Hi, new to the forum, thought someone might know the answer to this. I want to add a small extension ( 2m x 4m ) to the rear of the house, which is build up with block retaining walls. Due to this I want to keep the weight down, and am considering using a timber frame, UPVC clad on the outside, insulating packing and plasterboard interior, on a concrete slabe. Is this feasable/ permissable ? Jeff605