IIRC planning is all down to floor area etc. Built my 17' x 24' garage 3 years ago, needed planning and building regs. ££££'s!

Foundation trench will need to be AT LEAST 1 metre deep depending on sub soil etc and then have a min. of 8" of concrete poured in.

Ask "righthooker" (Clive) for some advice, he's a builder.

Not the sort of thing you can do in a weekend mate, the amount of earth you'll remove doing the founds will be very surprising, and skips aint cheap anymore!