Free Tool
ADU Size Calculator
Find the maximum ADU size allowed on your property in any of 100 California cities. Real data from official city planning departments, not generic state minimums.
Includes setbacks, height limits, lot coverage rules, and units-per-lot allowances.
California ADU Size Rules at a Glance
State law sets a floor. Cities can be more permissive but not more restrictive than these statewide minimums.
Detached ADU
Up to 1,200 sq ft
State floor: 850 sq ft (1BR) or 1,000 sq ft (2BR+). No minimum lot size required.
Attached ADU
Up to 50% of home, max 1,200 sq ft
Must be physically attached to the primary dwelling. State floor still applies.
JADU
Up to 500 sq ft
Must be within the existing home. Requires separate entrance and efficiency kitchen.
Garage Conversion
Follows detached ADU rules
Converting an attached garage to an ADU does not require setback compliance per state law.
ADU Size Rules Explained
How big can an ADU be in California?
California state law (Gov. Code §66321) guarantees homeowners the right to build a detached ADU of at least 850 sq ft (one bedroom) or 1,000 sq ft (two or more bedrooms), regardless of local restrictions. Most cities allow up to 1,200 sq ft for detached ADUs. JADUs are capped at 500 sq ft.
Can my city limit ADU size below 1,200 sq ft?
Yes, but only down to a floor. A city must allow at least 850 sq ft for a studio or one-bedroom ADU and 1,000 sq ft for two or more bedrooms, and it can never block an 800 sq ft ADU with 4 ft setbacks. Many cities allow 1,200 sq ft. Use the calculator above to see your city's limit.
What are the setback rules for ADUs in California?
State law requires cities to allow ADUs with a maximum 4-foot side and rear setback. Front setbacks typically match the primary dwelling's requirement. Some cities have adopted exactly 4-foot setbacks; others are more permissive. Setbacks within existing structures may be reduced further.
Does lot size affect how big my ADU can be?
State law removed minimum lot size requirements for ADUs. You can build an ADU on any residential lot, regardless of size. However, lot coverage limits (if your city has them) and physical setback requirements may constrain how large you can build on a small lot.
What is a JADU and how big can it be?
A Junior ADU (JADU) is created within a single-family home, typically a converted bedroom or attached garage. JADUs are capped at 500 sq ft statewide. They need a separate entrance and an efficiency kitchen, and can share a bathroom with the main home (in which case the owner may be required to live on site).
How many ADUs can I build on my property?
Most single-family lots in California can have up to three units: one ADU converted from existing space, one JADU, and one new detached ADU of up to 800 sq ft. Lots with an existing multifamily building can add up to 8 detached ADUs (not more than the number of existing units) plus conversions of non-livable space for up to 25% of existing units. Some cities allow more.
Know Your City's Rules. Now Build.
Get quotes from licensed contractors who know your local requirements.
Get Free Contractor Quotes