California, US | Multiplex Guide

The California Multiplex Guide: What You Can Build on a Single-Family Lot in 2026

If you own a house on a single-family lot in a California city, state law already lets you add homes to it. Two sets of rules apply. SB 9 (2021) lets you build two primary homes, split the lot in two, or both. State ADU law lets you add accessory dwelling units (ADUs), which are smaller extra homes on the same lot. Used on their own or together, they can put up to four homes where one stood. This guide explains each law, what changes on January 1, 2027, and how six large cities apply the rules.

VanPlex is a multiplex company in Vancouver, British Columbia. We do not build, lend or sell property in the United States. We wrote this guide as a reference for California owners, and every rule on it is linked to the statute, the state agency or the city that wrote it.

Four homes from the HCD SB 9 Fact Sheet. Ten homes from Gov. Code 66499.41. Sixty days from Gov. Code 65852.21. ADU count from the Department of Finance E-1 release.

The State Laws in Force Today

California housing law is written mostly at the state level, and cities must follow it. The table below lists the laws a single-family lot owner can use as of October 2026. Most of them use ministerial approval, which means city staff check your plans against fixed written rules and must approve them if they comply. There is no public hearing and no vote by a planning board.

SB 9 start date from the Los Angeles Department of Building and Safety. All other rows from the code sections listed.

SB 9 (2021) applies only to lots in a single-family zone inside a city that includes part of a US Census Bureau urban area. ADU law applies on any lot zoned for homes, including multifamily lots. The other laws apply to narrower groups of land: SB 684 and SB 1123 cover multifamily-zoned lots and vacant single-family lots, and SB 79 covers land within a half mile of qualifying transit stops in Alameda, Los Angeles, Sacramento, San Francisco, San Mateo, Santa Clara and San Diego counties.

The builder's remedy applies when a city's housing element (the housing plan the state requires every city to keep current) is out of compliance on the day your application is complete. In that case, Government Code 65589.5 lets a qualifying project exceed local zoning. A project of 10 or fewer homes on a site under one acre, at 10 or more units per acre, qualifies with no affordable units required. The permit deadlines page covers it in full.

What Starts January 1, 2027

California passed the housing laws below in 2026, and each one takes effect on January 1, 2027. Until then, the current rules apply. Plan an application filed in 2026 under today's law, and plan one filed in 2027 under the new one. For SB 1116, the new rules cover applications a city receives on or after January 1, 2027.

Two of these bills change what most owners can build. AB 956 raises the number of new detached ADUs a city must approve on a single-family lot from one to two. SB 1117 changes how impact fees (one-time charges a city or district collects to pay for public facilities) apply to an ADU larger than 750 square feet: on a lot with up to two ADUs, the fee will apply only to the floor area above 750 square feet. The ADU Handbook of the California Department of Housing and Community Development (HCD), updated in March 2026, does not yet describe either bill, so expect new state guidance before they start.

Two follow-up bills to SB 79 also start on January 1, 2027. AB 2576 removes some historic sites from SB 79, and SB 1361 bars a city that has a station from taking certain actions against transit agencies and their projects. The SB 79 page explains both.

How Many Homes Fit on One Lot

The table shows the common ways a single-family lot reaches two, three or four homes under state law in 2026, and the state option for more than four. Your city may allow more by its own rules. It cannot allow less than state law requires.

The four-home limit comes from HCD's own reading of the law. Its SB 9 Fact Sheet says that "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, ADUs, and Junior ADUs." After an SB 9 lot split, the city has to allow only two units on each new lot, and ADUs count toward those two. The duplex to fourplex page walks through each combination, including what an existing duplex can add under ADU law.

Some cities already allow more on their own. Sacramento's Missing Middle Housing ordinance sets no maximum density in its R-1 zones. Berkeley's Middle Housing rules allow up to 70 units per acre in R-1. San Francisco allows four units on RH lots and six on corner lots, and Oakland's RD zone allows four homes on a lot of 4,000 square feet. Read your city's page before you assume the state limit is the only one.

Two Different Bills Named SB 9

Two unrelated California laws share the name SB 9. The lot split law is SB 9 of 2021 (Chapter 162, Statutes of 2021). A separate SB 9 of 2025 (Chapter 510, Statutes of 2025) deals with local ADU ordinances: HCD's ADU Handbook explains that a city ADU ordinance not sent to HCD within 60 days is "null and void," and the city must then apply state ADU law alone. This guide always writes "SB 9 (2021)" for the lot split law so the two are never confused.

How the State Enforces These Laws

The state has given these laws deadlines and an enforcer. HCD has had power to enforce SB 9 since January 1, 2024, under AB 434 (Chapter 740, Statutes of 2023), according to its SB 9 Fact Sheet. Since January 1, 2025, a city must approve or deny a complete SB 9 application within 60 days, or the law treats it as approved. ADU law sets its own deadlines: the city has 15 business days to say whether your ADU application is complete and 60 days to decide it when a house already stands on the lot.

Charter cities, which run under their own local charter, have tested how far state law reaches. Five of them (Redondo Beach, Carson, Torrance, Whittier and Del Mar) won a trial court judgment against SB 9 in 2024. On November 12, 2025, the Court of Appeal reversed it and sent the case back to the trial court. ADU law and SB 79 both say in their own text that they apply to charter cities. The SB 9 page has the full case history.

What the Numbers Show

California had 15,064,131 housing units on January 1, 2026, according to the Department of Finance. Single detached houses were 8,565,186 of them, or 56.9 percent. Buildings of two to four units held 1,157,166 homes, or 7.7 percent.

The Department of Finance reported 29,710 ADUs added in 2025, an 11.42 percent increase over 2024, and said the state passed 15 million housing units for the first time. In their annual progress reports to HCD, cities and counties reported 41,846 ADU permits for 2025, against 5,336 permits for homes in two- to four-unit buildings. Those totals are VanPlex's own sum of HCD's published Table A2 data, from 503 reporting cities and counties.

SB 9 has been used far less. The Terner Center at UC Berkeley surveyed 13 cities after its first year and found Los Angeles received 211 applications for new SB 9 units in 2022, while San Diego received seven. The Center concluded that "the impact of SB 9 has been limited so far." HCD does not publish a statewide SB 9 count in its open data.

Five Steps to Your Answer

1. Step 1

   Find your zone and your city

   Look up the zoning of your parcel on the city or county zoning map. SB 9 (2021) needs a single-family zone. SB 684 needs a multifamily zone or a vacant single-family lot. ADU law works in any zone that allows homes.

2. Step 2

   Run the exclusion checks

   Fire hazard zones, flood zones, historic listings, tenant history and past lot splits can each block SB 9 or SB 684. The eligibility page lists all eleven checks.

3. Step 3

   Pick the law that fits your goal

   To keep one lot and rent the homes, ADU law fits: a city cannot require you to live on a lot with an ADU. To sell homes on separate lots, you need an SB 9 lot split or SB 684, or a city that has opted in to selling ADUs as condominiums.

4. Step 4

   Read your city's own rules

   Cities add written standards on top of state law and sometimes allow more homes. Start with the Los Angeles, San Diego, San Jose, San Francisco, Sacramento or Oakland and Berkeley pages.

5. Step 5

   Check fees, tenants and the loan

   Impact fees, the statewide rent cap and lender rules decide whether the homes pay for themselves. See impact fees, rent and tenant rules and financing 2 to 4 units.

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Official sources (checked October 2026)