California Senate Bill 9 (2021) — allowed splitting single-family lots into two parcels and constructing a duplex on each, effectively permitting up to 4 units on what was previously a single-family lot.
SB-9 is separate from but complementary to ADU law. A homeowner can do an ADU + JADU under ADU law, OR an SB-9 lot split + duplex on each lot. The two paths have different permit processes and different outcomes for property tax and ownership.
Part of the BackyardADU California ADU Glossary — plain-English definitions of the regulatory, construction, and financing terms you'll encounter during a California ADU project.