The State of California allows “accessory dwelling units” on residential properties. The implementation can vary from city-to-city, but this basically allows a small apartment-type unit to be either attached to the main residence or detached from the main residence. Recent changes to the ordinance even allow an ADU that is attached to the residence IN ADDITION TO a detached ADU. An additional parking space may be required, depending on proximity to public transportation stops.
For some ideas about how to get past initial roadblocks to A.D.U. design and access to a Pre-Design Primer, check out this page.