PP is concerned with how your dwelling looks from the outside. If you are therefore thinking about putting dormers into the roofline (and hence changing the shape/appearance) then you may need PP. In many cases if you are merely putting in flat roof windows (Velux or similar), you won't need PP. However another part of PP guidance is about ensuring that any new development doesn't unduly overlook a neighbour. So depending on the orientation of the windows are (flat or otherwise) and how high up in the room they are (flat ones) also drives the need for PP application or not. In the first instance do as Chris suggests above and talk to Development Control (Planning Dept) at your Local Council - they are there to help and don't bite.
The second thing to be aware of is the need for Building Control approval. This is different for PP, and is dealt with by another department at the same Council. BC ensures that minimum stds of construction quality are built into the structure (insulation, safety, foundations and lots more besides). You will definitely need this, irrespective of the PP situation - and its your guarantee of minimum quality.
For a loft conversion, I'd go with Chris's suggestion above to talk to a specialist company who will be aware of the Regs and accustomed to making applications for both elements on behalf of clients.