There's a small covered-over area outside the back door (4.8 m2), which I want to convert into living accomodation (ie extending the kitchen, basically.) It has an existing flat concrete slab for a roof, single-skin brick wall to the left and in front of the back door, and is open to the elements down the right side. Plan is to fill the right side by moving the back door and adding a window and some brickwork, then to bring it all up to habitable standard.
I'm doing this by the book which means a Building Notice, and I'm now trying to work out the specs needed for insulation to comply with the regs, which is where I'm coming a bit unstuck. I know I'll need lashings of the stuff in the roof, walls and floor, but how do I best work out the thicknesses? I've got bogged down with U values in Approved Doc Part L and am wondering if I'm going too far trying to take this back to first principles. Would John Q. Builder really do all that? Isn't it a case in practice of 'oh you need X mm of product A on the ceiling, Y mm of product B on the walls, etc?
Haven't been to see Building Control yet, but I'm assuming that they will expect to see and approve my proposals, rather than me expecting them to just give me all the answers?
Can anyone point me at an easy-to-follow source of info to guide me through all this?