Just a few points from experience.
You will need more space than you might think. There are things that require space that are not areas that you can use such as, proper low frequency treatment that will consume space in many directions, sound locks etc. Even the HVAC ducts will require special baffles.
I know of one built out of an airport hanger that could hold a large Dougles cargo plane, and when finished, was much smaller inside than the way it looked outside.
A studio acoustic consultant in the planning stage is well worth the investment, perhaps there are plans available for a (ground up) small studio already.
Less glass, less problems, and expense.
Install your snakes in a way that will allow easy removal. Be carefull to use power from the same rails of your power box, to prevent ground or polarity differences in various areas of the studio, between tracking room and main sound room etc.
Hope this helps a little,
and best of luck! It took me over a year to build a slab-up studio once, I had no regrets.
LTL