California Senate Bill 9 (signed 2021, in effect January 1, 2022) — 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 ADU law, and the two interact. On a parcel created by an SB-9 lot split, a city does not have to permit more than two units of any kind, ADUs and JADUs included. The two paths also 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.