I am planning an extension at the back of my house and from research here I intend to go down the insulated slab route with no screed, so the UFH would be in the concrete slab and then the slab is the finished floor level. The existing first floor has a concrete slab (I suspect it’s uninsulated as was built in 1950s) which I plan to dig up and then lay using same lay up as extension.   I plan to lay the slabs separately as my intention is to live in the old house whilst the extension i