Hi everyone my 1930's semi has a little room 3000x5000 mm built on the side that we use as a utility room, wash machine, shoes and all these stuff. it further has a toilet. unfortunately this room, built in the 1950's - 1960's has a single wall only and the concrete floor is just poured on the ground. thats very cold, and its damp. I guess no isolation and no damp prove. I hasn't a decent foundation, there's some concrete poured in a trench, maybe 400mm deep, and the walls are fitted on this slab, no cracks in the walls, none of the internal tiles cracked. I am planning to demolish this room and replace it with an extension the same size made of timber frame, OSB boards, insulation to current standard. I am planning to use this room as a spare room, maybe a room for the kids. the only trouble I have is its foundation. Our soil is clay, no trees nearby. I tried to dig already just for a try but there are lots of rubble buried , I think still from the time the houses were built, it makes digging nearly impossible. as these timber frame extension is not that heavy like a solid brick extension, what would be the requirement for my foundation? I can not dig 1 metre deep in this rubble/clay soil. would something like 400 deep x 400 wide suitable for my plan? I can not put the timber building on the existing concrete slab ( no insulation, no damp prove ) even if there are no cracks. does anybody have an idea or experience made with this?