Outside dimension of the addition is 28' by 20' 6". Ceiling height for the basement is 8', ground floor is 9', second floor is 8' at the walls, with a cathedral or tray ceiling above that. The atrium extends two floors and is entirely open to the staircase and the Great Room, so there are no interior load bearing walls.
I envision the external walls thickness as being simular to 2x6 framing. The roof/ceiling thickness should be typical for a southern/atlanta home. Interior walls I expect will be stick framed, though I am considering steel framing. Note too that there is a significant cantilever, 6', at the back of the roof, covering the balcony.