Window opening for escape - uK building regs

[Alexmash]

Does anyone know where to measure a window opening from when calculating what's a viable escape route from the first floor?

We have 500cm from front window edge to open window (top hung) but only 400cm from edge of external cil. I can't find any concrete language as to where the opening should be measured from to pass building regs. 

Many thanks, 

[Temp]

Google says..

 

Minimum Dimensions: Both the clear opening width and height must be at least 450mm.

 

Total Area: The clear openable area must be at least 0.33m^2. If you make the opening width the minimum 450mm, the height must be at least 735mm to 750mm to hit the area required.

 

Height: The bottom of the openable area should be no more than 1,100mm (1.1m) above the internal floor level for safe access.

[Temp]

I suspect the BCO may want a ball 450mm in diameter to fit through it. Not literally (that's not in the regs) but by measurement.

 

We have a window a fraction too narrow but the BCO allowed it provided we fitted a thicker more fire rated door to that room.

[torre]

3 hours ago, Temp said:

a fraction too narrow but the BCO allowed it provided we fitted a thicker more fire rated door

Bit surprised with the focus on fire safety you'd get any concession on this one. Maybe they thought you'd slim down sweating while waiting to escape? 

 

4 hours ago, Alexmash said:

only 400cm from edge of external ci

50mm is more than a fraction, I think you might struggle to get that past a BCO. There's no point being able to climb halfway out of a window but jammed on the cill. Maybe if it's picked up on (or you're on the larger side anyway) you can look at whether it's easy to make the stairs a protected corridor instead? 

[Temp]

On 02/06/2026 at 23:01, torre said:

Bit surprised with the focus on fire safety you'd get any concession on this one. Maybe they thought you'd slim down sweating while waiting to escape? 

 

 

🙂

 

 

[Spinny]

Could you have a hammer to smash your way out through the window ?

Seems to be allowed for trains.

 

How long is it going to take to knot the bed sheets together ?

[kandgmitchell]

As Temp says above: the window needs an opening which is at least 450mm in either direction as well as being at least 0.33m2 in area - so that negates 450mm x 450mm. The dimensions relate to the opening which is why some upvc window hinges do not comply as they do not allow the window to swing fully open. The bottom edge of the opening should not be more than 1100mm off the floor.

 

The idea is to allow someone to climb out of the window, hang from the bottom edge and drop to the ground outside - hence only being allowed for first floor windows.

[Russell griffiths]

On 04/06/2026 at 21:46, Spinny said:

Could you have a hammer to smash your way out through the window ?

Seems to be allowed for trains.

 

How long is it going to take to knot the bed sheets together ?

Not a chance 

it has to be an openable window. 
i would not fancy your chances smashing triple glazed windows with the outer pain in laminated glass. 
 

 

[Russell griffiths]

We have been given a bit of leeway with one of ours as it is higher than 1100mm from the floor, bco has stated due to the amount of furniture in the room egress through the windows could be gained easily by using a piece of furniture to stand on.