@dsm @pwm The deck is just a deck, I'll go with 1"x6" tongue and groove pine with rigid insulation underneath between joists. I'm going to put it on a bit of a slope with a couple feet clearance under the low side, so that I can fit a grey water & a centrex waste composting system, and have a functional sink & toilet without worrying about contaminating groundwater or digging a septic field.