Under regulations that came into effect from 1 october 2008, loft conversions are now considered as permitted developement, And generally means that no planning permission is needed unless you live in a conservation area.
A minimum of 2m headroom is generally required over the flight of the staircase and landing, although the Local Authority may permit a slight reduction height.
The average time of work will take:
3-4 weeks for a velux loft conversion on a typical 3 bedroom property
4-5 weeks for a dormer loft conversion on a typical 3 bedroom property
5-6 weeks for a wrap around loft conversion dormer on a typical 3 bedroom property