What are the minimum dimensions for a fire escape window?

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.

What should the sill height be for a fire escape window?

Per AD B Vol 1 Diagram 3, the bottom of the clear opening (sill height) must be no more than 1100mm above the floor. There is no Part B minimum sill height — a low sill is acceptable for escape. AD K (Protection from falling) sets fall-prevention requirements which may require window restrictors where sill is below 800mm and the drop exceeds 600mm. For loft conversions, a Velux-style escape window may be acceptable if it meets the opening dimensions.

When is a fire escape window required?

Fire escape windows are required in all habitable rooms on the first floor (or ground floor with no direct external door access) of dwellings where the room could become an inner room with only one exit route. Loft conversions always require escape windows unless a protected staircase is provided to ground level.

Can a window restrictor be fitted to a fire escape window?

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.

Do all bedroom windows need to be fire escape windows?

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. However, in a two-storey house, first-floor bedrooms accessed only via an open-plan ground floor (inner rooms) do require escape windows. Loft bedrooms always need them.

TradeCalculator.co.uk

Window Fire Escape Calculator

Check whether windows meet Approved Document B fire escape requirements for clear opening dimensions, sill height and egress compliance in UK dwellings.

Window TypeSide-hung casement is most common for escape

Room TypeDetermines escape requirements

Clear Opening Width

Width when fully open (min 450mm)

Clear Opening Height

Height when fully open (min 450mm)

Sill Height Above Floor

Max 1100mm (AD B); no lower Part B limit

Price per Window

If replacing non-compliant windows

How We Calculate This

This calculator checks whether a window meets the fire escape (egress) requirements of Approved Document B of UK Building Regulations.

Requirements (AD B)

Clear opening ≥ 0.33 m², minimum 450mm in any direction

Minimum clear width: 450mm
Minimum clear height: 450mm
Minimum clear area: 0.33 m²
Sill height: max 1100mm above floor level (AD B, no lower limit)
Restrictors must be easily overridable without tools

Frequently Asked Questions

Related Calculators

Window U-Value Calculator

Calculate window U-value from glass, frame and spacer thermal performance.

Double Glazing Unit Calculator

Calculate DGU thickness, weight per m² and thermal performance by gas fill.

Window Condensation Risk Calculator

Calculate window condensation risk from internal temperature, RH and dew point.

Sealant and Mastic Calculator

Calculate sealant cartridges from joint volume per window frame.

Fire Escape Window Calculator

Check Part B fire escape window requirements for loft habitable rooms.

Last updated: March 2026

All calculations are estimates. Verify with your supplier.