As a trusted contractor at All County Development, we’ve worked with homeowners across California to help them maximize their property’s potential through the construction of Accessory Dwelling Units, commonly known as ADUs.
With so many homeowners looking to add extra living space, boost property value, or generate rental income, understanding the legal limits and possibilities of ADU sizes is essential.
In this article, we’re breaking down one of the most common questions we hear: How big can an ADU be in California? We’ll walk you through the state guidelines, local restrictions, and what you need to know to plan your next project wisely.
Key Takeaways
- ADU size regulations are influenced by both California state laws and local zoning ordinances.
- The maximum size for detached ADUs in California is generally up to 1,200 square feet.
- Attached ADUs are typically limited to 50% of the primary residence’s square footage, up to a maximum allowed by local code.
- Junior ADUs (JADUs) must be no more than 500 square feet.
- Homeowners should always verify local jurisdiction rules, as they may be more restrictive or offer additional flexibility.
- Contact All County Development at 818-720-3316 to start planning your ADU project today.
Understanding California’s Statewide ADU Guidelines
In 2017 and again in 2020, California passed legislation aimed at making it easier for homeowners to build ADUs. These laws were designed to address the state’s housing shortage by encouraging the development of small, independent housing units on residential lots.
Under current California state law, the following size limits apply:
- Detached ADUs can be up to 1,200 square feet, regardless of the size of the primary residence.
- Attached ADUs are generally limited to 50% of the main home’s floor area, although some local governments allow up to 1,200 square feet.
- Junior ADUs (JADUs) must be created from existing space within the primary dwelling and cannot exceed 500 square feet.
These laws set a baseline—local jurisdictions must allow at least these minimums. However, cities and counties are also allowed to impose more specific regulations based on zoning, lot size, and other factors.
Local Zoning and Its Impact on ADU Size
Even though the state allows up to 1,200 square feet for detached ADUs, it’s important to understand that your local jurisdiction has the final say. Some cities have additional rules that may limit the size of an ADU based on:
- Lot coverage percentages
- Floor Area Ratio (FAR)
- Setback requirements
- Height restrictions
- Parking requirements
For example, while the state may allow a 1,200-square-foot unit, your city might require a maximum lot coverage of 40%, which could reduce your allowable ADU size depending on your existing structures.
As contractors, we frequently help clients navigate these local regulations. At All County Development, we start each ADU project by conducting a detailed site assessment to determine what’s feasible based on your city’s ordinances and your property layout.
Attached vs. Detached ADUs
There are two primary types of ADUs you can consider: attached and detached.
Attached ADUs
Attached ADUs share at least one wall with the primary residence. Their maximum size is typically 50% of the primary home’s square footage. So, if your main house is 2,000 square feet, your attached ADU could be up to 1,000 square feet, depending on local codes.
Detached ADUs
Detached ADUs are stand-alone structures. Under state law, you can build up to 1,200 square feet for a detached ADU. This type offers more privacy and design flexibility and is ideal for homeowners seeking to rent out the space or use it as a guest house or home office.
Both options have their pros and cons, and the right choice depends on your specific needs, available space, and budget. We at All County Development help clients compare the two options with a detailed feasibility study.
Height and Setback Requirements
While size is a key consideration, it’s not the only factor. Your ADU must also comply with:
- Height limits – Typically between 16 to 25 feet, depending on the city and ADU type.
- Setbacks – Minimum 4-foot side and rear yard setbacks are required for most detached ADUs.
These rules ensure that your ADU does not encroach on neighbors’ space and that it maintains a proper fit within your neighborhood.
It’s also worth noting that newer legislation has made it easier to build ADUs in certain zones by removing minimum lot size requirements and streamlining the permitting process.
Special Cases: Multi-Family Properties
If you own a multi-family property, you can often build more than one ADU. The general rules for multi-family properties are:
- Up to two detached ADUs allowed.
- You may also be able to convert non-livable spaces (like storage rooms or garages) into multiple interior ADUs, depending on local rules.
This presents an excellent opportunity for property owners looking to generate multiple rental income streams or provide housing for extended family.
Why Size Matters in Planning Your ADU
Determining how big your ADU can be isn’t just about legality; it’s also about functionality and return on investment. The right size:
- Ensures efficient use of space
- Improves livability and comfort
- Enhances resale value
- Meets your goals for rental income or family use
At All County Development, we consider all of these factors when designing your ADU. We believe every square foot should serve a purpose, and we work with you to create a space that adds real value to your property.
Ready to Build Your ADU? Let’s Talk.
Building an ADU in California involves more than just understanding square footage. It requires navigating local laws, maximizing design potential, and staying compliant every step of the way.
If you’re considering building an ADU and want to know how big yours can be, give us a call at 818-720-3316. Our team at All County Development is here to answer your questions, assess your property, and guide you from concept to completion.
We’ve helped countless homeowners turn their ADU dreams into reality, and we’d love to help you, too.
Let’s make the most of your property together.