2.7 - All habitable rooms in the upper storey(s) of a house served by one staircase should be provided with a window or external door which complies with paragraph 2.11.
2.11 - The window should have an unobstructed openable area that is at least 0.33m² and at least 450mm wide and 450mm high. The bottom of the openable area must not be more than 1100mm above the floor. To comply with regulations the minimum effective opening is 450mm x 750mm in either direction.
Escape window regulations may vary between different UK regions - Check before specifying.
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The Conservation™ Range of Timber Windows & Doors