I have built three detached garages over the years, and a couple of lessons I have learned from each of them. 24 x 40, 24 x 20, and 24 x 45. None were big enough after a couple of years. Make your entry door extra wide and tall, and if possible an exit door of same size. Make sure the floor/pad is extra thick 7"+, and 3-5000 PSI rated with 7/8" or so rebar in the floor. If you go concrete walls, then the foundation/footer needs to be over 18" deep everywhere, and especially if you want to have a lift/hoist or two, then reinforce that floor area before the concrete pour. Make the walls high, at least 12' for lift with room for hood to be opened on vehicle. Have a drain built into the floor in whatever area you plan on assembly or cleaning of parts or engines. Make sure your electrical power source has adequate amperage available(over 200 amps) for multiple units running at once: AC, welder, compressor, etc.. I would also incorporate/build a "safe room" for tornado or emergency if possible with a sink/toilet. In my current shop, I have half of it with a second story floor for storage, and I wish I had built the walls at least 14' high instead of 12'. I keep a vehicle on my lift and park one under it. Give yourself plenty of door opening room between stalls. If you have room at the exit door, give yourself a large apron for parking and pressure washing undercarriage/engine blocks, portable sand blasting, etc. Just some of my wishes for my next build. If you decide to build a floor pit for oil changes, etc, then incorporate a built in floor safe/gun safe. LOL, Joe-JDC