I am thinking of building one additional room above already build kitchen/bathroom. This existing part has been build approx 10-15 years ago. Simple, 12sq.m, flat roof, window of the bathroom. The problem I have is that i will have to "cut in to" slope of my roof in order to achieve desired height of the ceiling for this new built room. This brings to question if I need to apply for the planing permission. Any advice please.