ADU Size Limits in California: Minimum & Maximum ADU Sizes

Mar 2, 2022 | ADU

The development of ADUs in California has been on the rise. As a state battling homelessness with a housing production rate well below the rate of demand, ADUs offer a way to increase the housing supply in a way that is beyond traditional housing construction. However, these untraditional houses have certain size limits they cannot exceed in the state of California.

As a general rule, everywhere in California should allow an ADU to be built up to 800 square feet with a 16-ft height restriction and a 4-ft setback requirement. However, some places allow larger ADUs under certain stipulations.

Depending on the type of ADU you plan to build and in what city/county, your size limits may be more than 800 Square feet. Read on to find out how to figure out the size limits of your property and the best ADU size to build.

General Maximum Detached ADU Size Limits

California State code allows for a detached ADU to be built up to 1,200 square feet. About 391 cities and counties use this state code, meaning there is a good chance you live in one of these areas. However, cities and counties are allowed to place their own restrictions on the sizes of ADUs, meaning that other cities and counties have stricter ADU size rules.

About 150 cities and counties limit your detached ADU size based on your design plans. A detached ADU with two or more bedrooms will likely limit you to 1,000 square feet. A detached ADU with one or fewer rooms is likely limited to 800 square feet due to things like lot size and coverage.

It is important to remember that this is just in general. Your specific city may have regulations that differ from the mentioned above.


General Rules for Other Types of ADUs

As you are probably already aware, there are different types of ADUs. The four main types of ADUs are:

  • Detached
  • Attached
  • Conversion
  • Internal

If you are converting an existing structure, such as a garage, there are usually no maximum size limits. This is because the structure already exists. If there are limits, the maximum size is usually determined by the largest accessory structure currently in the jurisdiction. However, if you are converting a structure into a JADU (junior accessory dwelling unit), it has to be under 500 square feet.

If you are building an attached ADU, the general rule is that it cannot exceed 50% of the square footage of the home. It also cannot be taller than the existing home.

Internal ADUs are inside of a home and are usually constructed within the existing floor plan. Therefore, there are not a lot of general regulations for them, just that they must be within the pre-existing home.


Minimum Size Limits for ADUs

The California State code says that all ADUs must be at least 150 Square feet regardless of the type of ADU they are. However, there are exceptions. You can apply for a discretionary permit to build a smaller ADU, but there is a good chance that you will not get approved.


What ADU Size Limits Apply to You?

The California Department of Housing and Community Development (the HCD) states that: “A local government may apply development and design standards that include, but are not limited to, parking, height, setback, landscape, architectural review, maximum size of a unit, and standards that prevent adverse impacts on any real property that is listed in the California Register of Historic Resources.”

Therefore, general state law can be further regulated by individual local governments. As a result, 8 counties and 482 cities have created their own regulations within state law that their community must follow when building an ADU.

This section of the law allows local governments to do things like loosen restrictions by allowing ADUs to be built larger than 1,200 square feet. It also allows them to tighten restrictions by lowering the square footage allowed and increasing setback requirements. So, just going off the general size limitations we provided may not always work out for you.

The best way to find out your specific area’s ADU laws is to contact your local government or planning office and ask. Many cities also have websites available with information about their ADU regulations.


Loopholes To ADU Size Regulations

Although cities and counties can add rules and regulations surrounding ADU sizes, there are certain loopholes to be aware of.

For starters, the maximum square footage should not be below 800 square feet. If it is, you may have grounds to argue that those rules are unenforceable.

Secondly, state law states that although local ordinances can have their own restrictions, there need to be exemptions in place to allow individuals to build an ADU up to 800 square feet and less than 16 feet high. So, if lot coverage requirements limit you to an ADU smaller than 800 square feet, you can still build one that big as long as there are 4-foot rear and side yard setbacks and you do not exceed a height of 16 feet.


Best ADU Size in California

If you are looking to build an ADU in California, you may be tempted to go big. However, we recommended building your ADU under 750 square feet. ADUs under 750 Square feet are exempt from bulk & massing development limits (i.e., lot coverage, open space, and min lot size requirements). Being exempt from these regulations can make your planning and development process much easier.

Also, ADUs under 750 feet are exempt from impact fees, making building an ADU yourself a less costly endeavor.


Making The Most of Your ADU Space

If you live in one of the cities or counties that have very strict size limitations, you may need some help making the most out of the space you have. Here are a few tips and tricks to develop an ADU that doesn’t feel cramped.


Don’t Skimp on Windows

Although windows take up valuable wall space, they also provide an abundance of natural light. No one wants to live in a small, dimly lit area. The more natural light you let into a room, the homier it feels. Plus, the windows help with the illusion that the space is bigger than it really is.


Plan For Lots of Storage

Storage is key. Wherever you can make use of an area for extra storage, do it. The more belongings that are tucked away in storage areas, the less clutter there will be in the unit. The more cluttered a unit, the smaller it feels.

There are a number of ADU appliances and furniture that are specifically designed to maximize storage space. Look into these before you start planning your ADU to ensure everything will fit nicely together.


Get Rid of What You Don’t Need

Finally, during the design stage, think about what is necessary and what is not. Ask yourself questions like:

  • How many closets do I need?
  • Should there be more space in the bathroom?
  • How large does the common area need to be?

Be sure you check out these common ADU mistakes before deciding your final ADU design.


More ADU Resources

Meeting size requirements is important when building your ADU. Your plans will not be approved by your local government if they do not meet all regulations and requirements. Therefore, it is important to look up your city/county’s size regulations before starting your ADU design and planning.

This ADU guidebook and eCourse can help guide you through how to plan your ADU. As well as how to acquire the correct permits to build it. Without the appropriate permits, you could face heavy fines.


Work With ADU Professionals

If you don’t want to take the time to try to figure out all the rules and regulations surrounding building an ADU, you should work with an ADU professional. They can assist you in understanding your city/county’s size limit regulations and help you gain the permits necessary to build your ADU.

Here at Levi Design Build, we have years of experience developing ADUs in California. We offer a free consultation to help you navigate through ADU size limitations, along with other essential ADU planning aspects. From there, we can help you plan out your entire project, including ADU costs and financing. Then finally, help you construct the ADU of your dreams.

Contact us today to get started on developing an ADU that meets your needs.