How to Build an ADU in California: What Homeowners Need to Know

Adding an Accessory Dwelling Unit (ADU) is one of the most flexible ways for California homeowners to expand their property’s potential. Whether you’re planning a rental unit, a space for aging parents, or just more breathing room for your household an ADU offers smart long term value.

Thanks to recent updates to California ADU laws, the process is now more streamlined than ever. In this article I’ll break down what ADUs and Junior ADUs (JADUs) are, how the latest legislation has made them more accessible, what planning guidelines you’ll need to consider such as size limits, setbacks, parking, and utilities, and what the design and permitting process looks like.

I’ll also share how I can help if you’re a homeowner in Martinez or the East Bay and ready to get started.

What Is an ADU?

An Accessory Dwelling Unit (ADU) is a secondary living space located on the same lot as a single family home. It includes everything needed for independent living such as its own kitchen, bathroom, and sleeping area. And it comes in a few different forms.

Detached ADUs are standalone structures often built in the backyard. Attached ADUs share a wall with the main house, typically as a rear or side addition. You can also convert existing space into an ADU, such as a garage, attic, or spare bedroom as long as it meets certain requirements.

There’s also a smaller option called a Junior ADU (JADU). These are created entirely within the walls of the main home and are limited to 500 square feet. A JADU includes a separate entrance, a small kitchen or kitchenette, and it may share a bathroom with the main house. Both ADUs and JADUs offer homeowners new ways to adapt their property to changing needs.

Why California Is Encouraging ADUs?

California has faced a housing shortage for decades especially in areas with strong job markets like the Bay Area. In response, the state has passed several laws that make it easier for homeowners to build ADUs on their properties. The goal is simple: create more housing by using land and infrastructure that already exists.

ADUs are now recognized as an essential part of California’s housing strategy. Because they are built on lots that already allow residential use, they can be developed more quickly and at a lower cost than traditional new homes. State legislation has removed many of the older barriers that used to make these projects difficult including minimum lot sizes, strict parking rules, and owner occupancy requirements. Cities and counties are now required to review ADU applications within 60 days and cannot impose restrictions that go beyond what state law allows.

Whether you are adding space for family, creating a rental unit, or planning for future flexibility ADUs are now more accessible, more practical, and better supported by state policy than ever before.

Key Planning Considerations

Understanding a few core rules can help you design a code-compliant ADU without running into costly delays. While local jurisdictions may have their own review process, California state law now sets the minimum standards that cities and counties must follow.

Size Limits

State law requires cities to allow ADUs of at least 850 square feet for a one bedroom unit and 1,000 square feet for a two bedroom unit. Some jurisdictions allow up to 1,200 square feet depending on the zoning, lot size, and intended use. If you are converting an existing structure like a garage or barn, these limits often do not apply as long as the unit stays within the original footprint.

Setbacks

For detached ADUs, the state requires that only four foot side and rear setbacks be enforced. If you are converting a permitted structure that already meets existing codes, such as a garage additional setbacks usually are not required. Front yard setbacks may still apply, but they cannot be used to block a detached ADU that is up to 800 square feet.

Parking

In many cases, ADUs do not require any additional parking. If your property is within one half mile walking distance of public transit, in a historic district, or if you are converting space within an existing home or garage no parking can be required. When parking is required, the maximum is one space per unit or per bedroom, whichever is less, and it can be located in tandem or within setback areas.

Utilities and Fire Safety

If the ADU is created by converting existing space, it typically is not considered a new residential use for the purpose of utility connections. That means new or separate utility fees often do not apply especially for smaller units. Fire sprinklers are only required if the main residence is also required to have them. Newly built detached ADUs must meet California’s solar requirements, but conversions are usually exempt.

Fees

Impact fees are waived for ADUs under 750 square feet. For larger units, fees must be proportional to the size of the ADU relative to the primary residence. For example, if your ADU is half the size of your main home the impact fee cannot be more than 50 percent of what a new full size home would be charged.

The ADU Process - Step-by-Step

Building an ADU involves more than just design and construction. It is a process that moves through several clear phases, from documenting existing conditions to final inspection. Understanding the typical steps can help you plan ahead and avoid delays.

The first step is to document the existing conditions of your property. This includes taking site measurements, photographs, and notes in order to create accurate as-built drawings of your home and lot. These drawings form the foundation of your project and help make sure everything that follows is grounded in reality.

Next comes design. You will work with an architect or designer to lay out the ADU’s floor plan, elevations, and site plan in a way that meets your needs and complies with local and state codes. If your project includes a detached structure or any structural changes, you will likely need to coordinate with a structural engineer. Most projects also require energy compliance documentation, commonly referred to as Title 24, which must be prepared by a certified consultant.

Once the design is complete, your permit set is submitted to the local Planning and Building Departments for review. Under current state law, the city or county is required to respond to your application within 60 days. This is often called a ministerial review. They may make comments for clarification during this review, but they cannot request changes outside of the ministerial checklist. Once those comments are addressed and the permit is approved, you are ready to move forward.

At that point, you can hire a licensed contractor, pull the permit, and begin construction. A well managed ADU project typically takes several months from start to finish depending on your location, the design complexity, and contractor availability.

How I Can Help

As a licensed architect based in Martinez, I help East Bay homeowners design and permit Accessory Dwelling Units. Whether you are early in the planning stages or already thinking about construction I can guide you through each step of the process.

My work starts with documenting existing conditions using precise field measurements and photographs. I use that information to create accurate as-built drawings of your home and site. From there, I develop clear floor plans, elevations, and site plans that reflect your goals and comply with local and state requirements.

I coordinate with structural engineers and Title 24 energy consultants as needed to complete a permit-ready drawing set. I also stay involved through the submittal and review process to help address comments or revisions. My goal is to keep the process clear, efficient, and well organized so you can move forward with confidence.

The goal is to reduce delays, avoid surprises, and give you the peace of mind that your project is compliant and ready to build.

Ready to Start Your ADU?

If you are thinking about building an ADU or JADU on your property, now is a great time to explore your options. With streamlined laws, simplified permitting, and flexible design choices homeowners have more control than ever over how they use their property. Whether your goal is to create rental income, support multigenerational living, or simply make room to grow, I would be happy to help you take the next step. Visit www.bluesquare.studio to learn more or reach out directly to schedule a consultation. Together, we can turn your ideas into a clear buildable plan.