ADU + SB 9: The Lot-Split Frontier

There's one more density law every serious ADU student should know: SB 9, which since 2022 has allowed two things on most single-family lots that were unthinkable before — and which interacts with ADU law in ways that create real strategy decisions.

SB 9's two tools:

1. The duplex provision: convert or build so a single-family lot holds two primary units — ministerially, like ADUs.
2. The urban lot split: divide one single-family lot into two parcels (each ≥40% of the original, minimum 1,200 SF each) — also ministerially. Each new parcel can then hold up to two units.

The theoretical maximum (two units per parcel × two parcels) is four primary dwellings where one house stood. Owner-occupancy applies to lot splits (the applicant must intend to live on one parcel for three years), units and parcels have size rules, and — honestly — cities have implemented SB 9 with widely varying enthusiasm, so local execution matters more here than anywhere else in this course.

How SB 9 and ADU law interact — the strategic part: the laws are separate doors, and you can walk through both, with limits. On an un-split lot, SB 9's duplex can combine with ADU/JADU allowances (state guidance requires cities to allow at least some combos). After a lot split, cities can cap each parcel at two units total — counting ADUs toward that cap. So the combos don't infinitely stack; but the floor is still remarkable: a split lot with a home + ADU on each side = four units on separately-sellable parcels.

That last phrase is the whole game: separately sellable. A lot with a house and two ADUs is one asset with rental income (an ADU generally can't be sold separately — outside AB 1033 opt-in cities). A split lot is two assets — each parcel independently sellable, financeable, and heritable. SB 9's paperwork is heavier (a parcel map, survey work, utility separations, the three-year owner-occupancy intent), but it manufactures a thing California almost never makes anymore: a new legal parcel.

The decision heuristic: want maximum income on land you'll keep → the ADU/JADU stack (13.5) is simpler and faster. Want maximum asset value and exit flexibility on a big lot → price out SB 9, ideally with someone who's executed it in your city. Big corner lots and deep lots with two street exposures are the natural candidates.