If it's just a storage space (ie. it won't be used as a living room or bedroom) and
(1) you dont put in a dormer window anywhere
(2) you dont put in a velux-type roof window on the side or to the front of the house and
(3) you dont change the profile of the roof,
then it should be exempt from requiring permission.
The existing ceiling may need to be strengthened if it is going to be used as a floor.
If new supporting steelwork is being inserted, it needs to be fire protected as it would be considered a structural element.
If you want to put in more than 2 roof windows, then you may require permission.
Note: If the house is located in a "Conservation area", (to find out, ask the Local Authority or look up the Local Development Plan map), you have to be more careful- roof lights to the rear may require permission. LIkewise if the house is on the list of protected structures- don't make any alterations without seeking advice from the L.A. or a professional.
If at least 1/2 the floor area of the new attic room has a ceiling height of at least 2.4m,
then you have the option of considering the room to be a "habitable" room. (ie. It can be used as a bedroom or living room).
If you do, the requirements of the building regulations will need to be met.
At least one rooflight (If permitted by the Local Authority- some don't allow habitable rooms to receive daylight solely from velux-type rooflights) is required to be an "escape" rooflight. Also a fixed staircase will be required.
There are a number of issues which are spelled out here: