California’s ADU boom is in full swing, with over 80,000 units approved since 2016—and now, it’s easier than ever to build one. New laws in 2024 have removed owner-occupancy rules, fast-tracked permits to 60 days, and expanded financing options, including up to $40,000 in grants.
But is your property ADU-ready? From size limits and zoning laws to utility requirements and financing options, this guide will walk you through everything you need to know before starting your ADU project. Let’s dive in!
Understanding ADU Size Requirements in California
When it comes to building an Accessory Dwelling Unit (ADU) in California, size matters—and not just for comfort and functionality. ADU size regulations dictate what you can build, where you can build it, and how local jurisdictions apply state-mandated flexibility. Whether you’re planning a detached backyard unit, an attached ADU, or converting an existing space, understanding these rules is the key to a smooth approval process and maximizing your property’s potential.
Maximum ADU Sizes: How Big Can You Build?
California has statewide ADU size allowances, but local jurisdictions may impose additional zoning restrictions. Here’s what the state law permits:
Detached ADUs – Can be built up to 1,200 square feet, regardless of the primary home’s size.
Attached ADUs – Limited to 50% of the main home’s living area, but cannot exceed 1,200 square feet.
Standard ADU allowances – Local agencies must allow at least 800 sq. ft. with a 16-foot height limit, ensuring homeowners can build even in areas with stricter zoning.
Multifamily properties – Can now have up to eight detached ADUs, provided they don’t exceed the number of existing primary dwelling units on the lot.
Garage conversions & adaptive reuse ADUs – These do not have to follow the 1,200 sq. ft. cap, making them a great option for homeowners with underutilized spaces.
Minimum Square Footage: What’s Required for Livability?
ADUs must be functional and livable, so California enforces minimum square footage requirements to ensure basic living standards.
150 sq. ft. is the absolute minimum size for any ADU to qualify as a legal dwelling.
Efficiency dwelling units (small studio-style ADUs) must include:
- A separate closet
- Kitchen facilities (sink, cooking appliance, refrigerator)
- A private bathroom (toilet, sink, shower, or bathtub)
One-bedroom ADUs must be at least 850 sq. ft., while two-bedroom ADUs require a minimum of 1,000 sq. ft., unless local ordinances allow smaller units.
Junior ADUs (JADUs) – These must stay under 500 sq. ft. and be built within the main residence. They provide a more affordable, compact housing option while following distinct regulations from standard ADUs.