mine is 30' x 20' and 13' high for a 2 post lift.
floor is about a foot deep concrete all over and part of it is reinforced with metal for lift.
it took LOADS of hardcore to build up the ground as its on a hill. its blocks harled at the bottom, then tin sheets for sides and roof, in a H beam (think thats what there called) frame, and wood to screw the sheets too. i did nothing myself and its was 8 n a bit grand.
thats as just for shed with 2 doors, 1 personell door and one 10' x 15' door.
had to do all electrics w airlines etc. myself