After a garage fire on April 6, 1996--i built my own 24 x 36 garage. Having it built for the same money (insurance payment), 24 x 24 would have been a stretch. Learned a lot, but there are things i could have done better. That said, it's still standing and solid. Original shingles were on it until last spring (March 2025), when a tree fell on the roof.
Now...i'm 70 instead of 40. Two years ago i had an itch to build a worksop (or add on to an end of the garage). 12 x 16 would have made a nice turning shop, and cheap to heat. Materials to stick-build a 12 x 16 shell on a framed floor (sitting on an existing concrete pad) compared to having the same size pre-built shed dropped on the same spot were considered. I'm not seeing my notes, but the material cost was 85% to 90% of getting a factory-built shell. Now...i'd have had nominal 2 x 4 studs instead of 2 x 3, but otherwise pretty similar build. Not enough savings to build my own, and my knees would have hated me!! In any case, i never went through with adding the turning shop, which is okay. But the exercise in pricing was enlightening. The bulk-buy materials coupled with assembly line construction made for a pretty cost-effective building--and i've been in some that are around 15 years old and show no signs of age. For smaller buildings, it's hard to beat.
Good advice on checking codes as well. For my area (rural), the county allows one detached garage and one shed (up to 144 sq ft). Second shed requires a zoning variance and hearings, but an "addition" to the garage (think new wall attached to garage with a small passage) is a fairly simple building permit!! Knowing what you can do in advance helps weed out some options so you're not wasting time on those.