Well, just for comparison...
Some years back I designed a small video
studio that we used a number of times for 4 and 5 piece bands.
The studio was about 1000 square feet with a ceiling at about 20 feet. Obviously our needs for video were different than for a live room but my sense is that the space would have provided enough room for iso booths, and general separation.
The control room was about 360 square feet, on a couple of levels with technical equipment on the lower level and a raised area (in our case for graphics and producers but in a sound situation it would have held a client couch.
We also had a "
green room" of about 400 square feet which had a counter with microwave, drinks fridge, etc (which might be enough for your kitchen. Being a TV studio, we also had a dressing/make up room of about 150 square feet--this could be your office I guess.
Besides the above, the air conditioning plant room (quiet enough for studio use) was about another 200 square feet plus compressors outside the building.
Other things to consider:
For our 1000 square foot
studio the fire officer required use to have two exits--we turned the second one into a combined storage area and loading bay. You WILL need storage for cables, mics, stools, acoustic screens etc. etc. and you WILL need easy unloading access for unloading drum kits, keyboards, etc. etc.
You'll probably want a reception area so visitors don't blunder
straight into
the studio area.
In case it helps, from memory our budget for
acoustic treatment (designed by
a professional consultancy) and the super quiet A/C was about 80,000 pounds, say 150,000 dollars. This doesn't include any of the building works for things like floating floors, double skinned sound proof walls, etc. and certainly doesn't include wiring or equipment.
Good luck (deep pockets)!