Check your local council website for permitted development guidelines. This will probably indicate that velux roof windows are okay, providing that you do not live in a conservation area or have a listed building. However, if the latter does pertain, then Velux do have windows in their conservation line that could possibly offer a solution. Legislation is moving in the right direction for this kind of improvement, but it is best to check before making costly alterations that then fall foul of the rules and subsiquently need reversing. Also, Velux offer excellent u-value windows, so beware of cheap imitations, least you find they are un-HIP! Best of luck.