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  • Writer's pictureADU Builders Group

The Ultimate Guide to Accessory Dwelling Units in California

Updated: Dec 8, 2023

Accessory Dwelling Units (ADUs) have become increasingly popular in California as a solution to the state's housing crisis. These small units allow homeowners to add more living space to their property. It can be used for renting, housing family, or expanding their own living area.

This guide will help you understand important information about building an ADU in California.

What do ADUs stand for?

ADUs, also called granny flats, are extra housing units that can be added to homes. They are self-contained living spaces, equipped with their own kitchen, bathroom, and sleeping area. ADUs can be converted from existing structures like garages or built as new, detached units.

ADUs come in various forms:

Detached ADUs: These are standalone structures not physically attached to the main house, such as a small cottage or guest house in the backyard.

Attached ADUs: These units are attached to the primary unit, often added on or made by converting existing space.

Junior Accessory Dwelling Units: These smaller units within a home, known as ADUs, are usually made within the main residence. They have a separate entrance and limited kitchen facilities.

What Makes an ADU Different from a House?

ADUs are distinct from traditional single-family homes in several ways:

Size: ADUs are typically smaller in size compared to traditional homes. They have a design that makes them compact and efficient living spaces.

ADUs often serve specific purposes, such as providing housing for family members or generating rental income. Traditional homes are primarily designed for long-term occupancy.

Location: ADUs are situated on the same property as the main house, whether attached or detached. Traditional homes are standalone properties.

Zoning: ADUs are subject to specific zoning and land use regulations that differ from those governing traditional homes.

ADU Laws in California

California has been at the forefront of ADU legislation to address the housing crisis. The state's ADU laws have evolved to make it easier for homeowners to build ADUs. Some key legislation includes:

SB 1069 and AB 2299: These state laws reduced parking requirements, eased restrictions on ADU size, and accelerated the approval process.

AB 68: This legislation further reduced parking requirements, allowed for multiple ADUs on a single property, and simplified the approval process.

AB 671: This law clarified that ADUs can be built concurrently with a single-family home and eased utility connection fees.

SB 13: Extended the deadline for local governments to submit ADU rules and allowed homeowners to replace garages with ADUs.

ADU Zoning Laws

Zoning laws for ADUs in California vary by city and county. Some common zoning considerations include:

Zoning Type: Residential zones are typically where ADUs are permitted. Check with your local zoning regulations to confirm.

Lot Size: Some areas may have minimum lot size requirements for ADU construction.

Setbacks: Zoning laws may dictate how far your ADU must be from property lines and existing structures.

Height Restrictions: Height limits can vary depending on the zoning regulations.

Design Standards: Local authorities may have specific design standards that your ADU must meet.

To determine the zoning laws specific to your property, contact your local planning department or consult with a professional.

What are the ADU size limits?

The size of an ADU in California is subject to local regulations, but there are some general guidelines:

Attached ADUs: Are typically limited in size. This limit is usually 50% of the main house's size or 1,200 square feet, whichever is smaller.

Detached ADUs: The maximum size can vary, but it's typically between 800 and 1,200 square feet.

How Many ADUs Can You Have? Detached vs Attached

Accessory Dwelling Units (ADUs) are popular for expanding living spaces and addressing housing shortages. When considering building ADUs on your property, one of the key questions that arise is, "How many ADUs can you have?"

The answer to this question depends on whether the ADUs are detached or attached. This blog will discuss the rules regarding the number of ADUs allowed on your property. We will specifically compare detached and attached ADUs.

Detached ADUs

Detached ADUs are standalone structures that are not physically connected to the main house. They can be small cottages in your backyard, converted garages, or entirely new constructions. The number of detached ADUs allowed on one property varies based on where you are and local rules. However, here are some general guidelines:

One Detached ADU: You can typically have one detached ADU on your property in many areas. This means you can build or convert a separate structure for this purpose.

Additional ADU: In some cases, zoning rules may allow for the construction of multiple separate ADUs. However, this is uncommon and typically comes with specific conditions or requirements regarding lot size.

Junior ADU: Some properties allow a smaller ADU inside the main house, in addition to the main detached ADU. These are typically smaller living spaces with limited kitchen facilities and a separate entrance.

Check local zoning rules and permits to understand specific regulations for detached ADUs in your area.

Attached ADUs

The ADUs are connected to the main house, either by adding on or by converting existing interior space. The rules for attached ADUs also vary depending on your location and local regulations. Here are some general considerations:

One Attached ADU: Similar to detached ADUs, you are typically allowed one attached ADU on your property. This means you can add an extension to your primary residence or convert part of it into a separate living space.

Additional ADUs: Some areas may permit more than one attached ADU, but this is less common and subject to specific conditions.

Junior ADU: In addition to the primary attached ADU, some properties may allow for a Junior ADU within the main house, just like with detached ADUs.

Consult your local zoning authorities to understand the rules for attached ADUs in your area. Factors such as lot size, zoning type, and the existing structure's square footage can influence the number of ADUs allowed.

How to Build an ADU in California

The Process

Building an ADU in California involves several key steps:

Concept and Design: Start by planning the layout, size, and design of your ADU. Consider seeking advice from an architect or designer.

Permitting: Get the mandated permits from the local building authority. This often includes zoning, building, and environmental permits.

Construction: Hire a contractor to build your ADU according to the approved plans.

Inspections: During construction, your project will go through several inspections to ensure it meets safety and building code standards.

Utilities: Connect your ADU to utilities, such as water, electricity, and sewage.

Occupancy: Once your ADU is complete and passes final inspections, you can rent it out or use it as intended.

Permitting Steps

Permitting can be a complex process, and it's crucial to navigate it properly. The steps generally include:

Zoning Compliance: Ensure your project complies with local zoning regulations.

Design Review: If your property is subject to design review, submit your plans for approval.

Building Permit: Apply for a building permit that covers construction.

Environmental Review: In some cases, the project may require an environmental review.


Building an ADU in California involves several fees, including permit fees, impact fees, and connection fees. The specific costs will depend on your location and the size and scope of your project. Make sure to allocate funds for these costs.

Average Timeline

The duration for constructing an ADU may differ considerably. It can take a few months to a year or more, depending on your project's complexity, permits, and construction.

Pre-Approved Plans

Some municipalities in California offer pre-approved ADU plans, which can streamline the permitting process. Check with your local building department to see if this option is available in your area.

In conclusion, Accessory Dwelling Units are a promising solution to California's housing challenges. Building an ADU in California is now easier because of changing laws and a focus on affordable housing. It's essential to do your research, understand local regulations, and plan your project carefully. ADUs can make more money, raise property worth, and help solve the housing crisis in California.

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