My bedroom window is currently too small. Do I have to make it bigger?

Generally no. Replacing an existing window is not an opportunity for building control to require an improvement that was never there, and the duty is to provide escape at least equivalent to what existed. Enlarging the opening is structural work in its own right.

Does a ground-floor room need an escape window?

Usually not, because the room has a door to somewhere you can leave from. An inner room is the exception: if the only way out is through another room, it needs its own escape window whatever floor it is on.

Can an escape window be locked?

It can have a handle that locks, but anyone inside has to be able to open it in the dark without hunting for a key. Keyless egress handles exist for exactly this, and they are the right specification for the escape window in a house with children.

Regulations · 08 of 19

Which windows have to open wide enough to escape from?

If a room relies on a window as its means of escape, a replacement must provide at least an equivalent escape. Approved Document B gives the benchmark as an unobstructed openable area of at least 0.33 square metres, at least 450mm high and at least 450mm wide, with the bottom of the opening no more than 1100mm above the floor.

This is the regulation most likely to be broken by accident, because it is possible to replace a window with one that looks identical, performs better thermally, and no longer lets anyone out.

Which rooms this applies to

Not every window is an escape window. The requirement attaches to the room, not the opening.

First-floor habitable rooms in most houses, where the window is the alternative to the staircase.
Inner rooms, meaning a room you can only leave by passing through another room, on any floor including the ground floor.
Loft conversions and third storeys, which are generally handled by a protected stairway rather than by an escape window, and where the rules are stricter again.

The practical test is simple: if the existing window in that room opens wide and has a low cill while the others in the house do not, it was almost certainly specified as an escape window, and the replacement has to keep doing that job.

The numbers, and what they mean on a quote

Approved Document B gives the benchmark for an escape window as:

an unobstructed openable area of at least 0.33 square metres;
at least 450mm high and at least 450mm wide, and both at once, since a tall narrow slot meeting the area on its own is no use;
the bottom of the openable area no more than 1100mm above the floor.

Unobstructed is the word that catches people out. It is the clear hole a person fits through once the window is open, not the size of the frame, so the sash, the hinges and anything that stays in the way all come off the figure. A side-hung casement on standard friction hinges can lose a surprising amount of its opening to the sash itself, which is why escape windows are usually specified with easy-clean or egress hinges that swing the sash clear.

How this goes wrong

Three patterns account for nearly all of it, and all three are avoidable at quotation stage.

A like-for-like replacement with ordinary hinges. The frame is the same size, the clear opening is not, and nobody measured it.
Adding a transom or an astragal bar for appearance, which reduces the openable light below the minimum.
Fitting restrictors to every upper-floor window for child safety, including the escape one. Restrictors on an escape window have to be releasable from inside without a key — see safety glass and restrictors.

Because replacement windows are notifiable work, the installer certifying the job under FENSA or CERTASS is certifying this too. Ask, before they order, which windows they have identified as escape windows and what clear opening each one will give.

Questions

My bedroom window is currently too small. Do I have to make it bigger?: Generally no. Replacing an existing window is not an opportunity for building control to require an improvement that was never there, and the duty is to provide escape at least equivalent to what existed. Enlarging the opening is structural work in its own right.
Does a ground-floor room need an escape window?: Usually not, because the room has a door to somewhere you can leave from. An inner room is the exception: if the only way out is through another room, it needs its own escape window whatever floor it is on.
Can an escape window be locked?: It can have a handle that locks, but anyone inside has to be able to open it in the dark without hunting for a key. Keyless egress handles exist for exactly this, and they are the right specification for the escape window in a house with children.

Note This guide is general information about England and Wales, not advice about your specific property. TrustedGlazing is an introducer and does not install, survey or quote. Rely on your installer's survey and your local authority for decisions about your own home.