Cut and paste below from the Planning Portal site:
Means of escape
When replacing any window, the opening should be sized to provide at least the same potential for escape as the window it replaces. If the original window that is being replaced was larger than necessary for the purpose of escape, then the new window opening could be reduced down to the minimum as specified in the criteria below.
The means of escape should be considered for any new window installed to an extension or existing dwelling. If an escape window is required then criteria set out below should be followed. It is also generally good practice to replace any window on the first floor that is not used as an escape window with an escape window.
See below for the general criteria for egress windows:
- Width and Height - Either of these are not to be any less than 450mm
- Clear Openable Area - No less than 0.33m²
- Cill height - The bottom of the openable area should be no more than 1100mm above the floor area.
Only one window per room is generally required.