Building a house

Be careful with the 12 foot ceiling dimension relative to the other two of the rectangular box.  Understand the difference between sound isolation (soundproofing) and acoustics.  Isolation is a science, acoustics an art. Only thing to add about isolation is close up every hole and gap with acoustic sealant and put seals on the door because that’s where the sound will escape. And don’t be surprised if the low frequencies still get out after doing all of the other soundproofing.  Design for flexibility in speaker placement and seating position.  If your room is 12x12x12 change it.  First time post from an architect. 

Room dimensions?
Two options:
1. Golden Ratio - Used in architecture
2. Fibonacci numbers - Used in Nature
Google them...

+1 consonance

Take a lot of pictures during the process, I mean a lot.  Takes very little
effort and you'll be glad you did at some point.  Use a tape measure to show stud locations.  Don't forget the ceiling!  You may need those for hanging acoustic treatments.  Have fun.

Regards,
barts

YEs good point....locate your best equipment in the basement on that solid concrete foundation...best tweak ever is for floors to not vibrate .......finish with dense thin pad and carpet.

Good proportions, solid wood floors, large Persian rug with proper heavy felt underlay, wood paneled walls, rock wall, wall paper or flat paint, crown moulding, acoustic ceiling. And no glazed artwork.

Yes I said acoustic ceiling!
I know a everyone thinks these are out of style...and the soft, asbestos-laden ones are. But the early, concrete-like ones, are not just good, they're phenomenal.

My last house had one of the ceilings as well as all those other features including a rock wall. It was a mid century modern time capsule that I bought from the original owners. It wasn't perfect...but was easily the best sounding room ever I've been in.  Meaning the clearest mid-bass and lower bass I've ever experienced. Super clean. Deep and tall soundstage. Soundstage did lack width. But it was good enough. Excellent high end too. It was not a dead room. It was a room with a lot of diffusion and very little absorption but it worked. It was also 16' x 32'.

There was not a shiny thing in that room. All artwork was tapestry, wood-carved or textured.

I have moved all over the country for work and have taken my stereo with me. I've been fortunate to have some great spaces to listen in.
The best rooms had acoustic ceilings in common.