First Post! VERRRRRY preliminary question...

Assuming I had a small amount of space to work with, i.e. say 12 x 20 feet or so in a garage, what is the consensus on one bigger room vs two small rooms?

So what size should a control room be? These days people are building bigger and bigger control rooms because so much happens in the control room now. Often the bass player and keyboard player actually sit in the control room. I often have the vocalist singing in the control room while the bed tracks go down, so a good size control room is a good idea. So many people think that you need a big studio and end up putting a poky little room at the end and call it a control room. I suggest that a control room ideally should be at least 6m x 5m with a minimum ceiling height of 2.4m. This size room means a good sized working area with space for the musos, friends and hangers on. Also because you don't want rear reflections to interfere it is better to start with a longer front to back dimension than the side to side dimension. i.e. 6m x 5m. The studio, on the other hand, requires a different set of considerations. The first to consider is do you want just one room or more? The trend nowadays is for more than one studio so the you can get isolation between players and different acoustics for each room. The acoustics you want for a live drum sound is totally different than you want for a vocal for example. I recommend at least 2 rooms. You can put drums in one, guitars in the other, keys , vocals and bass (DI) in the control room or maybe the band in one and the vocalist in the other. Getting separation between guitars and drums is usually OK in a good sized room but keeping them out of the vocal track is hard if they are all in the same room. - John Sayers

IMPORTANT FOLLOW-UP: What would be a minimum space for isolating a loud guitar amp without making the acoustics just terrible? 4x4? What is the minimum 2nd room size I should be considering?

For an instrument to sound good you must hear a balance of its high and low frequencies. If your room is too dead in the high frequencies the instrument sounds lifeless and tubby, whereas if it's too live in the high frequencies the instrument sounds distant and has no presence. To balance a room you must have a true balance of the high frequencies and the low frequencies with the right amount of deadness to liveness. - John Sayers - SAE website

"Be careful, a dead square booth will have low end issues (you'll hear the issues on your mic.)

Modally, with a 6'4" ceiling you'd do well with wall dimensions (inside) of 5 and 7 feet. That combination shows no modal problems at all. This means that vocals in such a room shouldn't be "colored" by the room. You would want to keep the upper half, including the ceiling, absorbed with such as rigid fiberglass insulation boards, covered with a light plastic film (they shed) and fire proofed cloth (they're ugly) - you'd likely get better vocal sounds with the singer seated rather than standing, since otherwise they will be nearly at the ceiling. Being that close to a boundary (especially if it isn't absorbed) will cause comb filtering due to early reflections.

IMPORTANT FOLLOW-UP: What would be a minimum space for isolating a loud guitar amp without making the acoustics just terrible? 4x4? What is the minimum 2nd room size I should be considering?

I was wondering given the similar dimensions, do you have a rough sketchup of what you ended up going with? It would be really useful to have a template to start playing with ideas on.