I think you might want to post your questions around reddit as well. https://www.reddit.com/r/AccessoryDwellings/ https://www.reddit.com/r/RealEstate/comments/sbyzbz/new_california_sb9_law_and_new_home_build_question/

Well, lord knows we need more housing in the Bay Area, so i think whether you go the ADU route or the SB-9 route will depend entirely on how much you can afford for construction, and, frankly whether you want to be a landlord with multiple residents/units. I think you're going to have to spend some money upfront on design for both scenarios so you can get more accurate estimates for construction. Then you'll have to do a break-even analysis on the rent you can charge over time vs. the construction cost to see if it's worth it.

Civil engineer. Real Estate developer. Start by a chat, maybe a few with city planning.

Personally I would never sell off land and reduce what I own. Yes Building am ADU has costs and there is much to consider and figure out but it’s all still yours.

IMO we have no clue as to the rules where you live so the first step talk to the town plannning to see what is doable