ADU Construction in Costa Mesa

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.

Zoning & requirements

Setbacks

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.

Height

18 feet for detached ADUs, plus an additional 2 feet to accommodate roof pitch. Two-story ADUs are permitted in most residential zones.

Parking

Optional — Costa Mesa does not require additional parking for ADUs in most cases, following state exemptions. Properties near OCTA transit routes are explicitly exempt.

Lot Size

5,000–7,000 sqft typical residential lots. No minimum lot size required to build an ADU.

Owner Occupancy

No owner-occupancy requirement (permanent per AB 976).

Additional Notes

Detached ADUs capped at 1,200 sqft. Attached ADUs: up to 50% of existing home sqft or 1,000 sqft, whichever is greater. Minimum allowable ADU size is 150 sqft — among the smallest in Orange County.

How to permit your ADU in Costa Mesa

Review your property

Check your zoning and lot dimensions on Costa Mesa's GIS system. Most R1 and R2 residential zones allow ADUs by right. Confirm utility connections and easements with the Engineering Division.

Design your ADU

Create architectural plans meeting Costa Mesa's objective design standards. Two-story designs must comply with upper-floor setback requirements. Include structural engineering, Title 24, and MEP plans.

Submit application

Submit plans to Economic & Development Services. Costa Mesa processes ADU applications ministerially (no public hearing required). The city is required to review compliant applications within 60 days.

Plan check corrections

Address any corrections from the plan check review. Costa Mesa's planning staff can clarify design standard requirements. Resubmit corrected plans for final approval.

Obtain permit & build

Pay fees and pull your building permit. Begin construction with a licensed contractor. Schedule inspections at each construction milestone.

Final approval

Pass final inspection and receive Certificate of Occupancy. Your ADU is immediately available for occupancy or rental.

Costa Mesa Economic & Development Services — Planning Division • Fees: $5,000–$12,000 total (plan check, building permit, school fees). Impact fees waived for ADUs under 750 sqft. • Timeline: 4–8 weeks for plan check. State-mandated 60-day maximum for compliant applications.