Great concept if you can put together the systems to make it a profitable reality. I researched adding an ADU to a rental property I had in Oceanside, but at the end, although it would have made extra income on the rental side, it would have taken too long to recoup my expenses when I sold it because it just didn't add enough value to the SFH from an Appraiser's point-of-view. view. Of course I was planning on Stick building it and not importing it from China, which could be a whole other ballgame.
There is a contractor here in San Diego that specializes in ADUs. Maybe on BiggerPockets, I don't remember . You should connect with them. I'm sure they could help you navigate all the different regulations of all the different cities. As mentioned earlier, each city has its own regulations. Some, like Oceanside, have just adjusted an approved their new guidelines to fall in line with the new state guidelines, others are still in the progress.