The information provided below is from the current building control regulations
For a full description of the building control regulations relating to the replacement of external windows and doors in dwellings for the UK please click here
Requirement B1- means of escape in fire
All windows to habitable rooms (but not kitchens, utility rooms, dressing rooms, bathrooms, wc's or shower rooms) at floors above ground level are required to be suitable for escape in fire. In addition, rooms at ground floor level whose only escape route is via another room must be provided with suitable escape windows. A suitable escape window should have an unobstructed openable area at
least 0.33m2 and at least 450mm high and 450mm wide (a 450mm wide opening will need to be 735mm high) and fitted with suitable fire escape hinges that do not restrict the opening sizes.
The bottom of the openable area should be no more than 1100mm above the floor. Any key required to open the window should be readily available.