The council dont like you building below the surface because of water ingress I think. If you did that you might need to tank the walls inside below the ground level. I built mine up two courses of concrete block and then timber framed it. Saved quite a bit of money because I did it all myself. I didnt fancy trying brickwork that high but a couple of courses was easy enough with my Brickies Mate laying tool. Insulated all the walls and roof with 100mm thick polystyrene and then clad the outside with plastic cladding. Looks nice even though I do say so myself. I also made the roof trusses from 100mm x 50mm to take extra weight and I use the space for storage. I also modified the roof structure to allow a bigger access to get larger and longer stuff up there.
I will try to post some photos later.

Ian