I've done a loft conversion in a flat. First thing, there is no such thing as "permitted development" in a flat, so the plans will need to be submitted for full planning permission if there are to be any external changes (i.e. a dormer). Second thing, most builders will tell you that the council is unlikely to grant planning permission for a full width dormer, and will suggest building a smaller 2-3m dormer instead. They may well be right... but on the other hand it could be worth using a local architect to draw up plans and submit them, as they may be more willing to try their luck. And if you are only able to get permission for the smaller extension, then the architect should be more adept at making the best use of the available space. You also need to check that you definitely own the loft - if you're a lease holder, your landlord might insist that you buy it from them.