California passed a number of laws protecting landowners’ ability to build ADUs, and the San Diego council super-charged those:

https://calmatters.org/housing/2023/11/adu-san-diego/

Tons of San Diego houses have a ton of land thanks to the mid-20th century lawn fetish back when everyone was pretending that there was enough water so there are a lot of places where someone can turn some dead grass into as many as 5 ADUs.