My old garage was too low for my latest vehicle to fit through the door. I decided to do a more tried-and-true design with a simple dome, using rebar and nylon-cement (nylon fishnet and cement).
The floor slopes a little, so I can roll out of the garage before starting up the engine. One avoids breathing some exhaust that way. Also, if the battery is low, I can push start it easily just letting it roll out of the garage.
The roof is treated with a cement-base sealer called Thoroseal, which is then protected by a layer of colored cement. The sealer and color coat can weather away slowly, but they doesn't peal or blister like paint and elastomeric sealers can. I am very happy with the longevity of that system.
I decided not to do an electrical installation, since I have electricity nearby and extension cords. There is a round skylight, a recycled glass table top.
I decided to not put any doors on the garage, but I did set gate hinges into the sides of the door opening in case I change my mind in the future.
Step 1: The Old Garage -- demolition
The old garage was made for a lower vehicle than my present one. It was an experiment, using "nylon-cement" (nylon fishnet and cement) to make a roof without using iron rebar. The fishnet was stretched out like a tent. Peaks were formed where the tent poles were. After plastering the fishnet, the poles were removed.
Cement is good under compression and bad under tension. A dome is all in compression. This old roof had a complex form, like an egg crate, that was not as simple to visualize. There were complex curves everywhere, but cross-sections sometimes resulted in lines that arched upward (resulting in compression) and sometimes downward (resulting in tension). Any point on the surface was often a result of both factors.
Just like an egg's membrane can hold all the broken pieces of eggshell together, I imagined the fishnet would be strong enough to hold the form together in case of cracking. It did. There were a lot of big cracks, though. Since my present vehicle didn't fit in the door anyway, I decided to scrap the old garage and build a new one.
In order to drop the roof, I broke the cement all around the top edge of the walls. It didn't fall until I went back around and cut all the fishnet with a machete.