Hi lceverge - I was looking to build off a reinforced concrete slab thickened at the edges - so a bit like a raft foundation. I've just starting preping the site and have found it is overlain by about 300mm of topsoil onto quite a soft sandy clay soil so may have to go thicker with the concrete - say 150mm thickened at the edges. That's a good point about insulation - it will only get occasional use so ust needs to be able to be warmed / cooled when in use.