“With a stroke of his pen, Gov. Gavin Newsom has officially ended the over 100-year scourge of single-family-only zoning in California.
Single-family-only zoning laws make it illegal to build anything but a single-family home on a particular lot of land. Now (with small exceptions like for fire-prone areas) it is also legal to build duplexes.”
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“While overhauling single-family-only zoning might sound revolutionary, the bills are gentle attempts at increasing density: legalizing duplexes and quadplexes and making it easier to build small apartment buildings that provide up to 10 homes. This doesn’t mean single-family homes are outlawed or can no longer be built, but it provides homeowners the option to convert their homes into duplexes or sell their homes to people who want to do so. Before now, it was illegal for someone to convert their home to a duplex on a lot zoned for single-family zoning. Not anymore.
This isn’t a panacea for housing production. UC Berkeley’s Terner Center for Housing Innovation found that SB 9 (the bill that legalizes duplexes) will “modestly accelerate the addition of new units relative to the status quo.” Other laws that restrict the building of new and more affordable homes are still in effect — in particular, local laws around minimum lot sizes will continue to make it illegal to turn single-family homes into duplexes if the existing lot is too small to subdivide while still adhering to the size regulations.
However, the Terner Center finds that “approximately 700,000 new, market-feasible homes would be enabled under SB 9.” That’s a lot! But because many people won’t want to sell their homes or subdivide them themselves, “only a share of that potential is likely to be developed, particularly in the near term. … As such, while important, the new units unlocked by SB 9 would represent a fraction of the overall supply needed to fully address the state’s housing shortage.””
https://www.vox.com/2021/9/17/22679358/california-newsom-duplex-single-family-zoning