I have just bought a barn to convert (so expect a lot of requests for advice!), and I havee to build the double-garage first in order to comply with planning regs & bats.
The general yard area (surrounding the barns) is about 1m higher than the field where the garage is to be located. I understand I need to use hardcore as a base under the slab/footings etc, but I was not sure if there was a recommend maximum height of the hardcore & how it should be retatined.
I think that I need to dig a retaining wall (again using hardcore & concrete as a base/footing), which will hole the hardcore in place. I can then lay the slab DPC etc as normal.
You will need to build the outer walls of the garage first thereby forming an enclosure for the hardcore. The depth of hardcore needed (and the foundation depth) will depend ground conditions - the softer the ground the deeper the depth of hardcore needed.
If your garage floor is going to be 1m out of the ground take elwood's advice and form a block and beam floor. The alternative would be to construct 225mm walls and fill with carefully compacted hardcore.