Multifamily ADUs in California: SB 1211 and Apartment Lot Rules
California lets the owner of an existing apartment property add two kinds of accessory dwelling units with a ministerial permit: units carved out of non-livable space inside the building, and new detached units on the lot. Conversions must be allowed for at least one unit and up to 25% of the existing units. Since SB 1211 took effect on January 1, 2025, a lot with an existing multifamily building may also get up to eight detached ADUs, but never more than the number of units already there.
For an investor, that turns a storage room, an old boiler room or a strip of surface parking into potential rent. Whether the extra units are worth building depends on construction cost, utility and fee charges, how the new units fit under state and local rent rules, and what a buyer will later pay for the added income. This guide works through each of those for apartment owners and buyers in Sonoma, Napa, Marin, San Francisco and the East Bay. It sits within our development and entitlement guides.
What state law allows on a multifamily lot
California’s ADU rules now sit in Government Code sections 66310 through 66342; SB 477, chaptered March 25, 2024, renumbered them. The multifamily provisions are in section 66323, which requires a city or county to approve qualifying units ministerially in a residential or mixed-use zone. Ministerial means staff checks the application against objective rules, with no hearing and no discretionary design review.
| Route | What the statute allows | Key limits |
|---|---|---|
| Conversion of non-livable space (66323(a)(3)) | ADUs inside parts of an existing multifamily structure not used as livable space, such as storage rooms, boiler rooms, passageways, attics, basements or garages | At least one unit, and up to 25% of the existing units; each unit must meet state building standards for dwellings |
| Detached ADUs, existing building (66323(a)(4)) | New detached units on a lot that already has a multifamily dwelling | Up to eight, not exceeding the number of existing units on the lot; height limits under 66321(b)(4); side and rear setbacks of no more than four feet |
| Detached ADUs, proposed building (66323(a)(4)) | Detached units built along with a new multifamily project | Up to two |
HCD’s ADU Handbook (March 2026 edition) says local agencies must allow the 66323 categories to be combined. In practice, an owner can usually pursue conversions inside the building and detached units in the yard on the same lot, subject to each route’s cap.
How the caps work at different building sizes
The detached cap tracks the existing unit count until it reaches eight. The conversion cap is a percentage, so small buildings get one unit and larger buildings get more. The table below applies the statutory limits; it is arithmetic, not a promise of what a specific lot can fit.
| Existing units on the lot | Conversions of non-livable space | Detached ADUs |
|---|---|---|
| 2 (duplex) | At least 1 | Up to 2 |
| 4 (fourplex) | 1 (25% of 4) | Up to 4 |
| 6 | At least 1 (25% is 1.5; ask how the city rounds) | Up to 6 |
| 8 | Up to 2 | Up to 8 |
| 20 | Up to 5 | Up to 8 |
Setbacks, fire access, utility easements, trees and the building’s own footprint often limit a lot well before the statutory count does. An architect’s site plan is the real test.
Height, setbacks and other standards
Section 66323 lets a city apply only the standards the section itself authorizes, and it bars the city from requiring you to correct nonconforming zoning conditions as a condition of approval. The minimum heights a city must allow come from section 66321(b)(4). Detached units under section 66323 are subject to the first three; the fourth covers attached ADUs:
- 16 feet for a detached ADU on a lot with an existing or proposed single-family or multifamily dwelling.
- 18 feet on a lot within one-half mile walking distance of a major transit stop or high-quality transit corridor, plus up to two more feet so the roof pitch can match the main building.
- 18 feet on a lot with an existing or proposed multistory multifamily dwelling.
- 25 feet, or the local height limit if lower, for an attached ADU, without having to allow more than two stories.
Two other rules matter on older apartment buildings. Fire sprinklers aren’t required in the new unit if the primary building doesn’t need them, and adding an ADU doesn’t trigger a sprinkler requirement for the existing multifamily dwelling. Where the property uses an onsite wastewater system, the agency may ask for a percolation test done within the last five years, or the last ten if it has been recertified.
Converted units still have to meet the building code for dwellings: egress windows, ceiling height, ventilation, fire separation and accessible paths where required. A basement or boiler room that looks large on a floor plan can lose much of its usable area to those requirements.
Parking: what can be removed and what can’t be required
Parking is usually the first objection a buyer raises, and state law answers most of it. HCD’s handbook says local agencies may not impose parking requirements on units approved under section 66323. Separately, section 66314(d)(11) says that when a garage, carport, covered parking structure or uncovered parking space is demolished for an ADU or converted to one, the agency can’t require the lost spaces to be replaced.
That last item, uncovered spaces, is one of the 2024 changes HCD lists for AB 2533 and SB 1211. It matters for apartment lots, where open surface parking is often the only realistic site for detached units. Section 66322 adds further cases where no ADU parking can be required at all, including a location within one-half mile walking distance of public transit or within one block of a car share vehicle.
Check the existing leases before you plan on taking spaces. If a tenant’s lease or rent includes a parking space, removing it can be a reduction in housing services under local rent ordinances, and it raises the questions covered in the rent section below.
Local ordinances and the Sonoma County example
State law sets a floor. Local ADU ordinances often lag behind it. Under Government Code 66316, a local ordinance that fails to meet the state article’s requirements is null and void, and the agency must apply the state standards until it adopts one that complies. HCD reviews local ordinances and sends findings letters when it sees conflicts.
HCD’s January 9, 2025 letter to Sonoma County is a useful example. It found that the county’s ADU ordinance, No. 6458 adopted December 5, 2023, did not reflect the SB 1211 limits for detached ADUs on multifamily lots. It also flagged the 18-foot height rule near transit and a sprinkler requirement HCD said state law doesn’t allow.
The letter asked the county to respond by February 8, 2025, by amending the ordinance or adopting findings explaining why it complies.
We have not confirmed how Sonoma County’s ordinance reads as of October 2026, so check the current version with Permit Sonoma. The same applies in any city: ask the planning department for its ADU standards in writing, compare them with section 66323, and keep the correspondence. For the wider permit process in Sonoma County, see our land entitlement guide.
Rent control, just cause and existing tenants
Adding units to an occupied building is a landlord-tenant project as much as a construction project. Three layers of law can apply, and they don’t always agree.
- State rent cap and just cause (AB 1482). Civil Code 1947.12 and 1946.2 exempt housing with a certificate of occupancy issued within the previous 15 years. Whether a new ADU on an older lot qualifies as its own newer housing is a question to put to counsel, and the existing units keep their status either way. Our AB 1482 guide for landlords covers the exemptions.
- Costa-Hawkins. The state Costa-Hawkins Act generally lets owners set rents on units with a certificate of occupancy issued after February 1, 1995, with exceptions. The exemption doesn’t apply where the owner has agreed by contract with a public entity in exchange for a direct financial contribution or for assistance under the state density bonus chapter (Government Code 65915 and following).
- Local ordinances. San Francisco’s guidance on its Local ADU Program says new units are subject to the Rent Ordinance when they are built under the Local program, added to a building that already has a rent-controlled unit, or approved with a waiver; the city’s guidance says such units are registered under a Costa-Hawkins agreement. San Francisco says state law decides whether some State program ADUs can be rent stabilized. Oakland, Berkeley and San Jose have their own rules; start with our East Bay rent stabilization comparison.
Rentals of 66323 units must be for terms longer than 30 days, so plan on long-term tenancies, not short stays. Construction also disrupts current residents through noise, utility shutoffs and lost storage or laundry. Rent ordinances often treat that as a loss of housing services, so have a landlord-tenant attorney review the construction plan and tenant notices before work starts.
Costs, fees and financing
The statute caps what a city can demand, but the budget still includes design, structural work, utilities and permits. Government Code 66324 bars impact fees on an ADU smaller than 750 square feet and requires proportional fees on larger ones. Water and sewer connection and capacity charges follow separate rules in the same section, so ask each provider for a written estimate per unit. Our ADU investment guide walks through the fee and utility rules and a worked yield calculation for a single ADU.
Financing changes with unit count. A fourplex that adds detached ADUs may end up with more units than standard one-to-four-unit residential loan programs cover, and a lender may then underwrite it as a commercial multifamily loan with different rates, terms and reserve requirements. Ask lenders early how they will count the ADUs.
Our guides to financing 2-4 unit properties and apartment building loans compare the options. Diamond may refer you to partners we work with; you’re free to choose any provider.
Property tax also rises. New construction is assessed at its value when finished while the rest of the property keeps its existing assessed value; see property tax for investors.
How added units affect value and a future sale
Buyers of apartment properties price income, so a new ADU adds value roughly to the extent it adds reliable net operating income. Capitalize the added NOI at a cap rate a buyer would actually pay and compare it with the all-in cost. Our guides to net operating income and cap rates show the method.
- Paper the permits. Keep the permit, final inspection, address assignment and utility letters for each unit. A buyer’s lender and appraiser will ask for them.
- Show the rent status. Document whether each new unit is under a local rent ordinance or a recorded agreement. Uncertainty here lowers what buyers will pay.
- No separate sale by default. An ADU can’t be sold apart from the property unless a local ordinance under Article 4 allows it, so value it as part of the building.
- Unpermitted units. If a building already has unpermitted units, read about unpermitted ADUs before adding more; legalizing them may be the better first project.
If you’re buying a building with ADU potential, treat the extra units as an option, not part of the price you pay. Our guide to buying an apartment building covers underwriting, and our page on selling a multifamily property explains how sellers can present an approved or permit-ready ADU plan.
This is general information, not legal advice. Before you design or price ADUs on an apartment property, have a land-use attorney confirm what the city must approve and a landlord-tenant attorney review how the new units and the current tenants will be treated.
Questions people ask
How many ADUs can I add to an apartment building in California?
Under Government Code 66323, at least one and up to 25% of the existing units can be created from non-livable space inside the building. A lot with an existing multifamily building may also have up to eight detached ADUs, as long as that number doesn’t exceed the existing units.
What did SB 1211 change for multifamily ADUs?
SB 1211, effective January 1, 2025, allows up to eight detached ADUs on a lot with an existing multifamily dwelling, capped at the number of existing units. HCD also lists the rule against requiring replacement of demolished uncovered parking spaces among the 2024 changes.
Do I have to replace parking I remove to build an ADU?
No. A city can’t require replacement when a garage, carport, covered structure or uncovered space is demolished for an ADU or converted into one, and HCD says no parking requirement may be imposed on 66323 units.
Are new ADUs on a rent-controlled building also rent-controlled?
Sometimes. In San Francisco, Local program ADUs and ADUs added to a building that already has a rent-controlled unit are subject to the Rent Ordinance. Elsewhere it depends on state law, the local ordinance and any agreement you sign, so have counsel review it.
Can I rent multifamily ADUs as short-term rentals?
No. Section 66323 requires rentals of ADUs created under it to be for terms longer than 30 days.
Does my city’s ADU ordinance override state law?
No. Government Code 66316 makes a noncompliant local ADU ordinance null and void and requires the agency to apply the state standards until it adopts a compliant one. HCD also reviews ordinances. Get the city’s standards in writing and compare them with section 66323.
Can I convert garages in an apartment complex to ADUs?
Garages are among the non-livable spaces the statute lists, along with storage rooms, boiler rooms, passageways, attics and basements. The conversion count is limited to at least one and up to 25% of existing units, and each unit must meet building standards for dwellings.
If you own an apartment property in the North Bay or East Bay, or you’re buying one for its ADU potential, we can help you test the added income against cost and resale value. Talk to Diamond about multifamily investing.
Sources & notes
- California Public Law: Government Code section 66323 (multifamily ADUs; updated Jan. 1, 2026)
- California Public Law: Government Code section 66321 (ADU height limits)
- California Public Law: Government Code section 66322 (parking exemptions)
- California Public Law: Government Code section 66314 (ADU standards and replacement parking)
- California HCD: ADU Handbook (March 2026)
- California HCD: Accessory Dwelling Units (SB 477 renumbering)
- California HCD: Sonoma County ADU ordinance findings letter (Jan. 9, 2025)
- California Public Law: Government Code section 66316 (noncompliant ordinances)
- California Public Law: Civil Code section 1954.52 (Costa-Hawkins)
- City and County of San Francisco: Check rules for adding an ADU to a building using the Local Program
Unit counts in the sizing table apply the statutory caps; site conditions, fire access and utilities decide what a specific lot can hold.
Last reviewed October 2026 by Matthew Martinez.
