Fire Escape Window Regulations UK: Minimum Egress Window Sizes
UK egress windows need a clear openable area of at least 0.33m², a minimum 450mm opening, and a cill no higher than 1,100mm. See where they are required and how to check your quote.
A fire escape window is one of the least glamorous and most important parts of a loft conversion or new bedroom. It is also one of the easiest things for a quote to skip, because the window looks fine until you check the openable area and the cill height. Here are the numbers from Approved Document B and how to check your quote meets them.
The dimensions
An egress (fire escape) window must provide a clear openable area of at least 0.33 square metres, with the height and the width of the opening each at least 450mm. The bottom of that openable area (the cill) must be no more than 1,100mm above the floor, so a person can climb out. All three conditions have to be met at once: area, minimum dimension, and cill height. Confirm against the current Approved Document B, as the figures are periodically reviewed.
The catch is that "0.33 square metres" is the clear opening, not the size of the glass. A window with a restricted opening, or one hinged in a way that blocks part of the gap, can have plenty of glass but fail the escape rule.
Where escape windows are required
- Ground and first floor habitable rooms (bedrooms, living rooms) generally need an escape window or direct access to a protected escape route.
- Loft conversions usually need escape windows in the new rooms, or a fully protected stairway down to a final exit, because the extra height triggers stricter rules.
- Bathrooms, kitchens, and other non-habitable rooms are not subject to this requirement.
This is why our loft conversion headroom and staircase guide and this rule go together: the loft conversion has to solve both the stair and the escape.
The mistakes a quote hides
Two failures recur. First, the cill is too high: the window meets the area rule but sits more than 1,100mm off the floor, so nobody can climb out. Second, the openable area is too small once you account for how the window hinges, even though the glazed area looks generous. A loft or extension quote that lists windows by size but says nothing about egress compliance is one to query, because retro-fitting a compliant escape window after the room is built is expensive.
For checking the windows themselves once installed, see our guide to checking windows and doors.
What your quote should confirm
- That habitable rooms have a compliant escape window or a protected escape route
- The clear openable area (at least 0.33m²) and minimum opening (450mm each way)
- The cill height (no more than 1,100mm above floor)
- For loft conversions, how the fire escape and stair strategy works as a whole
- Mains-wired, interlinked smoke alarms (a related Part B requirement)
Check your quote
Fire escape is a building regulations requirement that is cheap to design in and expensive to fix later. If you are holding a loft conversion or extension quote, upload it and we check whether the escape windows and the wider fire safety points are addressed, or whether they are a missing building regs item that Building Control will catch. It takes under a minute.
Got a loft or extension quote? Check the fire escape
See how your costs compare to thousands of real UK quotes.
Get My Estimate