The Design Home is reader-supported. When you buy through links on our site, we may earn an affiliate commission.
In California, Accessory Dwelling Units (or ADUs for short) have been called a “revolution in progress.”
Have you ever thought about building an ADU of your own? Are you curious about the steps you’ll need to go through to build one in the Golden State?
This guide breaks down everything you need to know about California ADUs, from the various ADU laws in major California cities to the average cost of building your own.
What Are ADUs?
Accessory Dwelling Units go by several other names, including secondary units, in-law units, and granny flats. They can be located in the backyard, basement, or garage of another property, or they may be built as standalone, detached buildings.
What Are JADUs?
Junior Accessory Dwelling Units (JADUs for short) are a specific type of ADU that is entirely contained within a single-family residence. An example could be a basement apartment or a converted garage.
What Makes an ADU Different from a House?
An ADU is smaller than a standard house. In fact, there are limits on how large the structure can be (discussed in more detail later).
ADUs also differ from houses when it comes to buying and selling. Even if an ADU is a completely separate structure, it is still part of the same property as the primary home, and the primary home’s owner also owns it.
ADU Laws in California
Before investing time and resources to construct an ADU on one’s property, it’s important to understand the latest laws surrounding them.
Here are some highlights from 5 of the most important laws:
- AB 3182: Prevents Homeowner’s Associations (HOAs) from prohibiting homeowners from renting out ADUs on their property (except for short-term rentals for 30 days or less).
- AB 68: Expands the definition of ADUs and reduces the maximum length of the approval period for ADUs from 120 days to 60 days.
- AB 881: Prohibits local agencies from establishing a maximum size of 850 square feet, or 1,000 square feet for ADUs that contain more than one bedroom.
- SB 13: Prevents local agencies from requiring owners to occupy either the ADU or the main residence until January 1, 2025.
- AB 671: Requires local agencies to include plans for incentivizing and promoting ADUs as affordable rental options.
ADU Zoning Laws
ADUs can be constructed on many properties throughout California. However, restrictions may be put in place based on water adequacy and sewer service accessibility. Restrictions may also result if the ADU impacts the flow of traffic or proves to threaten public safety.
ADUs are also allowed to exceed local zoning densities. They do not count toward the density of an area because they are accessory units built on the land of the main property’s owner. As a result, multiple ADUs may be allowed on one person’s property.
ADU Size Restrictions in California
The following are 5 key ADU size restrictions for Californians to keep in mind:
- There is no minimum lot size requirement.
- Local governments can establish minimum and maximum size requirements for ADUs; however, maximums must be at least 850 square feet or 1,000 square feet if the ADU has more than one bedroom.
- Local governments can establish maximum unit sizes that exceed 1,200 square feet.
- A percentage of the primary dwelling can count toward the maximum size of the ADU.
- However, the percentage of the primary dwelling used cannot restrict the ADU’s size to less than 850 square feet (or 1,000 square feet for multi-bedroom ADUs).
How Big Can an ADU Be?
In many places, the maximum size for an ADU is 1,200 square feet. Some areas allow for larger maximum unit sizes, though, especially in rural locations.
How Many ADUs Can You Have?
California law allows for 1 detached ADU and 1 JADU.
If you own a multi-family building, you may be allowed to add more ADUs.
If your building has space that’s not used as living space (storage areas, parking, etc.) you can add 1 conversion ADU for every 4 existing units. You can also add up to 2 new detached ADUs.
Detached vs Attached
Attached ADUs (including JADUs) have different size requirements than detached ADUs.
JADUs have a maximum size of 500 square feet and a minimum size of 220 square feet. However, if the JADU shares a bathing facility with the main dwelling, the minimum decreases to 150 square feet.
Local agencies can implement setback requirements for those wanting to construct ADUs on their properties. However, setbacks cannot exceed 4 feet for the side and rear yards.
How to Build an ADU in California
This section breaks down everything residents must know about building an ADU in California:
Start by choosing the ADU type that works best for your property (attached, detached, etc.). Then, consider the amount of space available on your property for an ADU, keeping size requirements, setback requirements, etc. in mind.
From here, you can begin creating building plans with the help of an architect who specializes in ADUs.
You don’t have to hire an architect according to California law (only buildings that are two stories or higher require this). However, you’ll likely still need an architect’s help unless you have prior drafting experience.
In addition to a building permit, you may also need the following permits before you can get started:
- Location-specific permit (i.e., Geological Hazard Clearance if your property is in a landslide zone)
- Site-specific permits (such as an easement permit if you’ll be building near a utility easement)
- Address assignment requests (this gives the resident of the ADU a different address from the resident of the main property)
If an ADU is less than 750 square feet, it is exempt from impact fees from local agencies, special districts, and water corporations. If the ADU is larger than this, the impact fees will be proportional to the structure’s square footage.
The average building timeline for an ADU typically spans between 12 and 18 months.
This allows time for the property owner to secure financing, complete the design process, obtain necessary permits, and complete the construction process. It also allows for potential delays related to supply chain issues, labor shortages, etc.
Pre-approved, standardized plans may help to speed up the construction process. Some examples of pre-approved plans are linked here.
Who Are the Best ADU Builders in California?
The following are some of the best and most popular ADU builders in California:
Abodu constructs beautiful backyard homes that align with California’s ADU guidelines. The company’s team is composed of both construction and real estate veterans who can provide a unique perspective.
Cottage touts simple, custom ADUs for California residents. The company is made up of builders, architects, and technologists, all of whom are committed to improving housing affordability and making it easier to build ADUs.
Modal focuses on building modular ADUs that are sustainable, modern, adaptable, and affordable. They are committed to improving the quality and functionality of ADUs while also supporting the planet.
Villa provides access to full-service ADU construction experts and transparent pricing. With other 15 floor plans, they make it easy for California residents to choose the perfect ADU for their property.
prefabADU prides itself on being a Northern California leader in ADU construction. They offer turnkey solutions built in 90 days and have floor plans ranging from 150 to 1,200 square feet.
Who Are the Best Prefab ADU Companies in California?
A prefab ADU is built in a factory and then shipped to your property. If you prefer this approach, here are 5 of the best prefab ADU companies worth considering:
Cover uses the latest technology to speed up the construction process. They create custom digital 3D models of each prefab home, then manufacture it using high-precision, quality-controlled processes typically used in the automotive industry.
All of Method Homes’ prefab ADUs are designed with sustainability in mind and have net-zero features. They align with many certification standards, including ENERGY STAR, LEED, and Passive House.
Clever Homes Prefab assists with all aspects of building a prefab ADU, from design and cost analysis to appliance installation. They pride themselves on efficiently constructing unique and sustainable homes.
All Ecosteel homes are built on bases made of steel-insulated panels. This allows for maximum durability, versatility, and energy efficiency.
SageModern offers an efficient and aesthetic approach to ADU construction. All modules are constructed in a qualify-controlled factory, then are delivered to the construction site to be assembled.
California ADU Grant Program
California’s ADU grant program provides residents with up to $40,000 in reimbursements for pre-development and non-reoccurring closing costs (site prep, architectural designs, permit fees, impact fees, energy reports, etc.).
Who Is Eligible?
This program is available to any California homeowner who meets the state’s low-income requirements are eligible for this program. Click this link to see if you qualify.
Los Angeles ADUs: Laws, Builders, Costs
The specific laws and costs associated with building an ADU may vary depending on where in California you live. Here are some key things to consider if you live in Los Angeles:
- 1 ADU is allowed per residential lot.
- Single-family residential lots can have 1 ADU and 1 JADU
- Existing multi-family dwellings can have up to 2 ADUs per lot
- Existing accessory buildings cannot be constructed into ADUs if they’re located in Very High Fire Hazard Severity Zones
- Detached ADUs are not permitted on the front half of a lot unless they’re located at least 55 feet from the front line
Who Are the Best Los Angeles ADU Builders?
Eano offers all-in-one custom ADU design and construction. All dwellings are built on-site for increased customization and flexibility.
Neo Builders is a local LA company that prides itself on working with each client individually. The team brings a unique perspective to every building project to allow for maximum customization.
What Are the Costs of Los Angeles ADUs?
The cost per square foot for an ADU in Los Angeles ranges from $250 to $400. A garage conversion may range from $80,000 to $120,000, whereas a detached ADU can cost between $100,000 and $400,000 on average.
San Francisco Bay Area ADUs: Laws, Builders, Costs
If you live in the San Francisco Bay Area, you’ll need to keep these ADU construction factors in mind:
- ADUs must be at least 220 square feet with ceiling heights of at least 7.5 feet
- Windows or other glazings are required in bedrooms or livings rooms that are 120 square feet or larger
- Windows should be larger than 10 square feet or half the size of the room and be at least 7.5 inches from the floor
- At least half of the ADUs required front setback should be permeable, and at least 20 percent should be unpaved and landscaped with plant material
- Every bedroom or sleeping area should have a rescue opening of at least 5.7 square feet and at least 44 inches from the floor
Who Are the Best San Francisco Bay Area ADU Builders?
Acton ADU specializes in designing and building high-quality, personalized ADUs. It is based in Campbell, California, and has served the South Bay area for more than 30 years.
What Are the Costs of San Francisco Bay Area ADUs?
The cost of a San Francisco ADU ranges from $100 to $200 per square foot. With this information in mind, a 500-square-foot ADU would range from $50,000 to $100,000 to complete.
San Diego ADUs: Laws, Builders, Costs
San Diego residents thinking about building an ADU should consider the following:
- The maximum allowed space for all detached ADUs is 1,200 square feet
- The maximum allowed space for attached ADUs ranges from 725 square feet to 1,200 square feet depending on the size of the primary dwelling
- New proposed ADU on lots in unincorporated county areas that have an existing and permitted single-family home may qualify for the San Diego County Fee Wavier Program
- Up to 2 detached ADUs are allowed per lot on multifamily complexes, but only if they have rear and side setbacks of at least 4 feet and are no taller than 16 feet
Who Are the Best San Diego ADU Builders?
Marrokal assists with custom construction, ADUs, remodels, and more. They have worked on a variety of ADU projects over the years and are known for their efficient, high-quality performance.
What Are the Costs of San Diego ADUs?
On average, costs for San Diego ADUs range from $300 to $400 per square foot. With this in mind, a 1,200-square-foot ADU would cost between $360,000 and $480,000.