How many ADUs can I build? Counts by lot type in California and San Diego

How many ADUs can I build? The quick table

State law sets the minimum every city has to approve by permit, with no hearing. Cities can allow more, and San Diego does. Find your lot type below, then read the section for the details.

Sources: Government Code § 66323, the state housing department's ADU handbook, AB 956 and San Diego's ADU code. Each new unit still has to meet building codes and fit on the lot. For every other ADU rule, see our guide to California ADU laws.

ADUs on a single-family lot

On a lot with one house, section 66323 requires every city to approve these together:

• One conversion ADU inside the existing house or an existing accessory building, such as a garage. It can grow up to 150 sq ft for an entrance.
• One new detached ADU of up to 800 sq ft, with side and rear setbacks of no more than 4 feet.
• One junior ADU, up to 500 sq ft inside the house, which is limited to one per lot and only on a lot with a single-family home.

That's three units plus the house. Not every lot can use all three: you need existing space to convert and room in the yard for the detached unit. Garage conversions are the most common way to use the conversion slot; see our garage conversion ADU guide and the junior ADU guide.

How many ADUs can you have on a property in San Diego without the bonus?

The City of San Diego writes the state minimum into its code. The city's ADU bulletin also has a crossover rule: a legally permitted multi-unit building in a single-family zone uses the multifamily counts, and a lone house in a multifamily zone uses the single-family ones. Above the state minimum, more units come through the city's bonus program, covered below.

Two detached ADUs from 2027

Governor Newsom signed AB 956 on September 29, 2026. Starting January 1, 2027, cities must approve two new detached ADUs on a single-family lot instead of one. The trade-off: a city isn't required to approve a junior ADU on a lot where two new detached ADUs have been built, though it may.

If you're designing two backyard units now, time your permit for the 2027 rules or confirm your city's ordinance allows a second unit today. Our guide to California ADU laws for 2027 covers AB 956 and the impact fee change that came with it.

ADUs on a duplex or apartment lot

A lot with two or more attached homes counts as multifamily. State law lets you combine two kinds of ADU there:

• Conversions of non-livable space such as storage rooms, boiler rooms, attics, basements and garages: at least one, up to 25% of the existing units.
• Detached ADUs: up to eight, but not more than the number of existing units. On a lot where the multifamily building is still being permitted, up to two.

HCD's handbook says the two kinds stack, and that a JADU is only for single-family lots, so a duplex doesn't get one. Rounding of the 25% figure and other details are in our multifamily ADU guide.

San Diego's ADU Home Density Bonus

The City of San Diego lets some owners build more ADUs than the state requires, in exchange for affordable ones. Under SDMC § 141.0302, inside the city's Sustainable Development Area you get one bonus ADU for each ADU rented at restricted affordable rents. Bonus and affordable ADUs need fire sprinklers, one parking space each outside a transit priority area, and a Community Enhancement Fee.

The 2025 caps by lot size

The City Council cut the program back in June 2025. A Sheppard Mullin summary in the National Law Review lists the caps on single-family lots:

The same summary says the program is now barred in the RS-1-1 through RS-1-4 and RS-1-8 through RS-1-11 zones unless the lot is in a High or Highest California Tax Credit Allocation Committee (CTCAC) Opportunity Area, stays in the RS-1-5, RS-1-6 and RS-1-7 zones, and isn't changed in multifamily zones. Check your zone on the city's zoning map, then confirm eligibility with Development Services before you design for bonus units.

Adding SB 9 to the count

SB 9 is a separate state law that lets you build two primary homes on a single-family lot, split the lot in two, or both. It changes the ADU count in different ways:

• Two homes, no split. HCD's fact sheet says the lot can have up to two primary homes under SB 9 plus the ADUs ADU law allows. In the City of San Diego, Information Bulletin 409 caps that combination at four units in total.
• A lot split. Each new lot can have two units, and the lot split statute counts ADUs and JADUs as units. A city also doesn't have to permit any ADU on a lot that uses both the split and the two-unit law, under section 65852.21. San Diego doesn't allow it.
• ADUs first, split later. The ADU handbook says a city doesn't have to approve an SB 9 split if ADU law, alone or with SB 9, has already put more than four units on the lot.

Our SB 9 vs ADU guide compares fees, eligibility and selling for each path.

What else limits the count

The legal maximum is a ceiling. These often decide the real number:

• Space to convert. The conversion ADU needs an existing structure. No garage or spare room, no conversion slot.
• Yard and setbacks. Detached units need room for 4-foot setbacks, access paths and separation from other buildings. See ADU setbacks in California.
• Utilities. Each unit needs water, sewer and power capacity; several units may need a service upgrade. See ADU utilities.
• Fees. No impact fees for an ADU of 750 sq ft or less, but costs multiply with each unit.
• Fire and coastal zones. Extra rules apply; see ADUs in fire zones and coastal ADUs.

Sizing each unit is its own decision: our guides to the 1000 sq ft ADU, the 1200 square foot ADU and ADU floor plans compare what fits and what it costs. Run a budget in our ADU cost calculator. A builder who has done multi-unit projects will spot the real limits fast; compare verified ADU builders in San Diego or our best ADU builders in San Diego, check a contractor's license and read how to hire an ADU contractor.

Common questions