm.

Approved Document B Volume 1 (para 2.10) requires the bottom of the openable area to be no more than 1100mm above the floor of the room. If the sill is higher than 1100mm, a permanent step or raised platform can be provided to meet this requirement. Approved Document B sets only this maximum — building-control guidance such as CNC Building Control CNCD-049 additionally advises a practical lower bound of about 800mm for standard escape windows (600mm for a loft rooflight) so the opening is reachable but not so low it becomes a fall hazard needing Part K guarding. The bottom-of-opening figure is the only height limit in AD B; there is no 1700mm "top of opening" rule.

Approved Document B Volume 1 accepts emergency escape windows for storeys whose FLOOR is no more than 4.5 metres above external ground level (paras 2.2 and 2.4 to 2.6) — typically up to and including a first floor, and often a second floor depending on storey heights. The 4.5m figure is the height of the room floor above ground, not the sill-to-ground drop. Where the floor is more than 4.5m above ground level, an escape window alone is not acceptable: a protected stairway (minimum REI 30 fire resistance) leading to a final exit must be provided instead.

Loft conversions in dwellings must have an escape window in each habitable room unless a protected stairway is extended from ground level to the loft. A roof window is acceptable as an escape window provided it meets the minimum clear opening (450mm wide, 450mm high, 0.33 square metres area) and remains open without being held — so use a top-hung or escape-hinged roof window (e.g. Velux GPL/GPU), not a standard centre-pivot type. The bottom of the opening must be no more than 1100mm above floor level (building-control guidance also gives a 600–1100mm band for loft rooflights). A dormer window is often preferred as it provides easier access. AD B Vol 1, Section 2 applies.

Generally no. A centre-pivot roof window (such as a Velux GGL) rotates about its mid-height axis, so the open sash blocks roughly half the aperture and, critically, does not remain open without being held. Approved Document B Volume 1 para 2.10(d) requires an escape window to be capable of remaining open without being held, so a standard centre-pivot window is not accepted as a sole means of escape. For loft conversions and other rooms relying on a rooflight for egress, fit a top-hung or escape-hinged roof window (such as a Velux GPL or GPU), which lifts the whole sash clear and stays open by itself. Always confirm the actual clear opening against the manufacturer’s egress data.

Yes. Approved Document B Volume 1 (para 2.1) requires habitable rooms (other than kitchens) at ground storey level to have either an opening directly onto a hall leading to a final exit, or an escape window / door as described in para 2.10. Even though the drop height is minimal, the requirement ensures occupants can escape without having to use internal corridors that may be smoke-filled. The same minimum opening dimensions apply (450mm x 450mm, 0.33 square metres).

AD B Vol 1 Appendix A defines a habitable room as one used, or intended to be used, for people to live in, which for Part B purposes includes a kitchen but excludes a bathroom. Even so, paragraphs 2.1 and 2.2 carve kitchens back out of the escape window duty: a kitchen does not need its own escape window or door. In practice that means an escape window (or equivalent route to a final exit) is needed in every bedroom, living room, dining room and home office that does not open directly onto a protected hallway. Bathrooms, WCs, utility rooms, halls and landings are not habitable rooms at all, so they never need their own escape window, and a bathroom, WC or shower room is itself permitted as an inner room under para 2.11(d), reached only through another room, with no escape window of its own required. For a full check across every room, see our Means of Escape Calculator.

Yes. A first floor bedroom in an ordinary two-storey house is covered by AD B Vol 1 para 2.2: because the room’s floor normally sits within 4.5m of ground level, it can rely on EITHER a compliant escape window (0.33 square metres, 450mm minimum width and height, sill no more than 1100mm above the floor) OR direct access to a protected stairway. A loft conversion is different because it usually adds a third storey more than 4.5m above ground, which moves the room out of para 2.2 and into paras 2.21 to 2.23. There, the standard solution is a protected stairway extended from ground level to the loft (fire doors minimum E20/FD20, partitions minimum REI 30), and where that stairway is provided, no escape window is required in the loft room itself. An escape window only reappears under the para 2.23 open-plan alternative, where sprinklers protect the open ground floor and the escape window sits on the first storey, reached via a fire door, rather than in the loft room. See our Loft Fire Safety Calculator for the full breakdown of which solution applies and what it costs.

Often yes. Building Regulations Regulation 4(3) requires that once replacement work is complete, the dwelling must not end up less compliant with the other parts of Schedule 1, including Part B, than it was before. AD B Vol 1 para 2.19 sets out what that means for windows: where the existing window would count as an escape window in a new dwelling and is already big enough to be used for escape, the replacement must either be sized to provide at least the same potential for escape, or, if the old opening was larger than the AD B minimum, it can be reduced down to the para 2.10 minimum (0.33 square metres, 450mm wide and high, sill maximum 1100mm) but no smaller. In practice a FENSA or Certass registered installer, or a building control surveyor, should check the existing window’s escape function before fitting a smaller-opening replacement, particularly where a side-hung casement is swapped for a sealed sash or fixed light. See our guide to window replacement costs for what a FENSA-registered installation typically involves.

Yes, they are the same thing. Cill is simply the traditional British spelling used across the glazing and window trade (FENSA installers, window manufacturers and many building-control guidance notes use cill), while sill is the everyday spelling used in dictionaries and most general building texts. Approved Document B itself uses neither spelling: para 2.10 simply refers to the bottom of the openable area. Both describe the height, measured from the finished floor of the room, up to the bottom of the openable part of the window, which is what governs the AD B para 2.10 maximum of 1100mm. It is not necessarily the same as the height of the window’s outer frame or masonry ledge if the openable light sits above a fixed lower pane: what matters for escape purposes is the bottom of the part that actually opens. AD B sets no minimum cill height, only the 1100mm maximum, though building-control guidance also points to roughly 800mm (600mm for a loft rooflight) as the practical lower bound before a low opening needs its own Part K fall guarding.

No, not in the same way. This calculator and the 0.33 square metre, 450mm and 1100mm figures throughout it come from Approved Document B Volume 1, which covers dwellings (houses and flats) only. Schools, offices, shops and other non-domestic premises are covered by Approved Document B Volume 2 (buildings other than dwellings), which does not use escape windows as a means of escape at all. Instead, Volume 2 sets maximum travel distances to a protected escape route or final exit and minimum exit widths based on occupant numbers, and school buildings also draw on Department for Education technical guidance alongside Volume 2. See our answer on how many emergency exits a non-domestic building needs for those occupancy-based figures.