Key Takeaways

• BE SURE TO CHECK LOCAL SIZE LIMITS FOR DETACHED AND ATTACHED ADUS IN CONTRA COSTA, WHICH VARY BY DISTRICT, LOT SIZE, AND ZONING.
• You want to keep a sharp eye on minimum lot size, height, and lot coverage requirements, since these all strongly impact the viability and design of your ADU project.
• Whether you opt for detached, attached, or garage conversion ADUs, each bears unique advantages, design possibilities, and regulatory implications that impact privacy, cost, and property integration.
• You might qualify for the expedited approval process for ADUs, which helps lower wait times and ease permits, if your project fits certain requirements.
• You still need to make sure your plans conform to zoning, setback, and design standards — to prevent expensive snags in the approval and construction process.
• You can optimize your investment with ADU by designing for long-term value, rental income potential, future adaptability, and sustainability.

You can construct a detached ADU up to 74 m2, whereas attached ADUs can be 50% the size of your primary residence’s floor area but no larger than 74 m2. For junior ADUs, the cap is 50 square meters. Your lot must comply with setback, height, and parking regulations imposed by the city or county. These limits ensure ADUs remain at a scale and style appropriate to the neighborhood. Being aware of these regulations will allow you to navigate your project to minimize expensive delays. The following section categorizes ADU types and provides you with an overview of what to anticipate in Contra Costa.

Contra Costa ADU Size Limits

ADU size limits vary based on your property’s location, the zoning district, and whether your accessory dwelling unit is attached or detached. Understanding these ADU requirements is crucial if you plan to design or construct an ADU that complies with local regulations. Familiarity with these standards not only saves time and costly mistakes but also aids in making informed choices.

1. Maximum Size

In Contra Costa County, the adu requirements for detached accessory dwelling units (ADUs) dictate that they cannot exceed 93 square meters (1,000 square feet) for most residential lots. However, if your property is over 1,115 square meters (12,000 square feet) or located in an agricultural district, you may construct up to 111 square meters (1,200 square feet). In the Kensington (-K) district, the regulations become even stricter, limiting detached ADUs to a maximum of 56 square meters (600 square feet) or 60% of your primary dwelling unit’s floor area. For larger units in this district, obtaining a special building permit is necessary, which adds complexity to your project due to additional processing steps. Attached ADUs in Contra Costa are also limited to either 50% of your home’s existing living space or 1,200 square feet (~111 square meters), whichever is less. Always confirm your property’s zoning requirements before proceeding with your ADU plans, as these can significantly impact your project.

2. Minimum Size

Contra Costa has no minimum ADU size, allowing homeowners to explore various accessory dwelling unit options. Technically, your ADU needs only one habitable room of at least 6.5 sqm (70 sq ft), giving you the freedom to keep things tight. To include an ADU in addition to your primary residence, your lot must be a minimum of 557 square meters (6,000 square feet). In the Kensington neighborhood, the cap increases to 929 square meters (10,000 square feet). These minimums can limit ADU possibilities in denser neighborhoods or on older, smaller lots, so consult your zoning map and local ADU requirements before planning your project.

3. Height Rules

Height limits contour your accessory dwelling unit’s style and what you’re allowed to construct above grade. In the Kensington neighborhood, your ADU cannot exceed 4.3 meters (14 feet) in height, adhering to specific ADU requirements. Other districts in Contra Costa may permit taller units, but you’ll need to check your zoning code. Height caps can influence your design choices, whether you create a loft, add storage above, or opt for a pit roof. These rules, if not followed, can delay or even jeopardize your permit application, so measure cautiously and collaborate with your builder to keep plans in check.

4. Lot Coverage

In typical single-family zones, your overall lot coverage—including the accessory dwelling unit—cannot exceed 40%. Some multi-family zones impose smaller caps, ranging from 25% to 35%. Coverage rules can restrict your ADU’s square footage, particularly if your primary dwelling unit and other structures are already coverage-heavy. Before finalizing your adu plans, check your existing lot coverage against allowed limits. Not checking can mean having to shrink or redesign your ADU mid-process.

Detached VS. Attached ADUs

Here’s what you must know before starting your project: detached versus attached accessory dwelling units (ADUs). All of which have particular size regulations, zoning requirements, and advantages. Detached units are stand-alone and often provide additional privacy and design flexibility. Attached units are connected to the main home and often share walls and utilities. This type selection will impact the utilization of your space, the feasibility of ADU construction, and your budget.

Detached Units

Detached accessory dwelling units (ADUs) in Contra Costa can be up to 112 sq m. (1,200 sq ft..). If your property is spacious or agriculturally-zoned, you might even be permitted additional room under the local requirements. You need a four-foot setback from side and rear property lines, and height is typically limited to 4.9m/16ft. These size regulations serve to maintain the structure’s scale relative to neighboring houses and preserve the privacy of neighbors.

A detached ADU offers you additional privacy and autonomy. If you intend to rent, a separate unit tends to be more desirable for tenants. Detached units are frequently permitted as full rentals, whereas attached units often have stricter owner-occupancy regulations. Its stand-alone characteristic means you’re free to choose whatever layout or style you like, which can include essential amenities like a kitchenette and bathroom. If your lot is large enough, you possess even greater design flexibility for ADU plans, allowing you to customize the area for extended visitors, family members, or tenants.

You’ll likely encounter more permit steps and local inspections for detached ADUs, including the need for an ADU permit. Prepare to submit plans and extra approvals, as this can impact your timeline and budget. Check your local codes before you begin, and if you want to maximize the space, collaborate with an architect who understands how to design small, livable spaces effectively.

Attached Units

Attached accessory dwelling units (ADUs) are required to comply with either the same size regulations for detached units or up to 50% of your main home’s living area, whichever is less. This could be as much as 112 square meters (1,200 square feet) in some locations, but is frequently capped at 79–93 square meters (850–1,000 square feet). Additionally, attached units generally must adhere to the same height and setback regulations as your primary dwelling unit, so it’s essential to verify what is permitted in your municipality regarding ADU permits.

If you want to save money, opting for attached ADUs is usually the wiser choice. Sharing walls, pipes, and power lines with the main house significantly reduces construction expenses. Moreover, you can share the heating or cooling system, making attached ADUs particularly attractive for multi-generational homes or as a means of adding space for work or guests without a massive investment in new construction.

Even adding an attached ADU can boost your property value. If you ever go to sell, buyers will view the additional living space as a perk. Consider how the addition will appear. If you integrate the design nicely, the entire lot feels like a single home. Bad design could damage curb appeal. Attempt to have rooflines, colors, and window styles correspond.

Garage Conversions

Converting the garage to an ADU means you don’t have to fuss with setbacks. We leave your existing structure in place, so you can utilize the entire footprint of your garage.

Garage conversions typically cost less than new builds and complete more quickly. You leapfrog certain things, like new foundations. This can make the project significantly more cost-effective and fast, so it’s a smart option if you want to add space quickly or keep costs low.

You still have to comply with size and design regulations imposed by your city. For instance, you can’t supersede the garage’s size or violate height limits. Your plan needs to conform to the envelope of the current structure.

Before you begin — inspect your garage. If the structure is compromised, you could end up spending more than you intend. Have a pro check it out first.

Standard VS. Streamlined ADUs

When you build an Accessory Dwelling Unit (ADU) in Contra Costa, you face two main approval tracks: the standard process and the streamlined ADU process. Standard ADUs have more steps, more reviews, and closer code compliance, while streamlined ADUs aim to simplify the permit application process, making it quicker to obtain required permits. Your decision between them significantly influences your schedule, expenses, and even the size, regulations, and design of your ADU projects.

Standard Process

You’ll begin with a comprehensive application that must include site plans, elevation drawings, and compliance documents to meet the ADU requirements. Local building officials will review your plans for fire safety, setbacks, parking, and environmental rules, ensuring that your project adheres to the necessary building permit guidelines. Inspections occur at multiple construction phases, all while remaining compliant with local codes and zoning regulations.

This process can be quite strict. Typically, the maximum size for a standard accessory dwelling unit (ADU) is capped at 50% of your main house’s total floor area. For example, if your primary residence is 120 m2, your ADU cannot exceed 60 m2. Local governments may require your design to fit the neighborhood style, which can limit your options for contemporary or unconventional architecture. Obtaining permits for standard ADUs can sometimes take over 60 days, with certain projects facing delays in paperwork or regulatory review due to compliance with local ADU legislation. Additionally, you might need to address neighbor concerns or satisfy other parking and landscaping requirements, which can further increase time and costs.

Preparation is key. A diligent application and thoughtful planning can prevent delays in your ADU development. Homeowners who don’t anticipate these hurdles often find their projects stalled. We advise consulting with an architect or planner early in the process to ensure that you meet every requirement the first time around.

Streamlined Process

The streamlined process is easier. If your project checks a few boxes—such as having been built in existing structures, on lots of specific size and zoning—you can bypass many of the lengthy reviews that a standard project undergoes.

The streamlined ADUs can go from application to permit in under 60 days. The standards are lower — particularly in California’s 2023 ADU laws, which promote construction in underutilized commercial areas and elsewhere to expand housing availability. Design rules are loosened, allowing you to utilize more flexible or contemporary styles. The size cap is frequently greater than for standard ADUs, occasionally allowing for larger units where local jurisdictions allow. Owner-occupancy rules to phase out in 2025, giving investors and property owners new flexibility in where and how to build.

If you desire to complete your project quickly and your property is eligible, this route provides obvious advantages. Less paperwork, fewer inspections, and quicker reviews = being able to build sooner. It’s a sensible option for most, particularly anyone looking to capitalize on rental income or rapidly house relatives.

Key Differences And Considerations

Standard ADUs have to follow more rules, so it’s a slower and frequently more expensive process. Streamlined ADUs reduce the threshold, in particular, for those hungry to construct quickly or leverage innovative design concepts.

Select what makes sense for you and your property type. If your lot is petite or you want to go big, streamlined ADUs can provide additional flexibility. Those with historic homes or in very strict neighborhoods may only be eligible for the standard process.

Local officials say streamlined ADUs can ease housing crunch, but others fear impact on neighborhoods. It’s crucial to balance these considerations, depending on your priorities.

Decision Factors

First, check your local requirements regarding accessory dwelling units. Know your primary residence size and inquire whether your lot is eligible for streamlined aADU permits.

Navigating The Approval Process

Getting approval to construct an accessory dwelling unit (ADU) in Contra Costa involves navigating a multi-step process influenced by local and state regulations, including specific ADU requirements. You must satisfy requirements for zoning, design, construction, and the use of the property. The local building department reviews your ADU plans, ensuring they comply with state law and city codes. With state ADU rules having evolved since 2020, stay up to speed with local changes to keep your project on track.

Planning Application

A planning application is your opening gambit in the process of obtaining an ADU permit. You’ll need to submit site plans, floor plans, elevations, and evidence of ownership. Fee amounts vary by city, and you typically have to demonstrate conformance with setbacks, lot coverage, and access. If your city has a pre-approved accessory dwelling unit program, leverage it. These programs, which are now prevalent in many regions, can expedite your review because your design has already cleared fundamental screenings.

How you fill it out can shift your timeline. If you bypass a required form or tick the incorrect box, your approval grinds to a halt. This delays not only your project but the 60-day permit decision clock as well. Cities are serious about this regulation, but only if you have a complete application. Double-check your forms and attach all required documents. Others stumble over fuzzy property lines or overlooked design standards — both of which are simple to avoid with a little preparation.

Planning reviews verify that your ADU is within the permissible size—up to 1,200 sqft for regular units or 500 sqft for junior units. Cities can also query parking, although AB 157 now exempts many streamlined ADUs from parking mandates. Design standards like setbacks and height arise. Know your city’s ADU ordinance before submittal, as rules can change, and violating a new rule means starting over.

Building Permit

After your planning application passes muster, you’ll require a building permit before construction of your accessory dwelling unit (ADU). This step matters: building without a permit can mean fines or tear-down orders. Your permit application will require technical drawings, structural information, and a comprehensive materials list. A few cities allow you to use pre-approved adu plans, which can reduce review time significantly. If you’re planning to rent or sell the ADU, see if there are additional requirements at this point.

Once your permit is issued, the building department checks your work. These inspections cover everything from foundation and framing to electrical and plumbing, culminating in a final fit-out. Local building codes and ADU requirements are verified at every stage. If your plans don’t conform to existing energy efficiency or safety standards, anticipate delays. Almost every city puts its inspection checklists online—review them early to avoid complications.

Not all problems are technical. Law or city policy changes can impact the approval window or what you need to provide. Keep in contact with your city’s planning staff and monitor online notices. A few cities provide resources to guide you through the process or provide workshops. If you run into a roadblock, professional assistance from a local architect or planning consultant can be time-saving.