Since you asked...
I'm fortunate enough to have two shops on my compound. One is a 24X48 that has a homemade paintbooth and is primarily used for body work and car storage. It was on the property when I moved in. The second one (pictured) was built 9 years ago by my dad, my brother, and myself. Doing most of the work ourselves, it cost around 25K including the hoist and lighting. It was originally designed as a 36X48 but because of a quirk in the set-back codes in my county it had to be reduced to 30X36. There is room to store 3 cars plus have a useful work bay. The tall door end has 18' ceiling and a 2-post hoist. There is just enough room to do an engine swap on a Suburban. The right side has loft above for storage. I converted 2/3 of it into a man cave. It's handy during parties if the house gets too crowded. You'll notice deadbolts, motion lights and an absence of windows. Always keep security in mind :? .
This is a view standing between the hoist posts...
It really looks more crowded than it is...
Upstairs. My wife's friend painted the skyline mural, the furniture was hand-me-down from my parents, $50 craigslist pool table...
I know you said you can't build up but the added square footage is
SOOOOO nice! Plus, the open TGIs work good for hanging things like frames and bodies. One rule is constant; you will never have a big enough shop. No matter how big it is - you'll eventually fill it up. I am currently working on "inventory reduction" in both shops. I'm also planning a move in the future and already have plans in mind for my next shop. These are the things I plan to do different or add to this design:
-a separate "clean room" for engine/transmission assembly and storage.
-an outside "mechanical room" to house the compressor, pressure washer, etc.
-a "dirty area" for fabrication, grinding, cutting, welding and steel storage separated by a curtain.
-heating system (preferably waste oil burning).
-running water...or even better, a bathroom!