How Big Can an ADU Be?
It depends on your state and city. California allows detached ADUs up to 1,200 square feet, while many other jurisdictions cap ADUs between 600 and 1,000 square feet or at a percentage of the primary home's size. Lot size, setbacks, and height limits determine what is actually buildable.
Best price · Quotes back fast · PE stamps backed by Apex Grid Engineering
State maximums
California's 1,200-square-foot allowance for detached ADUs is among the most generous in the country. Other states set lower caps or leave the number to cities, which commonly land between 600 and 1,000 square feet. Some jurisdictions also allow larger ADUs on bigger lots through tiered rules. The state maximum is the starting point, not the final answer.
Local caps that shrink the number
Cities add their own limits: floor-area ratios, lot-coverage maximums, and rules tying ADU size to a percentage of the main house. A 1,200-square-foot state allowance means little on a small lot with 40 percent coverage limits. Height limits, often 16 feet for detached ADUs, constrain two-story ambitions more than floor-area rules do.
What fits in practice
Model the buildable envelope first: apply setbacks, height, lot coverage, and parking to your specific lot, then see what floor area survives. Many homeowners discover the practical maximum is well under the legal maximum. Designing to the real envelope avoids the disappointment of a plan that is legal on paper but unbuildable on the lot.
The bottom line
The legal maximum and the buildable maximum are different numbers. Model both for your specific lot before designing.