Someone at a dinner party tells you they are splitting their lot and building four units. Someone else says their architect told them the ADU is the only realistic play. You go home and search, and every article treats these as a choice between two doors. They are not two doors. The state's own guidance says the two laws are complementary — and then draws one hard ceiling and one trapdoor.
The short answer: SB 9 and state ADU law stack, but never past four units on a single lot in any combination of primary units, ADUs and junior ADUs (ADU Handbook, SB 9). The one arrangement that costs you: splitting the lot and building two primary units on each half supersedes state ADU law entirely, leaving neither lot entitled to an ADU.
Key numbers
| Scenario | What you may get | Source |
|---|---|---|
| Overall ceiling under SB 9 | 4 units per lot, any combination of primary units, ADUs and junior ADUs | ADU Handbook, SB 9 |
| No lot split | A second primary unit under SB 9 plus up to two ADUs under state ADU law | ADU Handbook, SB 9 |
| Lot split taken | Up to 2 units per resulting lot — e.g. a primary unit plus an ADU or junior ADU | ADU Handbook, SB 9 |
| Lot split and two primary units on each lot | State ADU law is superseded — no ADU or junior ADU on either lot | ADU Handbook, SB 9 |
| Lot with an existing multifamily building | Up to 8 detached ADUs, not exceeding the number of existing units | ADU Handbook, SB 1211 |
The ceiling
Start with the sentence that governs every combination:
"In no case does SB 9 require a local agency to allow more than four units on a single lot, in any combination of primary units and ADUs or JADUs." (ADU Handbook, SB 9)
Four is the number, and it counts everything. A junior ADU — a small unit created within the walls of an existing single-family house — counts against it just as a full second house does. So does an ADU. So does a second primary unit.
Notice the exact wording: SB 9 does not require a local agency to allow more than four. That is a floor on what the city must permit, not a ban on a city choosing to allow more. Some jurisdictions are more generous than the state minimum; none may be stingier.
The three combinations
You do not split the lot
This is the path most homeowners actually want, because subdividing brings surveying, new legal descriptions, separate utilities and a much longer process.
Where no lot split has occurred, the lot is eligible to receive a second primary unit under SB 9, and to receive up to two ADUs under state ADU law. One original house, one new primary unit, two ADUs: four units, no subdivision, and the whole property stays under one deed.
You split the lot and build modestly on each
Where a lot split occurs, each resulting lot is eligible for up to two units — the handbook's own example is a primary unit plus an ADU or a junior ADU on each.
Two lots at two units each also reaches four, but you now own two separate parcels, each of which can be sold, financed and insured on its own. That is the real reason people take the split, and it is a different goal than maximizing unit count.
You split the lot and build two primary units on each half
Here is the trapdoor. Where a lot owner both splits a lot and builds two primary units on each resulting lot under SB 9, state ADU law is superseded — neither lot is entitled to an ADU or a junior ADU.
You still land at four units. But you have spent your ADU rights to get there, and you cannot later add a junior ADU inside one of those houses. In every other case, state ADU law applies on top, up to the four-unit cap.
That asymmetry is the whole reason to sequence the decision deliberately rather than approving each step as it comes up.
Apartment lots follow a different track
If your property already has a multifamily building on it, the SB 9 framing above is mostly beside the point — SB 9 is aimed at single-family lots, and ADU law has its own, more generous provisions for multifamily parcels.
A 2024 change authorizes up to eight detached ADUs on a lot with an existing multifamily dwelling, provided the number of ADUs does not exceed the number of existing units on the lot (ADU Handbook, SB 1211). It also settled a common fight: uncovered, off-street parking spaces demolished in conjunction with building an ADU do not need to be replaced.
The units created under that multifamily provision come with a strong protection attached. A local agency may not impose development or design standards on them — local standards or standards drawn from state ADU law — that are not specifically listed in the governing statute, and the state names the usual suspects explicitly: parking, height, setbacks, lot size, open space, floor area ratio (ADU Handbook, 66323 Units). Two conditions ride along: rentals must be for terms longer than 30 days, and fire sprinklers are not required for the ADU if they are not required for the primary residence — nor may building the ADU trigger sprinklers in the existing multifamily building.
One more useful allowance for conversions: an ADU created within the existing space of an accessory structure may be expanded beyond that structure's dimensions by up to 150 square feet, solely to accommodate getting in and out — a staircase to a second-story unit, for example. Junior ADUs get no equivalent: a junior ADU must be created within the walls of a single-family residence, not within an accessory structure, and may not be expanded beyond the existing dwelling's dimensions.
Parking is where the ADU path usually wins
One practical reason the ADU route beats a second primary unit on tight lots: state law bars a city from imposing any parking standard for an ADU in six listed situations, including where the unit is within a half mile walking distance of public transit, is within a historic district, is part of the proposed or existing primary residence or an accessory structure, where on-street permits are required but not offered to the ADU's occupant, where a car share vehicle is within one block, or where the ADU application is submitted together with an application for a new dwelling on the same lot (§ 66322).
That same section also bars a city from denying an ADU permit over the correction of nonconforming zoning conditions, building code violations or unpermitted structures that do not threaten health and safety and are not affected by the ADU construction — a common way older properties used to get stuck.
A warning about the name
A different bill also called SB 9 — Chapter 510, Statutes of 2025 — amended state ADU law regarding how local jurisdictions submit ordinances to the state, effective January 1, 2026. Among other things, it clarifies that where a local agency misses the deadline to submit an adopted ADU ordinance, or fails to respond to the state's findings in time, that ordinance is null and void and the agency must apply state ADU law when permitting ADUs (ADU Handbook Addendum, SB 9).
That is not the lot-split law. If a search result or a planner mentions "SB 9" and the subject is ordinance submission deadlines rather than duplexes and lot splits, you are looking at the newer bill.
What this article does not cover
This explains how the two laws combine. It does not explain whether your lot qualifies for SB 9 in the first place — eligibility criteria, minimum lot size, how a split must be proportioned, restrictions where tenants have lived recently or where the housing is rent-restricted, and any affidavit requirements are all outside the sections cited here.
Before committing to a path, check eligibility with your city's planning department, and — because the sequencing matters — decide the whole build-out before you file the first application, not after. If you believe a city is applying these laws incorrectly, the state maintains a complaint process; its own SB 9 fact sheet is the starting point.
Sources
- 2025 California ADU Handbook — Senate Bill 9 (SB 9), Chapter 162, Statutes of 2021
- 2025 California ADU Handbook — SB 1211
- 2025 California ADU Handbook — 66323 Units
- ADU Handbook Addendum — Senate Bill (SB) 9, Chapter 510, Statutes of 2025
- Government Code § 66322 — ADU parking standards and permit denials
