I built a shop some time ago, I was lucky to have been working in building trades at the time. I never got a building permit but one friend was a foreman who had done many similar projects and new all the proper way of making sure it was built to code in case for some reason an inspector showed up. The first thing we did was excavate the area for the shop down to hard pan or equivalent material, removing all organic material. We made the excavation about 4ft. larger than the shop dimensions all the way around . Then imported pit run and backfilled and compacted in one foot layers to achieve desired final prep grade for slab. To avoid having to do pony walls after pouring the slab we dug 1 foot thickenings around the perimeter to the dimensions where the the shop walls would sit , this was being done to strenghten the structure weight bearing walls. Next we made sure it would be square and used batter board to measure from, being square is very important for when forming and framing starts, if not square it is a night mare to build. We used 2"x12 to build the form where walls would sit. The top of the 2"x12's was installed 4 inchs above the slab grade to allow for a 4 inch concrete slab, used transit to level top of forms. Next we installed the rebar in a 1 foot grid, bit of an overkill with the grid spacing but better than under build. So at the perimeter of the shop you wind up with a 8 inch thickening footing built in. I had a friend who came and poured with crew and my help and, power troweled finished to a hard chip resistent glaze which is very sweepable which makes it easy to keep clean.
