It depends on a few factors if your house does'nt overlook another property then you don't need planning permission (not in Merseyside anyway) and if it's already got a skylight in. If the other adjoining properties have them also then it's likely you'd get permission, and just because someone objects does'nt mean you won't get approval..
It's easier just to ring up and ask and they'll tell you, even with it done you might not be able to claim it as bedroom due to the floorspace available etc, you may have to call it a study.