Unfortunately, flats and maisonettes don't fall within permitted development rights, however gaining planning approval should be relatively straightforward, especially if you're using like for like materials. That said, you can never second guess a Planning Officer. Even if other properties have had approval for similar works, every project/application is looked at as an individual entity.
You may also need to get consent from the freeholder.
EDIT: Apologies - may have just repeated what you already knew.