Hi. My new extension steps down from the current floor level of the existing house to a lower level. My question is - once I have worked out how far below the existing floor level the new floor level should be, how much more will I then have to dig out to allow for the construction of the floor slab. I think I'm right in saying that the slab comprises, compacted hardcore, sand binding, insulation, DPM, concrete slab and screed. Is there set thicknesses for each of these parts? Many thanks