Fire safety: Approved Document B regulation
In short:
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Ground floor habitable rooms (excluding kitchens) should either have a window/door that meets the fire escape window regulations, or lead to a hallway that has a fire escape window/door.
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First floor room that are no higher than 4.5 metres off the ground have the same rules as ground floor rooms or direct access to a protected hallway.
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Emergency escape windows
Windows should have an unobstructed opening area of:
A minimum area of 0.33m2.
A minimum height of 450mm and a minimum width of 450mm (the route through the window can be at an angle)
The bottom of the openable area is a maximum of 1100mm above the floor.
Windows must be able to remain open without being held.
Locks (with or without removable keys) and opening stays (with child-resistant release catches) may be fitted to escape windows.
Replacement windows need to comply with the above if they are big enough to comply. If not the situation should not be made worse.