Planning Permission is not always required, If you are constructing
new 'habitable space' i.e. space to be used as a bedroom etc and
not just storage space then technically you need to apply for PP
but be aware for 'habitable space' to be provided you need to meet
specific criteria regarding ceiling height laid out in the building
regs (part F). You will need PP for a Velux window to the side
or front of the building only the rear is exempt. If you want to
provide a dormer window to the front, rear or side you will require
PP. What most people do is build without obtaining PP, put a Velux
to the rear and officially the converted space is for "storage" where
as in fact it is used as a bedroom although this is not officially permitted
and may contravene the fire regulations.
A competent architect should be able to tell you
immediately your best course of action.