Probably similar construction to both our house and shed.
For the house, we very carefully dug down a couple of foot (took us to the bottom of the foundation stones, slight scary!), then I think we did fairly typical approach of compacted scalings-->DPM-->6 inches concrete-->Barrier-->UFH pipes buried in 4 inches of screed.
Cheers, not too bad then, I reckon if you can get a heifer through our door a micro-digger will fit.
I think ours will be something we do in the longer term, and something we’d have to get someone experienced to look at re new roof, wall loading etc.
Only thing I would say is to think about what you can do to the building now to make sure it doesn't fall down by the time you get round to it. We lost 3 or 4 outbuildings here as they were just left and nature took its course and pulled them down.
Probably similar construction to both our house and shed.
For the house, we very carefully dug down a couple of foot (took us to the bottom of the foundation stones, slight scary!), then I think we did fairly typical approach of compacted scalings-->DPM-->6 inches concrete-->Barrier-->UFH pipes buried in 4 inches of screed.