In California, an ADU can be up to 1,200 square feet, though some cities have their own local rules that adjust the maximum size. Certain cities adopt more generous regulations to encourage additional housing, while others enforce setbacks, height limits, or lot coverage rules that can reduce the buildable size. Statewide, ADUs under 750 square feet are exempt from impact fees, which is why many homeowners choose to cap their ADU at that size when budgeting is a concern.
