Late to this conversation!
My basic advice would be to do what you're suggesting, keep the room open and big. Those low ceiling are gonna cause problems, I fear.
If money were no major issue (ha!), I'd think about building the loft to be just partial, so that the garage would be full height, for at least some of the space. Put the control room upstairs, but use the garage for tracking drums, etc.
Low ceilings over a drum kit just make things difficult, unless you're going to install some serious absorption and go for a 70's dead drum sound.
I've recorded all over the place and my favorite recordings come from big open warehouse type spaces, usually untreated, just concrete walls, currogated metal roof, cement floor, maybe with some industrial carpet.
The BBC I think it was came to the conclusion that no room smaller than 1500 cubic feet was usable for recording. I tend to agree, especially with loud sources like drumkits.
Have fun, use your ears. I think studio design is 95% hype to get clients to spend more. There are a few situations where a well treated room is very important. ADR for film, soundtrack work involving live orchestra, etc.
But for rock music? Move the mics, throw up a few packing blankets if you need it. I've worked in every type of space and never has the room stopped us from making a good recording. By that, I mean, I don't believe that a 'better' room would have made a material difference in the final product.
Have fun!
