Accessory Dwelling Units (ADUs) have become one of the most popular ways to add livable space, boost property value, and create rental income. But before you begin designing your backyard cottage or in-law suite, one key question stands out: How big can an ADU be in California, legally?
The good news is that California has passed progressive laws to make ADU construction easier and more accessible, but there are still important limits on square footage, height, and placement. These rules vary depending on your city, lot size, and the type of ADU you’re building. So, overlooking them could lead to costly redesigns or permit delays.
In this guide, we’ll break down the statewide size regulations, explain when and how you can build a larger ADU, and share smart tips for maximizing space while staying within legal limits. If you’re planning to build an ADU in California, this is where to start.
Statewide Limits: How Big Can an ADU be in California?
Under California state law, homeowners are allowed to build an ADU of up to 800 square feet regardless of lot size or local zoning, provided the unit meets basic requirements for height, setbacks, and access. This rule was put in place to prevent cities from enforcing overly restrictive size limits that could block homeowners from adding ADUs altogether.
Here’s what the law currently allows:
- Detached ADUs can be up to 800 sq ft by right, with 4-foot side and rear setbacks and a 16-foot height limit.
- Attached ADUs may be larger, but the total cannot exceed 50% of the existing primary residence’s floor area unless local rules permit more.
- These units must be independent, meaning they include a full kitchen, bathroom, and separate entrance.
- No minimum lot size is required, and parking waivers apply in many areas close to transit.
These statewide standards serve as a legal floor, not a ceiling. Local jurisdictions may allow you to build larger depending on zoning, lot configuration, and project type.
When Can You Build a Larger ADU?
While 800 square feet is the standard baseline allowed by California law, many homeowners can legally build a larger ADU, often up to 1,000 or even 1,200 square feet, depending on local rules, lot size, and property layout.
Here’s when you may be allowed to build bigger:
- Larger Lots or Specific Zoning: Some cities permit ADUs up to 1,200 sq ft if the lot is large enough or zoned appropriately. Check your local development standards for thresholds.
- Detached vs. Attached ADUs: An attached ADU may be allowed to reach 50% of the primary home’s size. For example, if your main house is 2,000 sq ft, your attached ADU could be up to 1,000 sq ft.
- Multistory or Garage Conversions: Building above a garage or converting a large existing garage can increase usable space. Some cities permit two-story detached ADUs with square footage limits per floor.
- Flexible Local Ordinances: Certain jurisdictions allow larger ADUs outright, especially if they meet design criteria or provide long-term rental housing.
Keep in mind that even when a larger unit is allowed, you’ll still need to follow height, setback, and lot coverage rules. That’s where working with an experienced design-build team can help ensure compliance while maximizing space.
Junior ADUs and Size Considerations
In addition to traditional ADUs, California allows homeowners to build Junior Accessory Dwelling Units (JADUs) which is a smaller, more compact housing option that can be added within the walls of an existing home.
Here’s what you need to know about JADU size limits and rules:
- Maximum Size: A JADU can be up to 500 square feet and must be created within the existing footprint of a single-family home (often converted from a bedroom or part of the garage).
- Kitchenette Required: JADUs require a small kitchenette but do not need a private bathroom, they can share facilities with the main house.
- Owner Occupancy: The owner must live in either the primary residence or the JADU.
- Parking and Setbacks: Many JADUs qualify for parking exemptions, especially in areas near public transit.
Can you build both an ADU and a JADU on the same property? In many cases, yes, California law permits both on lots with single-family homes, giving you a creative way to add two housing units and make the most of your space.
How Local Jurisdictions Can Affect ADU Size
While California sets baseline rules for ADU construction, local cities and counties still play a major role in determining what you can build on your property. In some cases, local jurisdictions allow you to exceed the state’s 800 sq ft minimum, but they may also impose additional conditions related to design, placement, or infrastructure.
Here’s how your local government may impact ADU size:
- Zoning and Lot Coverage Limits: Some cities allow up to 1,200 sq ft for detached ADUs on large lots, while others cap size based on lot area or total building footprint.
- Height Restrictions: Local rules may limit your ADU’s height to 16 ft or allow more for two-story units, especially in low-density zones.
- Design Guidelines: Cities like San Francisco, Oakland, and San Jose may require your ADU to match the architectural style of the main house or meet special privacy or setback rules.
- Hazard Zones and Utilities: In areas prone to wildfires, floods, or other hazards, additional requirements may apply that influence where and how big your ADU can be.
- Permit Pathways and Reviews: Some jurisdictions offer streamlined approval for ADUs under 800 sq ft, while larger units may trigger discretionary review or neighbor notification.
That’s why it’s essential to check with your local planning department or work with a knowledgeable contractor who understands city-specific ADU ordinances. What works in one Bay Area city may not be allowed just a few miles away.
Build Smart, Stay Legal
Understanding the size limits for ADUs in California is the first step toward building a space that meets your goals without running into red tape. While state law guarantees up to 800 square feet by right, many homeowners can build larger depending on their lot, zoning, and local ordinances. Knowing your options, and your limits, helps you make informed decisions, avoid costly delays, and create an ADU that truly adds value.
At Green Group Remodeling, we specialize in helping Bay Area homeowners plan and build legally compliant, beautifully designed ADUs that maximize every square foot. Whether you’re converting a garage, adding a detached guest house, or exploring a larger two-bedroom ADU, we’re here to guide you from permits to final inspection.
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