I spent around 15 years in the first room i built so that helped me with the new room,
start with 2X as much money as you think you will need(and then double that amount)
And more square footage than you think you need
one thing you can never have enough of are wire ways that can be used in the future to run new wire ect.....
run extra AC lines for the future
Proper HVAC separate for the control room and recording area, and another one for the lounge, offices
Don't skimp on the soundproofing and isolation
As much bass trapping as you can fit
Larger Machine room than you think you will ever use
A tech room (with a tech ) maybe living quarters for the tech.
storage for all the gear the tech is waiting to fix............
make sure to have a large lounge for all the dudes to hang in
huge soffit mounted speakers
have someone else hang the multiple layers of drywall
and insulation
and caulking
and flooring
and framing
pretty much everything.........
more Mic lines than you think you will need
more headphone mixes (i ran 5) i use 2
no carpet anywhere. use area rugs
use wood floors
use 1/2" and 3/8 " glass (larger toward the control room side)
i will think of some more..................
as far as dimmers use stat-co variable transforms (501B ?)
So far i haven't ran into anything i would have done different
here, thats so far....................