heh. well, we haven’t really even started (hopefully breaking ground in july!) but a few things I think I’ve already learned include:
1. if you are in a cold (mostly heating bills) climate lusting after insulation and want to “superinsulate” (namely, R40+ walls, R60+ ceiling) then I highly suggest you skip the double wall approach. Why? Guess what, now you can pick a stock plan with 2×6 walls (keep it to 4-corners if you can) and add 4″ of rigid foam to the outside. And it helps you in 2 ways: 1) dew-point/warming the framing/condensation, 2) thermal bridging at joists. The main point is you have just saved yourself the misery of designing a custom home and all the time and energy (and wasted months renting, or 2 mortgages) that this involves. Sure finding a stock plan isn’t necessarily easy, but at least there are decent sites with plans from Ross Chapin
if you are going to build a passive house, then ok, this won’t work, but otherwise, heed this advice!
Another great plan is to buy a split-level ranch (that needs new siding and maybe new windows) on the cheap, and hopefully facing N or S. Then, do a deep energy retrofit on it ala Marc Rosenbaum. (see page 38 and on at this PDF)