When homeowners first learn they can build an ADU, their next question is almost always about ADU size. How big can an ADU be in California, what is the average ADU size, and how much square footage do you actually need for a comfortable layout? Understanding the typical ADU dimensions and the maximum ADU sizes allowed in California is essential before you choose a floor plan or commit to a design.
We’ve created a guide to the size limitations and considerations for your ADU project in Greater San Diego.
Understanding ADU Size Regulations
ADU size regulations vary by each jurisdiction, but the State of California sets a minimum standard that must be allowed. This means that while some cities may have more lenient laws, they cannot be more stringent than the state regulations allow. It’s crucial to check with your local city or county for specific ADU size limits and requirements.
Because ADU size limits in California can be confusing, many homeowners start by asking us to model different ADU square footage options on their specific lot so they can compare ADU dimensions early in the process.
Maximum ADU Size Allowed in California
California state law allows up to a 1200 sqft ADU on most residential properties, subject to 4′ side and rear setbacks. Municipalities may set more restrictive laws, but must at least allow up to an 850 sqft ADU with one bedroom, or up to 1000 sqft ADU with two or more bedrooms and up to 16′ in height (read more on height in next section). Many cities allow two story ADUs.
Attached ADUs often cannot exceed 50% of floor area of the main residence, though detached ADUs are not related to the size of the primary dwelling. ADUs are still subject to the lot coverage requirements of the underlying zone. However, you may still build one 800 sqft ADU even if the property is maxed out on floor area ratio (FAR) and lot coverage.
If you are converting an existing structure – like a detached garage or barn – into an ADU, there is no size restriction for your ADU. So that means you could potentially convert a 3000 sqft barn into a massive ADU. More about garage conversions vs new construction.
If you are trying to determine the right ADU dimensions or wondering what size ADU is most common, most detached ADUs in California fall between 600 and 1000 square feet because these sizes optimize layout, cost, and future flexibility.
Height Restrictions for ADUs in California
Generally an ADU may be 16′ tall per California state regulations. An update as of Aug 2022 is that SB-897 requires that local governments must allow the following:
- Detached ADU on a lot with an existing or proposed single family or multifamily dwelling unit: 16 feet
- Detached ADU on a lot with an existing or proposed single family or multifamily dwelling unit that is within one-half of one mile walking distance of a major transit stop or a high-quality transit corridor (defined in Section 21155 of the Public Resources Code): 18 feet. A local agency shall also allow an additional two feet in height to accommodate a roof pitch on the accessory dwelling unit that is aligned with the roof pitch of the primary dwelling unit.
- Detached ADU on a lot with an existing or proposed multifamily, multistory dwelling: 18 feet
- Attached ADU: 25 feet or the height limitation in the local zoning ordinance that applies to the primary dwelling, whichever is lower, but not to require a local agency to allow an accessory dwelling unit to exceed two stories.
Keep in mind that each local government will have its own ADU height restrictions in place. Some areas allow for two story ADUs, while others do not.
Height limits are often considered alongside ADU size and footprint because they determine whether a one-story or two-story layout can fit your desired square footage.
Largest Possible ADU Size in Greater San Diego
Most cities allow up to a 1200 sqft ADU, including San Diego, Carlsbad, Oceanside, Encinitas, Chula Vista, La Mesa, Vista and San Diego County. Meanwhile Del Mar, El Cajon, Escondido, San Marcos and Solana Beach have set the maximum size for an ADU at 1000 sqft. Poway actually allows up to a 1500 sqft detached ADU. Check out our city ADU regulation pages to see local size maximums that have been set by each municipality for accessory dwelling units.
These differences mean the maximum ADU size in California varies significantly by city, even though state minimums apply everywhere.
What to Consider When Choosing ADU Size
When choosing the right ADU size for your project, the key is balancing your intended use, budget, and the ADU square footage your property can accommodate. Choosing the right size for your ADU involves several considerations:
Fees & Requirements
Certain square footages trigger different requirements and fees. Key breakpoints include:
- <500 sqft: several cities have exemptions for soils reports or wastewater mitigation when the unit is under 500 sqft
- <750 sqft: all impact fees are waived for units under 750 sqft
- 800 sqft: even if you are maxed out on lot coverage ratios, you are still allowed to build an 800 sqft ADU
- 1000 sqft: some cities have set 1000 sqft as the upper size limit, or the upper limit that will enjoy reduced setbacks (Oceanside allows 1200 sqft ADUs, but 1000-1200 sqft would be subject to underlying setbacks)
These thresholds matter because ADU size is one of the biggest drivers of cost, permitting complexity, and long-term property flexibility.
Purpose and Intended Use
Your intended use will strongly influence the ideal ADU size. If constructing for a family member, personal preferences are crucial. If for rental purposes, maximizing the number of bedrooms within the allowed ADU dimensions can optimize rental income.
The size impacts the usability and future flexibility of the ADU. A larger ADU might offer more versatility and potential uses, from housing family members to generating rental income or providing a home office.
Space Availability and Property Size
Assessing the available space on your property is essential when deciding on ADU size. The physical dimensions of your lot, existing structures, and setback requirements will influence how large your ADU can be. Ensuring the ADU fits well within your property and complements your existing home is key to a successful project.
A property assessment will help determine how much space you need for an ADU and what ADU sizes will realistically fit your setbacks, access requirements, and existing structures.
What is a good size ADU to build?
There is no single typical ADU size, but most functional layouts start around 500 to 600 square feet for a one bedroom, then scale up based on bedroom count and desired openness.
There are certain considerations to the right size ADU to build. If you intend to rent out your new ADU, you will get the most bang for your buck by building the smallest unit that will accommodate your targeted number of bedrooms. This allows the homeowner to save money on the build, due to the smaller size, while also maximizing rental income with a greater number of ADU bedrooms available.
Here are some general guidelines:
- Two-bedroom ADU: Minimum around 600 sqft (a more spacious typical two bedroom is just under 750 sqft).
- Three-bedroom ADU: Minimum around 900 sqft (a popular three bedroom is around 1000 sqft).
- Four-bedroom ADU: Minimum around 1,150 sqft.
Check out the chart below for a general idea of how different sizes of accessory dwelling units will feel, from “tight” to “comfortable” to “spacious.”
CHART: ADU Size Ratings by Square Footage and Bedroom Count
Use our ADU Floor Plan Search to filter blueprints by size and bedroom count to see what options we have for you off the shelf. We can modify any of our plans to suit your needs, for instance expanding an existing plan to make it more spacious.
ADU Size FAQs
These are some of the most common questions homeowners ask about ADU size, ADU square footage, and how ADU dimensions interact with local regulations.
How much space do you need for an ADU?
The amount of space required for an ADU depends on setbacks, access paths, utility locations, and the ADU footprint you choose. Most homeowners can fit ADUs between 400 and 1200 square feet, but the usable area often comes down to the width of the lot and how much yard you want to maintain. Minimum lot size is no longer a concern thanks to California state law. A feasibility study is the best way to understand the ADU sizes that fit your land without creating awkward layouts or losing outdoor space.
Why are larger ADUs considered a better value?
In general, larger ADUs are cheaper to build on a per square foot basis. As you may imagine, many costs for the ADU are the same, whether you are building 1200 sqft or just 400 sqft. The same number of subcontractors must be mobilized to the job site, and they typically have a minimum fee no matter how small a job they are performing. There are also many of the same overhead costs, regardless of ADU size, for instance construction documents, soils report, survey, supervision, dumpster and temp toilet. All of these costs are spread over a smaller footprint, making small units must more expensive than larger units on a sqft basis.
Can an ADU be larger than the main house?
Yes, a detached ADU can be larger than the main house. One factor to be mindful of is that you will pay fees on the new unit that are much higher than for an ADU that is smaller than the primary residence. This is because most cities have a fee structure that sets ADU fees in proportion to the size of your primary home.
If your ADU is larger than the main home, you may be subject to fees akin to what is paid on the primary residence. It’s wise to check with the city for their specific interpretation before building an ADU larger than the main home. Another option is to reclassify the new structure as the primary and designate the existing home as the ADU. Again, the fees associated will vary by city.
Does the square footage of the ADU count towards the property?
Yes, ADU square footage is counted in the total habitable square footage of the property and will appear in your overall square footage calculations. The total habitable space of the property is subject to underlying zoning restrictions regarding floor area ratios (FAR), lot coverage, and open space requirements (known collectively as bulk and massing limits). As mentioned earlier, you are allowed to build an 800 sqft ADU regardless of lot coverage and FAR.
If you haven’t already, check out our standard floor plans for ADUs. Send in a Property Assessment so we can set up your free consult where we can look at different size ADUs on your property to see what will fit best for your needs and also comply with the size limitations for your jurisdiction and zone.
Does the square footage of a garage attached to an ADU count towards the size limit for the ADU?
No, the garage or other unconditioned space will not count towards the square foot size limit set for the accessory dwelling unit. Keep in mind though that any garages, decks, or other structures will typically not enjoy the same reduced setbacks that an ADU will. Read more about ADUs and garages.
Contact ADU Experts
Understanding the size regulations and considerations for ADUs in San Diego is crucial for a successful project. Whether you choose a standard, semi-custom, or custom plan, SnapADU is here to help you navigate the complexities and achieve your ADU goals. Contact us for a property assessment and personalized guidance on your ADU project.
The information presented in this technical blog is based on our extensive expertise in the field, drawing from our hands-on experience in design, development, and implementation. While we strive to provide accurate and up-to-date information, it’s essential to note that the rapidly evolving nature of the ADU space may result in changes that could impact the details discussed here.
At SnapADU, our goal is to equip our readers with valuable insights and practical knowledge to navigate the ever-changing accessory dwelling unit landscape. We encourage you to reach out to our experts for personalized guidance and to stay updated on the latest developments in the field.
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