I am thinking of converting my loft into a bedroom and I was wondering how to measure the cubic metres to see if I am within permitted development. I have looked on the planning portal and their volume calculator implies that the measurements are taken for the width, height and depth (on the outside). The roof though isn't a box or rectangle shape, it has slopes and will be wider at the bottom than the top. How do I take account of these variations? More importantly what would planners or building regs demand?