What is the minimum size for a fire escape window?

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.

What is the maximum sill height for a fire escape window?

The sill of an escape window must be no more than 1100mm above the floor level of the room. If the sill is higher than 1100mm, a permanent step or raised platform can be provided to meet this requirement. For roof windows in loft conversions, the bottom of the opening must be no more than 1100mm above the floor and no more than 1700mm above the floor at the top of the opening.

Do I need an escape window above the second floor?

No. Escape windows are only effective up to a height of approximately 4.5 metres (roughly the second floor). Above this height, Approved Document B requires alternative means of escape such as a protected stairway, because the drop height makes window escape too dangerous. Buildings over 4.5m to the escape window sill must provide internal protected escape routes instead.

What are the escape window requirements for a loft conversion?

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. Roof windows (Velux type) are acceptable as escape windows provided they meet the minimum clear opening requirements (450mm x 450mm, 0.33 square metres area). The window sill must be no more than 1100mm above floor level. A dormer window is often preferred as it provides easier access. AD B Vol 1, Section 2 applies.

Does a ground floor room need an escape window?

Yes. Approved Document B requires habitable rooms at ground floor level to have either an escape window or door leading directly to a place of safety. 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).

Fire Escape Window Calculator — Part B Emergency Egress

How We Calculate This

This calculator checks whether a window meets the emergency egress requirements of Approved Document B, Volume 1 (Dwellings). It calculates the clear opening dimensions based on window type and checks against the minimum requirements for width, height, area and sill height.

How it works

Clear opening: The actual unobstructed opening size is calculated based on window type. Side-hung casements provide full clear opening; sliding sash windows provide half the overall height; top-hung windows are restricted by opening angle.
Minimum requirements: AD B requires minimum 450mm width, 450mm height and 0.33 m² clear area — all three must be met simultaneously.
Sill height: Maximum 1100mm above floor level to allow easy access during emergency egress.
Floor level: Escape windows are only effective up to 4.5m drop height (approximately second floor). Higher floors need protected escape routes.

Key references

Approved Document B, Vol 1: Fire safety — Dwellings, Section 2
BS 5588-1: Fire precautions in the design, construction and use of buildings (dwellings)

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.