It kind of depends on what you're doing. Two of my friends have done the 'double decker garage' idea, which is to put the machine/wood shop on the ground floor

It kind of depends on what you're doing. Two of my friends have done the 'double decker garage' idea, which is to put the machine/wood shop on the ground floor and the office/electronics on the upper floor. My wife still believes that cars go in garages so I have yet to claim the full downstairs :-). Things I find most useful in my space are the electronics bench, the computer "station" (3 monitors on a desk), and the "small fab" bench (3D printer, mini-mill (Sherline), various holding jigs). I've got parts and such upstairs but they could reasonably be anywhere. My activities are primarily small circuit design/build/test, small mechanisms (robot) design/build/test, and algorithm research. These things don't have a lot of consumables per hour of engagement so they can go for a long time on relatively low additional investment. For larger fabrication having a garage door and a concrete floor are essential. But the kinds of things you want to work on and create is pretty intensely personal usually. A friend who likes to build airplanes built an assembly hanger that was roughly 40' x 60'. I have never found the space I have above the two car garage (and the same size square feet wise) to be an issue. What I have done over the years is look at the projects on the queue, and the things I want to learn, and then built to support those efforts.