On my garage they built the floor after completing the walls and roof. I think they should have done it when it was just a couple of bricks high. The result on mine was they got the level wrong for the fit of the garage door.
Hopefully yours will be a roller or your builder knows what to do to get a correct fit.
I had a damp proof layer put in under the concrete but no insulation. Possibly a mistake, but you would need to make sure enough soil is removed so the floor level will not be too high.
