simple as i can put it...
1. when the window is open there must be a clear unobstructed area of at least 0.33 sq m (with a minimum height or width of 450mm)
2. the window should be capable of remaining in the opening position (important for rooflights)
3. the bottom of this openable section must be between 600mm and 1100mm above floor level
4. the distance from the eaves of the roof to the bottom of the openable section must not be more than 1700mm
5. the area below the window on the ground should be left clear for someone to escape down to, or easily accessible for someone with a ladder.
a few points:
* simply use a rooflight designed for escape purposes, like the Velux GPL M08, which is 1400 x 780 and top hung.
* usually the purlin becomes a problem in a dormer situation. dont allow the carpenter to take shortcuts by simply running the purlin straight through, it may be necessary to cut it to achieve the opening.