I would check the floor joists size, if they are spanning the 12 foot width they should be 2x10 doug fir or better. If not, the joist could be changed to silent floor I-joists or sistered them up to beef up what ever is there. The subfloor should be 3/4" plywood glued to the joists.
The average height of the room sounds like 6 feet, so here possibly you could dormer one side of the roof for a better average height. Raising the entire roof is another, though, more expensive option.
Other construction items to consider are electric(dedicated outlets), AC, Heat, insulation, windows, skylights, doors, rough & finished carpentry, siding, roofing, wall and ceiling sheetrock and acoustic treatments.
I know it's alot to think about and pay for, but for the system you are thinking about, it's best to get the room right before you start moving in with expensive equipment.
One other heads up is that the room dimensions are problematic 16' x 12' x 8'?(average 6'). Room modes are stacking up so that base response will suffer. I would check out the site at Audio Asylum on Room Acoustics and see what Richard(Rives)and the others have to say.
Given all of this, I agree with Lbex2 that its probably best to set speakers up on the long wall. If the speakers are 3 feet off the back wall, 4 feet off the side walls and 7 feet apart, they will be approximately 7-8 feet from the listening position (assuming your listening seat is pinned to the wall). My point is make sure the speakers you select can integrate well in that distance. Best of luck with you plans. Ken