On June 4, 2015 new zoning rules (written in Oct 2014) will go into effect that make it easier to build small, detached homes on city lots outside the coastal zone.
http://www.sandiego.gov/development-services/pdf/industry/2014/smallotordinance.pdf
Here’s another article on the same topic: http://www.sandiegoreader.com/news/2015/jun/01/stringers-small-lot-subdivision-ordinance/#