Curious about how many ADUs you can build on a multifamily property in California? We’ve reviewed the latest multifamily ADU rules under SB 1211 to help guide you. More than a dozen of our ADU projects in progress or completed include multiple units. See our projects with multiple ADUs.
If you own a multifamily property – a duplex or up to a large apartment building – you are allowed to build as many ADUs as the number of existing units, up to eight new detached ADUs. You can also convert existing non-habitable space into ADUs.
Law changes in California over the past several years have made it easier to add an accessory dwelling unit (also known as an ADU, granny flat, casita, guest house, accessory unit, additional dwelling unit, accessible dwelling unit, mother-in-law suite) to most residential properties. Parking requirements have been reduced or eliminated, plus many impact fees have been waived on ADU projects. Setbacks have also been reduced to just 4’ on side and rear yards, meaning smaller lots may now accommodate an ADU. The ADU laws include single family residential properties, as well as multifamily properties.
Multifamily ADU Rules in California: New Opportunities with SB 1211
The newly passed SB 1211 was signed into law on September 19, 2024. Going into effect on January 1, 2025, SB 1211 introduces significant changes for multifamily ADUs by allowing property owners more flexibility and removing restrictive regulations. Whether you own a duplex ADU or a larger multifamily property, these updates impact the way ADUs are developed.
How Many ADUs Can You Build on Multifamily Properties?
The number of ADUs allowed on multifamily ADU properties in California depends on the number of existing multifamily units on the property.
New Detached ADUs for Multifamily
Prior to SB1211, most multifamily properties were limited to two new detached ADUs. With the new regulations, up to 8 detached ADUs can now be built, provided they do not exceed the number of existing units.
Conversion ADU for Multifamily
In addition to those detached units, you may also convert existing non-livable space in the multifamily structure. For conversions (think garages), the minimum number of ADU conversions allowed is one, up to a maximum of 25% of the number of existing multifamily units. This is in addition to the allowed number of detached ADUs. This means that if you have an eight-unit property, you could convert non-habitable space within the building into to ADUs (8 units * 25% = 2), plus up to eight detached ADUs.
San Diego County regulations state that any existing accessory structure can be converted into an ADU. This can include a garage, shed, or workshop; any non-habitable space, as long as it is not currently classified as a dwelling unit. We most commonly see garages and existing sheds being converted. These units are usually subject to local jurisdiction and setbacks just like any other ADU. However, if you have a non-conforming garage (meaning it is encroaching on the setbacks), you are allowed to convert the garage, or demolish it entirely and construct an ADU in its place. Read more about garage conversion vs. demo to start from scratch.
Restrictions on Size
In all cases, limits on your underlying zoning – such as floor area restrictions – will still apply to your ADU build. However, cities must allow a single-story 800 sqft ADU ‘by right’ if it meets building codes, according to state regulations. This holds true even if you are maxed out on your FAR (floor area ratio) or RFA (residential floor area). Also, unlike single family properties, you may not add a junior accessory dwelling unit (JADU) to a multifamily property.
ADU Affordability Bonus Program in City of San Diego
You may also be able to build more units under the City of San Diego’s Affordable ADU Bonus Program. The city allows you to build one ADU that is not deed restricted (e.g. you can rent it out at market rates) for every affordable ADU you build. This is of course subject to underlying zoning requirements on floor area ratio and lot coverage requirements. So in sustainable development areas, this means you could build more ADUs than the number of existing units. Learn more about the Affordable ADU Bonus Program.
What counts as a multifamily dwelling in California ADU development?
A multifamily dwelling refers to a residence that contains more than one housing unit, like a duplex or townhome. When you hear about multifamily, it is important to make a distinction between the property zoning and the structure itself. Note that the multifamily units need to be attached to each other to be considered a multifamily development under California state ADU laws. See page 12 of the HCD handbook for this clarification.
You may see a lot of different scenarios with multifamily zoned lots, including properties with the following structures currently built on them:
- Single family dwelling on a multifamily zoned lot
- Two single family homes that are not attached to each other
- Multifamily dwelling structure (two or more attached units)
Typically, you would be able to build multiple ADUs in the last example: a property with an existing multifamily dwelling already built on it.
City of San Diego Multifamily Dwelling Exceptions
The City of San Diego, however, will allow up to two detached ADUs on properties that are simply zoned for multifamily, even if there is only a single family residence currently on the property. Additionally, the City of San Diego regulations allows two ADUs on properties with an existing multifamily residence (like a duplex) even in a single family zone.
Can You Build an ADU on a Duplex in California?
Yes, you can build up to two ADUs on a duplex in California. Under SB1211, property owners with an existing duplex can add up to two detached ADUs, providing options for increasing living space and rental potential. In addition to the ADU duplex structure, property owners may also be able to convert non-habitable spaces like garages into additional ADUs.
How do you fit Multiple ADUs onto a property?
In some jurisdictions, ADUs must be standalone structures with minimum separation between them. In others, like the City of San Diego, you have the option to attach them. These ADUs can be built side by side or stacked on top of each other as two stories, allowing even smaller lots to accommodate two new units. Read more about stacked ADUs.
ADUs can be up to 1,200 square feet in many cities, providing ample space for larger units. With up to three bedrooms or even four bedrooms, these spacious ADUs are ideal for renters or extended family members, offering a highly flexible housing option.
How Much Does it Cost to Build Multiple ADUs?
Generally, the second ADU costs around 80-85% of the first – see example prices for each unit on our plans pages. Although shared mobilization and project management offer some savings, each additional ADU still requires high-cost components like kitchens and bathrooms. As a result, building multiple ADUs is not as cost-effective as increasing the size of a single ADU.
While you save by mobilizing construction crews just once and tackling more work at once, some expenses will still be significant. For example, fire-rated construction between units and dedicated HVAC and electrical systems for each ADU add costs. This keeps the cost per square foot from dropping as sharply as it would if you were simply expanding a single unit.
Read more about ADU costs, as well as more details on important (but often overlooked) components like utilities & sitework.
TABLE: Cost To Build Two Detached ADUs in San Diego, California
| Type | Size | Vertical Build Cost* | Build cost per sqft | Typical All-In Cost | All-in cost per sqft | View Example Plans |
|---|---|---|---|---|---|---|
| 1BR/1BA stacked | 500 sqft x 2 | $400K | $400 | $525K | $525 | 1BR <500 sqft |
| 2BR/1BA stacked | 750 sqft x 2 | $500K | $333 | $650K | $433 | 2BR <750sqft |
| 2BR/2BA stacked | 1000 sqft x 2 | $600K | $300 | $750K | $375 | 2BR >750 sqft |
| 3BR/2BA stacked | 1200 sqft x 2 | $650K | $270 | $800K | $333 | 3BR all sizes |
Source: Snap ADU (updated June 2024). *Vertical Build Cost includes finishes
What are the requirements for renting out ADUs?
ADUs may be rented for periods of more than 30 days. Owner occupancy requirements have been waived for ADUs built through 2024 and likely will be extended to 2030. This means that an owner of a multifamily property may build an ADU without having to sign an agreement that they will live on the premises (which was previously the case prior to legislation changes). Read more about owner occupancy requirements and renting out ADUs on our blog.
Read SB 1211 – Updated ADU Multifamily Rules for California
Want to dissect the relevant multifamily code, seeing how the SB1211 changes affected key state regulations? Red strike is removed wording, blue is the changed wording, and we’ve added bold to make things easier. We’ve also listed the whole piece of legislation in order, with our commentary noted at the top of each section.
Parking Spaces Need Not Be Replaced
ADUs built by replacing existing structures like carports or driveways no longer require replacement parking.
66314 (d) (11) When a garage, carport, or  covered parking structure  structure, or uncovered parking space  is demolished in conjunction with the construction of an accessory dwelling unit or converted to an accessory dwelling unit, the local agency shall not require that those offstreet parking spaces be replaced.
Conversions of Existing Space
For conversions of existing non-habitable space, the minimum number of ADU conversions allowed is one, up to a maximum equal to 25% of the number of existing multifamily units.
Height Restrictions
The height restriction is 16′, 18′, or 20′ depending on which section. Additionally, many cities allow for taller ADUs, so long as they conform to the underlying regulations for the local zoning.
Section 66321 (b) (4) (A-C) for easy reference
Number of ADUs
You may build as many detached ADUs as you have units in the existing multifamily dwelling, up to eight new detached ADUs.
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