Building a house

Professional room treatment begins with design. It is almost silly to give advice however because costs escalate so quickly. Just the advice to have plenty of outlets, which seems solid enough, but every hole in the wall is a sound portal you need to handle. It is not just an outlet, and everything is like that. The door is not just a door, it is a sound portal or acoustic surface. Windows? Gosh I hope you don't have windows but if you do- sound portal, acoustic treatment. On and on.

It will help to have some kind of budget, and realistic expectations. Also I know everyone says the room is so important, and it is. But never forget the room is really just one more component in a system in which every component matters. And quite honestly, they all matter about the same. Also if the room will be in a house, well then it is just another part of the house. No one even including Jay Leno would put a $5M garage on a $.5M home. Well Leno might. But no one else. And so as always, a sense of proportion will serve you well.

Had a house built and made an effort to get a first class sound room.  Lath & Plaster walls & ceiling. Soundproofing inside the walls and an exterior weight door to minimize disturbing the wife.  Dimensions not especially large (is it12'X11X8'?) but I'm a guy who prefers tone quality & soundstaging over bass & deafness. I'm mainly a classical guy. Gorgeous Eucalyptus bare floor sourced from local wood. Double-glazed windows. No acoustic treatment other than a couple thousand LPs. What can I say? I love the sound I'm getting. There's a picture (edcyn's system) in this crazy website's picture ghetto.

Check out my house of stereo system. It probably contains most of the info you are looking for.

Room is the most important piece of the audio puzzle. Go out to expert sites like Cardas, Acoustic Fields, and read Jim Smith’s get better sound book. Room dimensions are forever so get them right in the beginning.
Pull multiple 20amp dedicated circuits to the areas you will be setting up your equipment (read job m smith’s book on this too).
Use wired Ethernet for your audio equipment and make sure you have good access to 5ghz wifi to use your ipad/iPhone to control your music app like Roon or others.

Maybe  a bit more than  you need...maybe not.
And a bit of a different problem
I'm an architect doing larger scale projects, namely Educational. Often the program involves a Music Room....an instructional music class for instruments. Sometimes to listen to recorded music  as well, but not always.
This is different from a home in that has to put up with a certain level of use that you won't experience.
The construction is meant to isolate the space from other adjacent spaces. I have  done suspended and isolated floor systems, as much to isolate it from adjacent spaces but also  from  the resonant nature of the structure.  Ceilings also are a special consideration with very specific suspension methods.
At the front of all of this are the room proportions