Hi, I need to replace my back door as it’s in seriously bad shape and bottom of the frame had rotted. I want to replace frame and door with wood again. house is late 60’s. The roof above utility room is flat. Utility room wall is single skin solid - unsure what it’s made of as outside is rough cast render. The existing wooden door measures approx 1950 x 755 and the frame either side seems to be at least 1” wide. I haven’t taken off the architrave to measure the actual rough opening. it seems like all the available frames are far too tall to fit the existing opening. I’m assuming measurements changed around that time.. Looking for some guidance as to what the best thing to do is. I also have the following questions 1) as the roof is flat - can I increase the frame opening height with no major issues? 2) if I had to widen the opening slightly would a 9” angle grinder do the job? 3) do I have to get planning permission to make these changes with all this it seems like a custom door and frame might be the way to go. Any recommendations would be welcome