An underground house needn’t involve a hill. I hired a backhoe/frontend-loader operator to dig a 50 by 30 foot hole 5 foot deep at one end and 6 at the other, poured a cement floor with tall steel eye-beams to support a roof one third flat and a third at each end slanted to just above the ground, overlaid the sides with rebar and chicken-wire, had “shotcrete” (cement with fiberglass mixed in) sprayed over that and the side-walls under the roof, and presto, had a sorta house which has lasted me for 20 years. The exit ramp for the heavy equipment that dug the hole remains, going to a large front door. There are a couple of windows and sky-lights. It’s amazingly temperature-controlled, there’s a couple of second floor areas, a large one in the middle and a small one (about 10 by 20 feet) at one corner. A ledge around the top of the hole, so at ground-level, offers shelving. As I prefer life with my family in Thailand, and they don’t speak English, the longest I’ve stayed there is a couple months at a time. Usually it’s been only a couple weeks per year. But now a friend has been in residence for over three years and likes it quite a lot. Tried to sell the place; that didn't go so well...