I'm looking at expanding my property via the PD route. Part of the change will be a hip to gable conversion, a dormer and a ground floor extension. I know that under the rules, I have to abide by the 50m3 rule (schedule 2, Class B.1 (d) (ii)). My question then is related to the ground floor extension. As it's a flat roof, is it considered to have a volume that would have to be included in the 50m3 calculation? The current hip to gable, plus dormer, come out at just under the 50m3 limit. I've read somewhere that anything above eaves level would be considered roof space (i.e. eaves line up to the outer surface of the flat roof), but the rules don't seem particularly clear where it's not part of the original roof space.