Ok there is a LOT to discuss here. Since this seems like a new build from the ground up you have a few options to get started. The first is are you going to hire an Acoustical Architect to design the building and space or are you going to do it yourself? I’m assuming yourself since you posted here, and I would recommend you start with finding some books that have been written about this subject. There is far too much to consider to be fully answered here but I will give you some things to think about.
First go read these books:
These 2 books will give you enough information to likely build and treat your space.
These are the for-sures:-The building can be about 700 square feet with 10 foot high walls and a 16 foot high roof peak (max).-It has to be on the more square side of rectangle (i.e. 24’ ish x 28’ ish, 4 corners)-I want a large practice space (main use of the space) with a separate control room and a small storage closet.-Control Room needs to be big enough for a full sized couch and large chair, mini fridge, small desk, and one rack of gear, with enough space to move around comfortably.
700 sq’ or 20 x 35 is a workable size for what you want, but you will likely have many compromises in design to get the best use out of the space. I’d advise not having a mini fridge in the control room as it will add a lot of unwanted background noise.
These are the big variables in my mind:-Optimum sizes for the rooms for sound quality, while keeping in mind the predominant use is practice space-Shape of each room, building layout, and building dimensions to optimize sound prior to treatment
Some of this depends on construction materials, how isolated you need each space, Realize that “optimum room ratios” refer to 0% of the sound leaving the room, if you are not using 8" of concrete for walls, some sound will be lost through the walls (mostly low frequencies) so the optimum room ratios are not a definitive to getting a great sounding space. They are however a great place to start when planning like what you are doing now. In general your every attempt should be made to not have parallel walls in the “live” or “practice” room and try to diffuse the sound as much as possible. The larger the practice space the better. Since the use of the control room you say will be 25% it should be on the smaller side, or even incorporated into the Live room, you can use acoustic panels as dividers to balance out the room response and get the RT60 low enough for quality mixing. Now that I think about your situation a bit more I would definitely use one room for everything, it will save you a lot of money on construction, and you can build your own absorption panels to convert from live to mix space.
Vaulted ceiling, flat ceiling, suspended ceiling tiles, height???-Type of lighting (budget friendly)-Flooring (floor will be painted cement, but can be covered) (I was thinking a few area rugs for budget reasons)-opinions on a vocal booth (worth the real estate and money?)-any other input
No vaulted ceiling, that does nothing but focus the reflections instead of diffuse them. The ceiling height should be as high as you can afford to build it, cubic volume is what you are after. Depending on the design of the space you choose that will determine if you want to use ceiling tiles which will absorb and deaden the room. If you plan to record drums then (depending on height) you will want the room live but diffuse, longer RT60 times would be desirable.
You can build your own portable and movable vocal booth easy enough, removing reflections from vocals for recording is a pretty easy process, the main benfit to a vocal booth is isolation during live tracking, but even this can be done with some panels and proper use of a the rejection plan of a cardoid mic.
Hopefully this gives you something to think about