SB 9 Design in Los Angeles County

SB 9 lets the owner of a single-family lot in California add a second primary dwelling, and in many cases split the lot in two, through ministerial review: no hearing and no discretionary findings. It has two separate parts. A two-unit development under Government Code section 65852.21 adds a second home on the existing lot. An urban lot split under section 66411.7 divides the lot, with up to two units on each half. The first does not depend on the second. An owner can keep the lot whole and build the second home.

Design 1 Studio has designed more than 1,500 projects across Los Angeles County and Orange County since 2015, and SB 9 work has been part of that since the law took effect in January 2022. Our work is residential, up to four units and two stories.

Cities apply their objective standards to SB 9 projects: height, setbacks, floor area, parking and design rules written as numbers. The statute caps side and rear setbacks at four feet and requires no setback for an existing structure that stays in place. Since January 1, 2025, Senate Bill 450 has barred any standard that would physically keep an owner from building two units of at least 800 square feet each, and it requires a decision within 60 days of a complete application. The charter cities’ lawsuit over SB 9 has not produced a final ruling, so we check the current status before relying on SB 9 in a charter city.

What a city cannot do is review an SB 9 application as something it is not.

In July 2025, Walnut planning staff wrote that because the owner was not pursuing a lot split, the property was “only eligible for ADUs/JADUs” and “the opportunity for two primary dwellings is not permissible,” after discussion with the City Attorney. We answered in writing: a two-unit development under section 65852.21(a) does not depend on a lot split, and a primary dwelling under that section is not an ADU. We supplied the statute language and HCD’s SB 9 fact sheet on primary units on a lot that has not been split, and asked the city to evaluate the proposal under the route the statute describes. Later that month the planner wrote that the owner could proceed under SB 9, the first project of its kind in the city. The full record is in our case studies.

• The lot read comes first: zone, overlays, historic status, fire hazard zone, easements, and the lot’s permit history. SB 9 excludes some lots, among them historic districts and landmarks, certain hazard zones, and housing a tenant has occupied in the last three years, so eligibility is settled before anything is drawn.
• The unit strategy follows the lot and the owner’s goal: two primary units on the whole lot, a lot split with up to two on each side, or either of those with an ADU where the city’s combination rules allow it.
• The city’s SB 9 ordinance is checked line by line against the statute, and every departure is answered with the section that controls before the plans go in.
• A complete plan set coordinated with a licensed structural engineer, managed through plan check to permit issuance. When a city issues a correction the statute does not support, we answer it in writing.

Do I have to split my lot to use SB 9?

No. A two-unit development under section 65852.21 stands on its own. The urban lot split under section 66411.7 is a second, separate tool.

Can my city deny an SB 9 project?

Only on the narrow grounds the statute sets: an ineligible lot, or a written finding by the building official, based on a preponderance of the evidence, that the project would have a specific, adverse impact on public health and safety with no feasible way to mitigate it. Design preferences are not grounds.

Does SB 9 apply in charter cities?

Senate Bill 450 states that it does, as a matter of statewide concern. The lawsuit brought by four charter cities has not produced a final ruling, so we confirm the current status for a charter city before relying on it.

Can I add an ADU as well?

Often, within limits that depend on whether the lot was split and how the city applies the combination rules. That unit count is settled in the lot read, because it is where SB 9 projects most often go wrong.