Southern California underground 120 square foot guest room on 7 percent slope

I want a cool room for summer desert heat and winter night frosts 20F. Clay soil, no trees yet. There is a road at the highest end of 2 acre property. There the slope is 7 percent for 6 yards where I thought digging the room.The well is there also on the highest part near the road. A drive way has to be made with dirt from the excavation, I thought.
Any suggestions for a roof. Dirt is heavy. I like fog catching (which is there pretty often.)