There is also a huge regulatory burden to build in much of the bay area, which probably extends to other areas of California. Even when land is available, zoned, and purchased, builders often have a hard time actually getting the cities to sign off. Same for things like in-law units. My girlfriend's parent tried to build on in their backyard, but it turns out just getting the permitting done costs upwards of 80k, about as much as the building the unit.