We are converting part of a single storey stone built barn into living accommodation and we have removed a wooden partition between an old garage and a cow shed. This has left a gap between two concrete slab floors, only one of which is going to be part of the domestic accommodation. We therefore have to build a block wall, where the gap is. The wall will be to the height of the existing walls and will support a wooden floor, to provide a storage area in the roof of the non-domestic part. The barn is built without foundations but on an area of rock. Excavation the gap I have found a cobbled floor on top of large lumps of local stone, closely packed but I have not yet reached bedrock.
How deep should I go with my excavation? Can I put a concrete foundation on top of the layer of stones? How deep must the concrete be?