I'm planning to build a 12x10' habitable/insulated outbuilding after the rainy season and I'm thinking about foundation options.
I'm in the San Francisco Bay Area so temperatures are mild and there's no danger of frost. There are of course seismic concerns though. I am about a mile from the ocean so the water table is fairly high
First you need to check with the building inspector, they can discuss code compliant foundation options at length with you. In general, foundation possibilities include conventional concrete footings with a floating slab, post-tensioned concrete slab, concrete piers with grade beams, wooden foundation (may not meet code in your area), concrete block foundation (probably requires reinforcing due to seismic issues), and piling.