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How Big Can An ADU Be In California? Exploring Size Regulations And Limits

The intricacies of accessory dwelling units (ADUs) have changed recently in the Golden State due to updates to legislation. So how big can an ADU be in California under the new regulations? Uncover the guidelines that dictate ADU size, revealing insights for homeowners, developers, and anyone else interested in these developments in the California housing landscape.

Renée White

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December 26, 2023
Table of content
    • ADUs in California can be 800+ feet in size, typically with a 16-foot height and a 4-feet setback distance.
    • The state has general size regulations, but there are slight variations in different localities.ADUs are intended to be secondary in size to the main residence, but detached ADUs may be bigger in rare circumstances.
    • Some areas permit 2-story ADUs, so it is crucial to be informed of local ordinances.
    • We can provide support and guidance on the latest regulatory updates to help you plan, build, rent, and sell an ADU in Contra Costa.

    How Big Can An ADU Be In California? Understanding The Limits

    According to California state laws, the rough guidance for how big an ADU can be is as follows:

    • A floor area of up to 800 square feet
    • A 16-foot roof height
    • A setback distance of 4-feet

    However, in certain places and under certain conditions, they can be larger than this.

    For detached ADUs, there are approximately 391 cities and counties that allow sizes of up to 1,200 square feet. Attached ADUs must have a square footage that is no more than 50% of the main property’s square footage, while conversion ADUs are usually not limited by any size restrictions.


    The General ADU Size Limits In California

    As mentioned, there is some variation in how big an ADU can be in California. The general size limits look like this:

    • Every city and county should permit a detached ADU of at least 800 square feet.
    • Most locales allow up to 1,200 square feet.
    • Some cities will enforce strict limits based on the number of bedrooms.
    • Attached ADUs and converted structures have different rules, as described above.

    The number of cities and counties that have chosen to offer some variation on the state guidelines is over 400, so you will need to check local guidance. As a leading team of real estate professionals, we can help you do this in Contra Costa County.

    Specific ADU Size And Location Questions: Regional Differences

    The regional differences in maximum ADU sizes are quite varied. The reason for this comes down to local considerations like housing demand, preservation of the character of neighborhoods, and more.

    In the county of San Diego, the maximum size of an ADU is 1,200 square feet, with a 4-foot setback and a building separation of 6 feet. In Los Angeles County, meanwhile, the maximum permissible size is 800 square feet, though some local ordinances allow larger sizes and this is subject to change. Since you are now allowed to sell your ADU as a condominium, expect to see more demand and possible changes in regulations.

    How Big Can An ADU Be In Contra Costa?

    In Contra Costa County, the maximum size for a detached ADU is currently 1,200 square feet, but there are certain conditions attached. You will need to pay a separate property tax if the ADU is rented or sold. We can help navigate the intricacies of how big an ADU can be in California.

    With our experienced team and extensive professional network, we have access to the full range of knowledge about local guidelines relating to real estate. And we always have a finger on the pulse of regional developments, so we can offer up-to-date expertise on this ever-evolving housing market.

    How Big Can An ADU Be In Los Angeles?

    In Los Angeles, detached ADUs are not permitted to exceed 1,200 square feet, and their minimum size is 150 square feet. They are limited to 2 stories, and zoning height limits are usually 25 feet. Attached ADUs, meanwhile, may not exceed 50% of the main property’s square footage, though a 2+ bedroom attached ADU with a floor area of anything less than 1,000 square feet is always permitted.

    This guidance on how big an ADU can be in Los Angeles hasn’t yet changed with the new regulations. But more are likely to start appearing now that legislation has passed to make permits easier to obtain.

    How Big Can An ADU Be In Other San Francisco?

    In San Francisco, minimum ADU size is set at 220 square feet, while the maximum is 1,200 square feet. That maximum applies to both detached and attached ADUs, but attached ones must be no more than 50% of the square footage of the main residence up to that maximum. In terms of height, the maximum is listed as 16 feet.

    As you can see, these regulations fall close to the general guidance for how big an ADU can be in California. Regional variations will not deviate far from the statewide legislation, but it is always important to know the details of your locale, whether you plan to sell or rent your property.

    How Big Can An ADU Be In Other San Diego?

    In San Diego, ADUs are supposed to be secondary in size to the primary dwelling. An attached ADU can be 50% of the square footage of the main property, up to a maximum of 1,200 square feet. That maximum size also applies to detached ADUs, regardless of the size of the main residence.

    There is some variation in maximum sizes depending on the number of bedrooms in the ADU, so seek specific guidance before making any plans.

    ADU Size Relative To Primary Residence: What’s Allowed?

    In California, how big an ADU can be is somewhat dictated by the size of the main property. As mentioned, the ADU is intended to be secondary in size, and the main residence dimensions are particularly relevant for attached ADUs. Generally speaking, an ADU can be no larger than 50% of the square footage of the main residence it is attached to, with an absolute maximum size guideline in most counties. There are also maximum height restrictions.

    For detached ADUs, the maximum size is usually the only number you need. Your construction can be anything up to that size regardless of the square footage of the main residence. If your plans don’t adhere to these guidelines, it is likely your application will be denied.

    Can An ADU Be Larger Than The Primary Residence?

    There are restrictions in place if an ADU size is going to exceed your property’s floor-area ratio. For attached ADUs, the square footage can never be more than 50% that of the main residence. For detached ADUs, if the planned size exceeds that of the primary dwelling, the ADU size is limited to a maximum of 800 square feet.

    This is one example of the complexity of the laws around how big an ADU can be in California. If you are looking to build one to rent or sell in Walnut Creek, Richmond, or anywhere else in Contra Costa, contact us for guidance on the entire process.

    Are There Any Loopholes Regarding ADU Size Regulations In California?

    Finding loopholes in how big an ADU can be in California depends on local regulations and requirements. It may be possible to find loopholes relating to the number of bedrooms in your proposed property, or in projects relating to multi-family housing. We advise you to seek expert advice in your specific location to explore these possibilities further.

    The updates to the legislation around ADU’s is likely to have a substantial impact on the Contra Costa housing market, as well as local markets elsewhere in the state. Working with the best real estate professionals is key to making the right decisions.


    California’s ADU Construction Size Specifications

    Here are the need-to-know specifications for how big an ADU can be in California:

    • An ADU must have a minimum living area of 150 square feet.
    • Cities and counties cannot impose a maximum ADU square footage that is less than 800 square feet.
    • Attached ADUs cannot have a square footage that exceeds 50% of the floor-area ratio of the main residence.
    • The height limit for a single-story ADU is 16 feet, with a side and rear setback of at least 4 feet.
    • Height limits for 2-story ADUs vary by local ordinance, but the maximum is 24 feet.

    You will see variations within these numbers in different localities, and all is subject to change in the evolving California property market.

    Can You Build A 2-Story ADU In California?

    Some areas in California do not permit 2-story ADUs, but they are permissible in many municipalities across multiple counties. California state law SB 897 allows for 2 stories, but there are restrictions.

    • Detached ADUs can be up to 16 feet in height, or 18 feet if located within a half-mile of major transit stops.
    • Attached ADUs can be up to 25 feet tall, or the height limitation of the main dwelling – whichever is lower.

    Naturally, it is essential to work within these restrictions. Permits will not be granted for proposals that don’t meet local regulations, regardless of the number of stories.

    How Close To Property Line Can You Build An ADU?

    When planning for how big an ADU can be in California, it is important to note that a detached ADU is subject to the following rules:

    • It must be at least 10 feet away from the main residence.
    • It must be at least 4 feet away from property lines.

    There may be some local variation in these numbers, so be sure to check with local authorities before you make your plans.

    Revised Height Restrictions For ADUs

    SB 897 has brought new detached ADU height restrictions into law for 2024. The new height limits include:

    • A minimum requirement of 16 feet for all detached units to ensure sufficient space.
    • That minimum can be 18 feet in some circumstances.
    • For 2-story ADUs, under certain conditions, the height can be up to 20 feet.

    Attached ADUs can be up to 25 feet tall, or the height of the primary residence – whichever is lower. Contact local authorities to get details on specific guidance for your municipality and ensure compliance with the relevant regulations.

    California’s ADU Features And Layout: Designing For Efficiency

    The first consideration is to choose between an attached, detached, or converted ADU. The converted option, if available to you, has the least restrictions. But there are pros and cons to each, so explore which is right for you.

    To decide on a design, you will need to look into local regulations including zoning restrictions. And you must explore options for financing to understand what your budget will be. Space optimization is key, so open-concept layouts are a popular choice. Make sure your interior design meets the needs of the intended occupants and choose a design style that complements the main house.

    Tiny homes are an option in some areas of California, particularly with restrictions on how big an ADU can be. Explore the possibilities thoroughly before creating your design.

    How Many Bedrooms Can An ADU Have?

    California law does not allow for a limitation on the number of bedrooms for an ADU. However, 1-2 bedrooms are the most common choices, as the size limitations don’t allow for many more than this. The larger your ADU, the more bedrooms it could potentially accommodate, so design carefully.

    What Is The Best Size For An ADU?

    This really depends on your needs. For a single occupant or a couple, a smaller studio ADU of 800 – 1,000 square feet is often sufficient. But larger always offers more flexibility and may fetch a higher price, if you can afford to build it.

    The main factors that should determine your size choice are:

    • Local size limits
    • Intended use
    • Your budget for construction

    Let these factors guide your decision-making when creating your design.


    ADU Regulation Specifics: Keeping Up-To-Date

    The latest height and size restrictions for how big an ADU can be in California come from updates to statewide legislation around ADUs. The new rules are intended to make it easier for people to get permits to construct ADUs on their properties, so the easing of restrictions is all about providing more affordable housing.

    Different jurisdictions have found various loopholes to be able to impose variations on the statewide law. Thus, it is important to educate yourself on local requirements before starting a project. Also, keep up-to-date on changes as these are regulations that are always subject to updates and revisions.

    Frequently Asked Questions

    How Big Can An ADU Be In California?

    Local governments cannot impose a maximum size restriction on ADUs that is below 800 square feet. Many permit larger sizes, and there are intricacies to the rules based on different factors.

    Can An ADU Be Larger Than The Primary Residence?

    An attached ADU cannot be more than 50% of the size of the main residence. For detached ADUs, if the size exceeds that of the main house, it is capped at 800 square feet. ADUs are intended to be secondary in size to the main residence.

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