Here are some highlights and pitfalls:
I'll start with me. I am not an engineer, or an architect, or a famous studio designer. I learned acoustics and isolation over the last 20 years from my father-in-law, (an architect/engineer with a PhD and acoustics background) and by reading every book I could find on the subject (his private library included). I practiced on a project studio I built in my own basement in 1996 - and it really worked!

One of the most significant things I learned while building this studio is that you must be able to compromise. If you are building (or involved in) a studio, you want it to be this perfect thing - it won't be. I had to learn to compromise some, and to compromise on the right things. Sometimes you compromise to save the budget, sometimes you compromise to save your sanity...and sometimes to save your marriage.

A big problem with constructing a sound studio is that seemingly minor details can have a major impact in the performance of the rooms. My mantra for the construction crew was:
"Performance is more important than appearance".
I started saying this because I found that construction crews are used to doing things a certain way, and they concentrate on it looking good and being easy, and not necessarily in that order. I wanted them to concentrate on doing it right acoustically. How it looked was secondary; how easy it was didn't matter (by the way, NOTHING about it was easy).

Design
-Rooms to fit in the hole.
The idea for the studio kind of happened by accident. I had a project studio at home and I was building a new commercial building as both a real estate venture and to house our software consulting company. In an early discussion with the General Contractor (Roschon Corp.) and the Architect (Tim McCoy), I asked how big a deal it would be to add a storage area below grade (add a basement, in English). It was fairly expensive to dig the hole in the first place, but not much more expensive for it to be big. As it turned out it in our building, 4000-sq. ft. was only about twice as expensive as 500-sq. ft.! So, the wheels began to turn and pretty soon I was trying to fit in a recording studio in this hole in the ground.

Since I was riding the edge of the WAF (Wife Acceptance Factor) and the already large building budget, I couldn't afford to go all out and hire designers and consultants to "take care" of my project for me. Also, the economics of Music/Video/Film in Minnesota do not support building a sound studio with a huge overhead. This was DIY on a major scale! I'm here to tell you, you can do it. It will be difficult and you will make mistakes, but it can be done without going the mega-buck route.
(I know I will probably be flamed by people who believe that you can't do it yourself -but I'll bet most of them have never done this, or had a large pile of "Other Peoples' Money".)

O.K. - We've got this hole in the ground and we start compromising right away. This level would need two stairways, two bathrooms, an elevator (w/elevator equip room), a fire riser room, and an equipment room (electrical, mechanical) to meet code - then I could put in the studio part...
I wanted a Control room, a Tracking (or live) room, a couple Isolation booths, and a Client area/"Green room".
The room sizes were a compromise, and since I wasn't hiring an acoustic consultant to oversee the project, I decided on rectangular rooms because it is easier (possible!) to predict the resonance modes of rectangular spaces using fairly simple calculations. Controlling those resonances is another matter, so I dimensioned the rooms very carefully within the space I had to work with. I ended up with 10'1" finished ceilings as highest I could (afford to) go with the best modal distribution. When we were building our building, each foot of height added to the below grade ceiling would add about $50,000. The 10'1" required a minimum of 12'4" bottom of deck height -already fairly high. In Minnesota, this necessitated a hole over 17 feet deep!