Duplexes, Triplexes and Fourplexes

On a lot with more than one home, every building has a legal name, and the name decides the rules: a primary dwelling, an accessory dwelling unit, a junior unit, accessory living quarters, a guest house. A second primary home is allowed where the zone allows two units, or under SB 9 in a single-family zone. An ADU is approved under state ADU law with its own size and setback rules. Accessory living quarters are not a dwelling at all until the law and the permits say they are. Getting the labels right at the start is most of the work; getting them wrong is how a project stalls for a year. Design 1 Studio designs two-to-four-unit projects across Los Angeles County and Orange County, new and converted, up to four units and two stories, coordinated with a licensed structural engineer and managed to permit.

In April 2021 the City of Los Angeles deemed the application incomplete with one sentence: “A second Single Family Dwelling is not allowed on an RA zoned lot.” The choices offered were to remove the second house from the plans or call it accessory living quarters. SB 9 took effect the next January, and the project changed route: a two-unit development, with the accessory living quarters converted into a second primary home and the ADU and garage work reviewed alongside it. Fire access set the construction: NFPA 13D sprinklers, Type V-A construction and a 20-foot driveway built to the H20 standard. The last stretch was paperwork, because the certificate of occupancy could not issue until the dimensions, construction type and use codes agreed across every permit. The owner reported the certificate of occupancy for the second house in July 2025.

The building was damaged in the 1994 earthquake and never finished. In 2019 the owner wanted it legalized as four units and brought to a plan a contractor could build from. The City’s position was that a project this old is a new submission, not a renewal: current drawings, current engineering, an updated geotechnical letter. The design problem was accessibility. In July 2020 the plan checker wrote, “Pls. provide a code justification for not complying with CBC CH 11A Section 1109A,” and in August confirmed that Chapter 11A parking had applied from the start, so the parking layout had to change for an accessible space on a tight site. We asked for Building, Mobility and Planning in one conversation and redesigned the parking and path of travel to comply. Planning still needed the Architectural Review Board’s lighting sheets and consistent deck areas. On March 22, 2021 the City confirmed plan check complete and the permits ready to issue. Later revisions for the stair and roof deck followed in 2024.

Walnut’s first reading was that a lot without an urban lot split was “only eligible for ADUs/JADUs.” We answered with the statute: a two-unit development under Government Code section 65852.21(a) does not depend on a lot split, and a primary dwelling under that section is not an ADU. The City reconsidered and accepted the SB 9 submission, its first. The full record is case study 2. The SB 9 route has its own page.

• The unit inventory first: what each existing structure legally is, from the permits, before anyone says what it will become.
• The route that fits: the zone’s own density, SB 9 in a single-family zone, state ADU law, or a legalization of what is already there.
• The rules that arrive with more than one unit: fire sprinklers and construction type, fire access and driveway width, accessibility under Chapter 11A where it applies, parking, and separate utility services.
• Structural coordination with a licensed engineer, and permit descriptions that match the drawings to the square foot, because the certificate of occupancy is checked against both.
• Plan check managed to issuance, with any correction the code does not support answered in writing.

Can I put a second house on a single-family lot?

In a single-family zone, under SB 9, yes, with or without a lot split. Outside SB 9 the zone has to allow two units. An ADU is a different approval with its own rules, and it is often the better route.

Can an existing guest house or accessory living quarters become a second home?

Under SB 9 it can be converted into a second primary dwelling in a single-family zone, as the Reseda project was. Otherwise the path is usually an ADU, or a legalization if the structure was never permitted.

Do accessibility rules apply to a fourplex?

Chapter 11A of the California Building Code reaches privately funded buildings with three or more units, including the parking. It reached a four-unit legalization in Santa Monica and changed the site plan.

What stops a certificate of occupancy on a multi-unit project?

Mismatches. Dimensions, construction type and use codes have to agree across the drawings and every permit, including supplementals. The Reseda certificate waited on exactly that.