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Jul 23, 2024

How Big of An ADU Can You Build?

an accessory dwelling unit

When considering building an Accessory Dwelling Unit (ADU) in California, the allowable size can vary based on state laws and local regulations. As a rough guide, California state laws specify the following typical guidelines:

  • A floor area of up to 800 square feet
  • A maximum roof height of 16 feet
  • A setback distance requirement of 4 feet

However, it’s important to note that these guidelines can vary significantly depending on the specific city or county regulations. For example, detached ADUs in approximately 391 cities and counties across California allow sizes of up to 1,200 square feet. Attached ADUs, on the other hand, are generally restricted to no more than 50% of the main property’s square footage.

Conversion ADUs, which are converted from existing structures like garages or basements, typically do not have specific size restrictions imposed.

It’s crucial to consider that ADUs are intended to be secondary in size compared to the main residence, and the dimensions of the main residence are particularly relevant for attached ADUs. Generally, an ADU cannot exceed 50% of the square footage of the main residence it is attached to, although there may be exceptions with maximum size guidelines set by local regulations.

Additionally, height restrictions may also apply, ensuring that ADUs maintain a harmonious scale with existing residential structures.

Factors Determining ADU Size

Local Zoning Laws and Regulations

One of the primary factors influencing the size of an ADU is local zoning laws and regulations. Each municipality has its own set of rules governing the size and placement of ADUs. These regulations typically include maximum square footage allowances, lot size requirements, setback regulations, and sometimes height restrictions. It’s essential to consult with your local planning department or zoning board to understand the specific requirements applicable to your property.

Lot Size and Coverage

The size of your property and the percentage of the lot that can be built upon, known as lot coverage, also dictate the maximum size of your ADU. For instance, some areas may limit the total floor area of all structures on a lot, including the primary residence and any ADUs. Understanding these restrictions can help you determine the feasible size of your ADU project.

Primary Residence Size

In certain jurisdictions, the size of the ADU may be linked to the size of the primary residence. For example, regulations might specify that an ADU cannot exceed a certain percentage of the main house’s square footage or impose a fixed maximum size regardless of the primary residence’s size. This factor ensures that ADUs remain complementary to the existing residential character of the neighborhood.

Detached vs. Attached ADUs

Whether your ADU is attached to the primary residence or detached can also affect its size limitations. Detached ADUs often have more flexibility in terms of size compared to attached units. This distinction is important as it may influence the design and construction options available to you.

Cost Implications of ADU Size

The size of an ADU significantly impacts its construction costs. Typically, larger ADUs require more materials, labor, and possibly longer construction times. Here’s an estimated breakdown of costs based on size:

ADU Size (sq ft) Cost per Square Foot Total Estimated Cost
400 $200 $80,000
600 $200 $120,000
800 $200 $160,000
1,000 $200 $200,000

Understanding the maximum size of an ADU that you can build involves careful consideration of local regulations, lot size, and the type of ADU you plan to construct. By adhering to these guidelines, you can ensure compliance with zoning laws while maximizing the potential of your property.

Consultation with local authorities and professionals in ADU construction is crucial to navigating these requirements effectively and realizing your ADU project goals.

How Close to the Property Line Can You Build an ADU?

Building an ADU involves considerations beyond size, including setbacks from property lines. Understanding how close you can build an ADU to your property line is essential to avoid legal issues and ensure compliance with local regulations. To learn more about setback requirements and how they affect ADU construction, read our detailed guide on How Close to Property Line Can You Build an ADU?

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