Multifamily ADU rules in California: how many units you can add

Multifamily ADU rules in California

Government Code section 66323 lists ADUs a city must approve ministerially, meaning by building permit, without discretion, in any residential or mixed-use zone. Two of its four categories are for multifamily lots.

The two types stack. The state housing department's ADU handbook says cities must allow conversion ADUs and detached ADUs together on the same multifamily lot. It also says a multifamily dwelling, for ADU purposes, is a structure with two or more attached units on one lot. Several detached houses on one lot don't count.

Each detached unit is subject only to the height limit in section 66321 (at least 16 feet, or 18 feet on a lot with a multistory multifamily building or near major transit) and side and rear setbacks of no more than 4 feet. Our ADU height limit guide and ADU setbacks guide cover those numbers in detail.

What a city can't add

Section 66323 bars a city from imposing any objective development or design standard the section doesn't authorize on these units. HCD's handbook reads that to cover parking, height, setbacks, lot size, open space and floor area ratio. The statute also says a city:

• can't make you modify an existing building whose rear or side setback is under 4 feet as a condition of approving a detached ADU;
• can't require you to correct nonconforming zoning conditions to get the permit;
• can't require fire sprinklers in the ADU if the primary residence doesn't need them, and the ADU can't trigger sprinklers in the existing multifamily building;
• must require that rentals of these units run longer than 30 days.

For the full set of state rules on single-family and multifamily lots, see California ADU laws.

How many ADUs on a multifamily property

Add the conversion count to the detached count. The conversion side is capped at 25% of the existing units, with a floor of one. The detached side is capped at eight and at the number of existing units, whichever is lower.

Two cautions. The conversion count depends on having enough non-livable space that can meet building standards for a dwelling, and a 25% figure that isn't a whole number needs a rounding rule the statute doesn't give. The City of San Diego's ADU bulletin says one ADU is allowed where the 25% math yields less than one. Ask your city how it rounds anything above that.

Detached ADUs on a multifamily lot don't have to stand alone. HCD says they must be detached from the multifamily building, but they can be attached to each other or to other accessory structures. That lets you build several units as one small building.

What SB 1211 changed

Most of today's multifamily ADU rules come from SB 1211 (Chapter 296, Statutes of 2024), which took effect January 1, 2025. HCD's summary of the bill lists four changes:

The definition matters for conversions. Section 66313 defines livable space as space in a dwelling intended for human habitation, including living, sleeping, eating, cooking or sanitation. A storage room or boiler room isn't livable space, so it's eligible for a conversion ADU.

The 2027 law changes don't touch these counts. AB 956, signed in September 2026, raises the number of detached ADUs on lots with a single-family home; it doesn't change the multifamily categories. See California ADU laws 2027.

ADU on a duplex

A duplex on one lot is a multifamily dwelling under state ADU law. HCD's handbook says so directly. That changes the menu compared with a house:

• Up to two detached ADUs. The detached count is capped at the number of existing units, which is two.
• At least one conversion ADU. 25% of two units is less than one, so the floor of one applies, if you have non-livable space such as a garage, storage room or attic to convert.
• No junior ADU. HCD says a JADU can only go on a lot with a single-family home, and a duplex on one lot doesn't qualify. A half-plex is different: two homes on separate lots sharing a wall can each have a JADU in single-family zones.

That puts a typical duplex at up to three ADUs under section 66323. If you're weighing a lot split instead, compare the two paths in SB 9 vs ADU. More on JADUs in our junior ADU guide.

Adding ADUs to an apartment building

On an apartment property, the conversion route turns unused space into homes. The statute names storage rooms, boiler rooms, passageways, attics, basements and garages, and each unit must meet state building standards for dwellings. HCD adds that a leasing office can be converted too, and lists gyms and enclosed parking as examples of non-livable space.

Conversions vs. detached units

Conversions have to meet building, health, safety and fire standards for dwellings, and HCD notes that accessibility requirements may apply. Basements and boiler rooms can be harder to bring up to code than they look. Our ADU building code guide covers the standards, and garage conversion ADU covers the most common conversion.

Parking and replacement parking

Parking is usually the first worry on an apartment lot, and state law takes most of it off the table.

• No parking for § 66323 units. HCD says cities may not impose parking requirements on the units section 66323 guarantees, which include multifamily conversions and detached ADUs.
• No replacement parking. When a garage, carport, covered parking structure or uncovered parking space is demolished for an ADU or converted to one, section 66314(d)(11) says the city can't require you to replace those spaces. SB 1211 added uncovered spaces to that list.
• Other exemptions. Section 66322 bars parking standards within half a mile walking distance of public transit, and when the ADU is permitted together with a new multifamily building on the same lot, among other cases.

Replacement parking aside, losing a lot's surface spaces can affect tenants and leases. Read your leases before you build over assigned spaces. For every parking rule, see ADU parking requirements.

Multifamily ADUs in San Diego

The City of San Diego writes the state counts into SDMC § 141.0302 and adds its own rules. Information Bulletin 400 (January 2026) summarizes them.

Fees and thresholds that apply to bigger projects

• Inclusionary housing. The bulletin says ADUs aren't exempt: a project with one or more ADUs that results in 10 or more total units (5 or more in the Coastal Overlay Zone) is subject to inclusionary requirements.
• Transportation fee. Projects proposing more than one ADU pay the Regional Transportation Congestion Improvement Program fee.
• Mobility and trash rules. ADU development exceeding four dwelling units is subject to the Mobility Choices regulations, and ADUs must meet refuse and recycling storage rules.
• Landscaping. On sites with an existing multiple-dwelling structure, existing landscaping must be preserved or replaced to the extent feasible.
• Floor area ratio. The city counts ADU floor area toward the lot's FAR, with an exception for one ADU of 800 square feet or less.

Some of these local limits go beyond what section 66323 lists. HCD's position is that a city can't impose standards on § 66323 units that the section doesn't authorize. If a city rule would cut your unit count, ask the city in writing which rules it's applying to which units.

Bonus ADUs

San Diego's ADU Home Density Bonus can add units on top of the state counts. Under § 141.0302(d), inside the Sustainable Development Area you get one bonus ADU for each affordable ADU, rented at restricted rents for 10 years (very low or low income) or 15 years (moderate income). Bonus and affordable ADUs need fire sprinklers, one parking space each outside a transit priority area, and a Community Enhancement Fee. The 4, 5 or 6 unit lot-size caps apply to single-family zones, not multifamily ones.

There's also a crossover rule. IB-400 says a legally permitted multi-unit building in a single-family zone can use the multifamily ADU counts, and a lone house in a multifamily zone uses the single-family ones. In the Coastal Zone, a coastal development permit may be needed; see ADU coastal development permits.

Before you plan

• Count your legal units. The detached cap is tied to existing units, so confirm what's permitted on record.
• Map non-livable space. Walk the storage rooms, garages, laundry rooms and attics with someone who can judge exits, fire separation and ceiling height.
• Check utilities early. Ask your utility and the city what water, sewer and electrical service several new units will need.
• Plan the permit. The City of San Diego uses a different submittal track for multi-unit ADU projects than for houses; start with the ADU permit process in San Diego.
• Hire for multi-unit work. Ask for completed multifamily or multi-ADU projects, then check a contractor's license and read how to hire an ADU contractor.

For budget ranges, see what an ADU costs in San Diego, then compare verified ADU builders in San Diego.

Common questions