I built my shop on the slab of my old house, which was totalled out in a tornado in 2003. The slab is 63' x 38, which is an odd size, granted; but utilizing that saved my nearly $20K in concrete (at '03 prices, double that now). Plus, where my old patio was in front, became a carport/patio, which is 12' x 24'. The building is angle iron, with steel corrugated walls and foam insulation. For what you are wanting, I'd go with a 30'x40' at a minimum. After I had the shop built, I was still having the new house built adjacent to that, so I had a 34' fifth wheel TT, four cars, two tractors, and a lawn tractor inside. Plus tools and parts. Oh, and I had a bathroom built where the old one stood, so that is a 3/4-bath. And I have two BIG shelving units in one corner for all my heavy/big parts. No lift, even after nearly 18 years. That may change very soon.
I can park my '66 Imperial and my '66 Coronet end-to-end, with the roll-ups at each end, with one inch between the car bumpers and the doors, and about four inches to squueze myself through the car bumpers, parked nose-to-nose. The third roll-up is powered, and I can get five smaller cars, or four larger cars or pickups, (along with the tractor and mower) in that space and have decent room to work and manuever.
My Dad and I did the basic wiring, with four 220V circuits and several 110V circuits. I still need more lighting! I have two overhead doors in front and one in back (one has a power opener), two windows and two walk-in doors. Ceiling height is 14', with 12' doors. We utilized bits from the old house where we could, so one window, the bathroom door, and the 200A breaker box were all used in the shop build, as well as my heavy industrial shelving. It is NOT air conditioned or heated, so I can't simply walk out anytime to a 60* workspace, like some here have. Two BIG floor fans for the Summer, and one big propane patio heater for Winter. It is what it is.
Plan your shop for what you do now and for anything you can figure the future holds...including resale value added to the property.
Total cost for the building and doors was $33K. Wiring materials was another $3K. The bathroom build was $5K. So, $41K in 2003. In 2021 dollars (not including the slab!) is $60K. Add concrete, you are another $30K min to build my shop again.
I don't plan on moving anytime soon.
