Crank , taken from the Fensa guide 2014 ..
Summary:
A fire escape window is required on the ground floor in any habitable room that does not open onto a hall leading directly to an exit door e.g. an inner room.
A fire escape window is required on upper floors not more than 4.5m above ground level in every habitable room (unless the room has direct access to a “protected stairway”). This is usually the case for the upstairs of a conventional two-storey dwelling.
A habitable room does not include a kitchen or a bathroom.
Upper floors more than 4.5m above ground level should be accessed by a “protected stairway” or an alternative escape route and therefore fire escape windows are not required.
There is no requirement to have more than one escape window in a room.
A fire escape window should have an unobstructed openable area that is at least 0.33m² and at least 450mm high or 450mm wide. If one of the dimensions is at the 450mm minimum then the other dimension will need to be at least 734mm to achieve 0.33m². The route through the window may be at an angle rather than straight through.
The bottom of the openable area should be no more than 1100mm above the floor.
If the outgoing window meets the minimum openable area size of 0.33m² and 450mm dimension, then the replacement should meet these minimum requirements however, if the outgoing openable area exceeds the minimum requirements, there is no obligation for the new window to meet this larger size, as long as it meets the minimum requirement of 0.33m² and 450mm. Similarly, if the openable area complies by being less than 1100mm above floor level then the replacement should also comply by being less than 1100mm, but there is no obligation for it to be any lower than 1100mm above floor level even if the outgoing window is lower.
If the outgoing window does not meet the minimum openable area requirements, then the replacement window does not have to meet them, but the area, minimum dimension and height above floor level should not be made worse than the outgoing window
Its the bold bit at the bottom....