Building a house

In the design phase and planning on a dedicated listening room. Any advice on its construction, lessons learned?
Lots of good suggestions here. I incorporated many/most of them when I built a 38x28 addition on the end of our home. Concrete floor with extra foundation against earthquakes (it IS California!). Double offset stud walls using 2x6s, soundproofing, double door into room from remainder of house with one door constructed with internal sound deadening, etc. The only suggestion I would add that I did not see above is to avoid a flat ceiling (maybe I missed it); mine ranges between 8 and 10 feet peaking off center. Sonics are superb and my son's blues band uses it for practice and recording. Good luck.
It dawned on me that if you are setting up a permanent home-theater (i.e. will always be in same location), you should probably put some electrical outlets in the floor where your seating will be. Modern HT chairs have USB outlets, motors and regular outlets built in. and you don't want wires to trip over if you plug them into wall outlets.