First let me say I build on a sheet of drywall or building jig sitting on a card table, and have for years, in a spare bedroom or in my den. I'm inside the Heat/Air Conditioning so that is nice, the bad part is knocking everything down and cleaning up when I stop for the day. I lay the drywall on my garage work bench when not building and most of my most used building tools and paraphernalia are kept in clear plastic tubs. My stock of wood engines, extra are stored in my garage.
That said I wouls start in the garage but entertain several modifications. If you have steel garage doors without insulation an insulation kit is readily available and easy to install. Second depending on construction and cost you could look into a small wall mounted Heat & A/C unit for the garage, my brother found them to be a great solution for his turn of the century home and they weren't that bad price wise. You see several brands advertised on TV but most of the tradition unit manufacturers offer them now. If you are lucky enough to have a window in the garage maybe a window unit would be a viable solution? They come in a variety of sizes and they run for years with little maintenance. A window unit installed correctly could be made to be easily removed and a box fan replace it to exhaust paint fumes for a makeshift paint booth of plastic sheeting or drop cloths. I have seen other builders on this board and others adapt a foldaway engine stand to hold the airplane when painting. You can steal some great ideas cruising building threads!
Mike