How can I take a detached garage that has plumbing run (meaning water) and electricity and make it useful? It has an enclosed upstairs room that is no good for human activity (ceilings too short except down through the middle).

I am thinking of using that upstairs for a greenhouse in winter if I can install a wall of windows. Specifically, I'd like an "orangerie" with dwarf citrus and other tender trees. I am imagining lining the trees in a row in the tall part then in front (under the eaves) where the windows would be installed, I could put some potted plants and even trellis some beans and peas for the winter. The wall of windows would face south.

How can I tell how much weight the ceiling/floor support has?

How would I heat that area safely during our infrequent freezes?

How would I handle those trees in summer? I can't carry them up and down stairs once they are a certain size.

What else could I use that space for from a prepping/self-reliance perspective?