Building Types | Duplex to Fourplex

Duplex, Triplex or Fourplex on a California Lot: How the Laws Combine

A duplex has two homes, a triplex three and a fourplex four. You reach two, three or four homes on a California single-family lot by combining SB 9 (2021), which allows two primary homes or a lot split, with state ADU law, which allows accessory dwelling units (ADUs) and junior ADUs. This page uses only the statute text and the guidance of the California Department of Housing and Community Development (HCD) to show how the pieces add up, and where the state limit of four sits.

Every Way to Reach Two, Three or Four Homes

Sources: Gov. Code 66323, 66411.7, HCD SB 9 Fact Sheet and HCD ADU Handbook.

The Duplex: Two Homes

State law gives two ways to add a second home. Under SB 9 (2021), Government Code 65852.21 requires the city to approve up to two primary homes on a lot in a single-family zone, either as two houses or as one building with two homes. The city cannot apply standards that would keep either home under 800 square feet, and side and rear setbacks can be four feet at most. Under ADU law, you keep the house and add one ADU, either converted from existing space or built new and detached.

The two options differ in ways that matter for your plans. An ADU has no owner-occupancy rule and, if it is 750 square feet or less, pays no impact fees. Two SB 9 homes are both primary homes, and the lot can later be split so each home sits on its own lot. A city may also have its own duplex zone; San Jose, for example, allows two houses or a duplex on its R-2 lots under a 2026 ordinance.

The Triplex: Three Homes

ADU law is the most direct way to three homes. Keep the house, add a junior ADU of up to 500 square feet inside it, and add one ADU. Under AB 1154, in effect since January 1, 2026, the owner must live on site only when the junior ADU shares a bathroom with the main house. The other option is an SB 9 duplex plus one ADU, which HCD says is allowed when no lot split has occurred, because the lot "is eligible to receive ADUs and/or Junior ADUs as it ordinarily would under ADU law."

The Fourplex: Four Homes, the State Limit

On a single-family lot with one house, four homes is the most state law guarantees. You can get there three ways. With ADU law alone, Government Code 66323(a) requires the city to approve the house plus one converted ADU, one junior ADU and one detached ADU, all by staff review. With SB 9 and no split, two primary homes plus ADUs can reach four. With an SB 9 split, each of the two new lots can hold two units.

HCD draws the line in its SB 9 Fact Sheet: "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." Its ADU Handbook adds the other side: "if ADU Law is used on its own or in combination with SB 9 to create more than four total units on the lot, a local agency is not required to approve a lot split under SB 9." A city may still allow more by its own rules.

From January 1, 2027, AB 956 changes the ADU option. A city will have to approve two new detached ADUs on a single-family lot instead of one, and it will not have to approve a junior ADU on a lot where two new detached ADUs have been built. The ADU-only fourplex then becomes the house, a converted ADU and two detached ADUs.

SB 9 Homes or ADUs: How to Choose

Who can use it. SB 9 needs a single-family zone and a clean tenant history: no tenant in the last three years in any home you demolish or alter. ADU law works in more zones and has no tenant-history test, so an ADU remains possible on a lot that fails SB 9.

Living on site. An SB 9 lot split requires a three-year residence affidavit. Two SB 9 homes without a split, and every ADU, carry no owner-occupancy rule. A junior ADU requires the owner on site only if it shares a bathroom with the house.

Fees. An ADU of 750 square feet or less pays no impact fees, and a junior ADU of 500 square feet or less pays none. The SB 9 sections create no fee exemption for the primary homes, so expect the city's normal fees on them. See the impact fees page.

Parking and rentals. SB 9 allows the city to require one space per home, and none near qualifying transit or car share. The ADUs listed in 66323 need no parking at all. Both SB 9 homes and those ADUs must be rented for terms longer than 30 days.

Selling. An SB 9 split creates a lot you can sell. An ADU stays on the same lot as the house unless your city has opted in to AB 1033 condominium sales.

Starting From an Existing Duplex

A lot that already has a legal duplex is treated differently. HCD's ADU Handbook says "a structure with two or more attached dwellings on a single lot is considered a multifamily dwelling structure," and several detached houses on one lot are not. Under 66323(a)(3) and (4), a lot with an existing multifamily building can add up to eight detached ADUs, capped at the number of existing units, which is two for a duplex. It can also convert non-livable space, such as storage rooms, attics, basements and garages, into at least one ADU and up to 25 percent of the existing unit count.

That arithmetic can reach five homes: the two existing homes, two detached ADUs and one converted ADU, if the building has space to convert. HCD's handbook refers to this kind of case when it describes ADU law "used on its own or in combination with SB 9 to create more than four total units on the lot." Where the duplex was itself approved under SB 9 (2021), the SB 9 limit of four described above applies, and cities write their own versions: San Jose limits an SB 9 duplex built without a split to one attached ADU or two detached ADUs.

Cities That Allow More Than the State Minimum

State law sets the minimum. These cities allow more on single-family or low-density land by their own rules. Each city page has the details and sources.

For five to ten homes, the state options are SB 684 and SB 1123 for small-lot subdivisions on multifamily or vacant single-family land, and SB 79 for buildings of five or more homes near qualifying transit in seven counties.

Rules That Change With the Unit Count

Energy Code. The 2025 Building Energy Efficiency Standards define a single-family building to include "a residential building of Occupancy Group R-3 with two or less dwelling units." A duplex follows the single-family rules, including solar panels on new buildings. A building of Occupancy Group R-2 is a multifamily building, and new multifamily buildings up to three habitable stories also need solar. Ask your building department whether a triplex or fourplex design is R-2 or R-3.

Financing. HUD Handbook 4000.1 counts two homes plus an ADU, or three homes plus an ADU, as a three- to four-unit property for FHA loans. Fannie Mae's Selling Guide B2-3-04 says "ADUs are not permitted with a two- to four-unit dwelling." Freddie Mac allows one ADU on one-, two- and three-unit properties. See the financing page.

Rent cap. Under Civil Code 1947.12, housing with a certificate of occupancy issued in the previous 15 years is exempt from the statewide rent cap. An owner-occupied building with two homes is also exempt, but only if neither unit is an ADU or junior ADU, so a house plus ADU does not get that exemption. See the rent and tenant rules page.

Selling homes separately. An SB 9 split creates two lots that can each be sold. SB 684 creates up to 10 lots. AB 1033 lets a city opt in to the sale of ADUs as condominiums, with every lender's consent. From January 1, 2027, AB 2601 lets an SB 9 parcel map include a condominium plan.

Where to Go Next

Related on VanPlex: the missing middle guide, the laneway and ADU guide, and the Sacramento and Berkeley comparison pages.

Frequently Asked Questions

Official sources (checked October 2026)