State law permits ADUs up to 1,200 sqft for detached units. 5,000–7,000 sqft typical residential lots. No minimum lot size required to build an ADU.
18 feet for detached ADUs, plus an additional 2 feet to accommodate roof pitch. Two-story ADUs are permitted in most residential zones.
4-foot minimum from rear and interior side property lines for new detached ADUs. Second-floor/two-story ADUs may have additional setback requirements under Costa Mesa's objective design standards.
Optional — Costa Mesa does not require additional parking for ADUs in most cases, following state exemptions. Properties near OCTA transit routes are explicitly exempt.
No owner-occupancy requirement (permanent per AB 976).
Estimated cost $250K–$400K. Permit fees $5K–$12K. Impact fees: Waived under 750 sqft.
Costa Mesa sits at the geographic center of Orange County, bordered by Newport Beach, Huntington Beach, and Irvine. With a population of about 115,000, the city is home to South Coast Plaza — one of the largest shopping centers in the United States — and the Segerstrom Center for the Arts. Costa Mesa's central location and diverse neighborhoods ranging from the Eastside to Mesa Verde make it a strong rental market. The city allows a minimum ADU size of just 150 sqft (one of the smallest in the county), giving homeowners flexibility for compact studio units. Costa Mesa also permits two-story detached ADUs up to 18 feet, with additional height for roof pitch.