How Many ADUs Can You Build on a Multifamily Property?

Curious how many ADUs you can build on a multifamily property in California? If you own a duplex, triplex, apartment building, or other multifamily property, the rules are often more generous than owners expect.

California law allows you to add detached ADUs equal to the number of existing units on the property, up to eight detached ADUs total. You may also be allowed to convert existing non-habitable space within the building, such as storage rooms, boiler rooms, passageways, attics, basements, or garages, into additional ADUs.

This guide focuses specifically on multifamily properties. If your property has just one primary home, check out our separate guide on how many ADUs you can build on a single-family property, since the rules are different.

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 in many cases, setbacks for certain detached ADUs have been reduced to four feet from the side and rear property lines, and some impact fees no longer apply.

Note that California State ADU regulations set the floor for what local jurisdictions must allow. It’s also important to look up your local codes.

How Many ADUs Can You Build on Multifamily Properties in California?

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. But as of 2026, up to 8 detached ADUs can 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 10 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.

SnapADU Townhouse Style ADUs

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

Source: Snap ADU (updated April 2026). *Vertical Build Cost includes finishes & appliances

Is it Cheaper to Stack or Attach ADUs?

If you’re building two identically sized ADUs, you might wonder whether stacking them (two stories) or attaching them side-by-side will save money. In most cases, the cost is about the same overall, but for different reasons. Here’s how the trade-offs typically balance out:

Stacked ADUs (Two-Story Construction)

Cheaper:

• Roof, gutters, and foundation: You’re covering the same square footage with one roof and one foundation instead of two. Even though the footings need to be deeper to support the added weight, the total area of concrete and roofing materials is less.
• Smaller footprint: Less site coverage can help with setbacks and yard space, and may reduce grading or retaining wall needs.

More expensive:

• Fire-rated floor assembly: Code requires fire separation between stacked units, which adds layers of drywall and specialty materials.
• Access and safety: Stairs, guardrails, and an exterior landing all add cost.
• Labor: Crews need scaffolding and fall protection to work on the upper floor, increasing both setup time and complexity.

Attached ADUs (Side-by-Side Construction)

Cheaper:

• Shared walls: You save on siding, gutters, and insulation for the common wall.
• Simpler access: Both units can be on grade, eliminating stairs and upper-level circulation.
• Construction efficiency: Easier to build and inspect since crews stay at one level.

More expensive:

• Fire-rated wall assembly: The shared wall must be fire rated, but this is typically less costly than the fire rated floor/ceiling separation of stacked units.
• Sitework: A wider footprint may require more grading or retaining to create a level pad large enough for both units.

Since stacked and attached ADUs generally come out to a cost wash when you’re comparing two units of the same size, the decision usually comes down to site constraints (available flat area, setbacks, height limits) and design preference (yard space vs. stairs). On smaller lots, stacking can be the only way to fit two full-size units while maintaining outdoor space. On larger, flatter lots, attaching side by side can simplify construction and accessibility. Read more about stacking ADUs.

Key Considerations for Expanding to 5+ Units on a Property

Before maxing out the unit capacity of your property, it’s important to understand the potential downstream implications that expanding to five or more units can trigger.

Financing Complexity: Once you exceed four units, your project typically falls outside the scope of conventional residential financing. Most traditional mortgage lenders cap at four units, meaning you’ll likely need to pursue commercial lending. These financing structures come with different terms, rates, and qualification requirements, which may affect your overall project feasibility.

Building Code Implications (CBC vs. CRC): While the California Residential Code (CRC) governs most ADU projects, transitioning to five or more total units on a lot can introduce added requirements. State law (California Business & Professions Code §5537) requires that a licensed architect or engineer stamp the plans when a property contains more than four total dwelling units. Note this requirement relates to who is qualified to prepare the plans, not necessarily which building code applies.

However, if your project also meets any of the following conditions, the California Building Code (CBC) will apply:

• A single structure contains more than two dwelling units
• A structure exceeds three stories in height
• The design includes non-conventional construction (e.g., steel or concrete)
• Multiple buildings are connected or otherwise classified as one structure

Why does this matter? CBC introduces stricter structural, fire safety, energy, and accessibility requirements than CRC. These can significantly impact your design choices, construction methods, and overall budget.

Uncharted Territory: While many ADUs have been added to properties, larger developments are a bit more unusual. This means you may be navigating new regulatory and construction challenges, which could introduce unexpected complexities. Be prepared for additional unknowns that arise during the process.

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 all ADUs. 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.

All information included in this blog is based on our past design/permit/build experiences and our best understanding of local and state regulations as of this time. With our clients, we do our best to identify the most likely scenarios and interpretations so that we can accurately advise on budget and scope. In the end, we don’t want anyone to be surprised… at least not by the costs or process (though it’s a good thing to be surprised by how smoothly things go and how great the ADU looks after it’s completed!).