Most of California, even the densest regions like the city of San Francisco, are covered in a sea of single-family houses. This is primarily because in the 1970's, all other kinds of housing were banned. See, for instance, the map at:

http://sfzoning.deapthoughts.com/

If we allowed four-story apartment buildings again like we used to, the state's population could easily double in size without even having to raise a single height limit anywhere.

(However, I should mention that I, for one, think six-story apartments are where it's at.)