Everything homeowners need to know about building an accessory dwelling unit in Costa Mesa - size limits, setbacks, permits, and the state review that pushed the city to rewrite its ADU ordinance in 2025.
Last verified: August 2026
6/10 · ADU-friendly, with a rocky compliance history
1,200 sq ft*
Max ADU size
18-27 ft*
Max height
Not required
Owner-occupancy
By-right
Permit type
This page summarizes California's statewide ADU law (Government Code Sections 66310-66342) together with Costa Mesa's local standards under Municipal Code Section 13-35, for general reference - it is not legal advice. As explained below, state housing regulators found the city's prior ADU ordinance out of compliance on 21 separate points in 2024, and the city adopted a substantially rewritten ordinance in 2025 to address most of them. Always confirm current requirements with Costa Mesa Economic and Development Services before designing or applying for permits. See sources below.
Zoning & size requirements
What you're allowed to build
ADU type
Max size
Max height
Notes
Detached (single-family lot)
Up to 1,200 sq ft
Up to 27 ft or two stories; at least 18 ft is always allowed, and a second floor or two-story ADU is allowed if it meets five specific siting conditions
A smaller 800 sq ft version with 4 ft side/rear setbacks and an 18 ft height cap is guaranteed regardless of other standards
Attached (single-family lot)
Greater of 50% of the primary dwelling's floor area or 1,000 sq ft
Same general height rules as a detached ADU
Must generally match the primary home's design, materials, finishes, and colors
Limited to the existing structure's height unless enlarged, in which case standard height rules apply
Up to 150 sq ft may be added to a converted accessory structure solely for ingress and egress
Junior ADU (JADU)
Up to 500 sq ft
Within the primary home's existing height
Must be entirely within an existing or proposed single-family home, including an attached garage
Allowed zoning districts
Single-family, multifamily, and mixed-use zones citywide, per the city's Citywide Land Use Matrix, on any legal lot with a residential dwelling that will remain on site. Unlike some neighboring Orange County coastal cities, Costa Mesa sits inland of the immediate coastline and does not appear to carry a Coastal Zone overlay, so a separate Coastal Development Permit generally isn't a factor for ADU projects here - confirm your parcel with Planning if you're near the city's edge.
Additional standards
Setbacks, parking & other rules
Setbacks
4 ft from side and rear property lines, plus full compliance with front-yard setbacks for the underlying zone, unless that would block an 800 sq ft ADU with 4 ft setbacks; none for conversions or same-footprint rebuilds; 6 ft minimum separation between a detached ADU and any other structure on the lot
Parking
No additional parking is required beyond what's already required for the primary home in most cases; where parking is required, it can be tandem or located in a setback area, and demolished parking doesn't have to be replaced
Units per lot
On a single-family lot, one ADU (detached, attached, or conversion) plus one JADU; qualifying multifamily lots can add more under state law, including up to eight detached ADUs under SB 1211
Owner-occupancy
Not required for a standard ADU. A JADU still requires the owner to occupy either the primary home or the JADU, unless the owner is a government agency, land trust, or qualifying housing organization
Building code
Must meet the California Residential Code as locally adopted; the city allows ADUs as small as 150 sq ft, below the general 220 sq ft efficiency-unit minimum
Rental terms
ADUs and JADUs may only be rented for terms of 31 days or longer. The city kept this rule for JADUs even after state housing regulators flagged it as inconsistent with state law, adopting its own findings that the rule serves the state law's housing-supply purpose
Ordinance history
How Costa Mesa's ADU rules got here
2017Statewide by-right ADU approval begins. California law requires cities to ministerially approve qualifying ADU applications, without discretionary review or a conditional use permit.
Feb 22, 2023Costa Mesa adopts Ordinance No. 2023-03. The city's standalone ADU ordinance under Municipal Code Section 13-35.
Mar 25, 2024State ADU law is renumbered. SB 477 relocates the statewide ADU statute from Government Code Section 65852.2 to a new Sections 66310-66342, immediately making the city's old code citations outdated.
Sep 11, 2024State housing regulators find the ordinance noncompliant. HCD's written findings identified 21 separate respects in which Ordinance No. 2023-03 fell short of state ADU law, including outdated statute references, an unlawful limit on combining ADU and JADU types, an owner-occupancy requirement for standard ADUs, a subjective "compatible" design standard, and overly restrictive front setbacks. The city had until October 11, 2024 to respond.
Feb-Apr 2025City rewrites the ordinance. The Planning Commission recommended a fix on February 24, 2025; the City Council introduced it March 18, 2025 and adopted Ordinance No. 2025-02 by a 7-0 vote on April 1, 2025, taking effect 31 days later. The rewrite addressed nearly every HCD finding, but the city explicitly kept its 31-day minimum rental rule for JADUs, adopting legislative findings defending the rule under the state law's own stated housing-supply purpose.
2025New programs launch alongside the fix. The city rolled out a Safe ADU Legalization Program under AB 2533, letting owners of ADUs built without permits before January 1, 2020 bring them up to code, plus a pre-approved ADU design program to speed up review.
What this means for an ADU applicant today: the 21-point noncompliance finding was a real, documented problem, not a rounding error, but the city moved on it quickly, adopting a comprehensive fix within about seven months. The standards summarized on this page reflect that April 2025 rewrite. The one open question is the 31-day minimum rental rule the city chose to keep for JADUs against HCD's specific objection; whether that holds up hasn't been tested. That combination, real problems found and largely fixed, with one disputed holdout, is why this page scores Costa Mesa in the middle of the pack rather than at the top.
Permit process
How Costa Mesa ADU permitting works
1
Pre-application check
Confirm your zoning and which ADU category (detached, attached, conversion, JADU, or a combination) fits your lot with Economic and Development Services.
2
Prepare your plan set
You'll need a site plan, floor plans and elevations for the ADU, and floor plans of the existing house if your size calculation depends on it.
3
Submit for ministerial ADU review
Applications that meet the standards are approved without a public hearing or discretionary decision.
4
Plan review & decision
State law caps ministerial review at 60 days from a complete application.
5
Building permit & inspections
Pull your building permit and schedule the standard framing, plumbing, electrical, and final inspections as construction proceeds.
6
Record the required covenant (JADU)
For a JADU, a recorded covenant restricting sale and size is now required before final inspection and occupancy rather than before the building permit is issued, giving applicants more flexibility on timing.
Fees and timelines are approximate - confirm current figures with Costa Mesa Economic and Development Services at 77 Fair Drive.
Budgeting
What it costs to build an ADU in Costa Mesa
Garage conversion
Roughly $70,000-$185,000, the least expensive path since the shell already exists
New detached ADU
Roughly $150,000-$350,000+ for a fully built freestanding unit, with Orange County land, labor, and material costs pushing many projects toward the higher end
Permit & plan-check fees
Local ADU designers report typical fees somewhere in the $5,000-$12,000 range depending on unit size and scope; confirm the current fee schedule with the city
Impact fees
ADUs of 750 sq ft or less are exempt from most impact fees under state law; larger units may owe fees proportional to their size, plus separate school district fees the city doesn't control
These are general ranges from local contractor estimates and regional reporting, not quotes - actual cost depends heavily on lot conditions, finishes, and whether utilities need to be extended.
Before you apply
Tips for a smooth approval
Confirm which ADU combination your lot qualifies for, in writing. Combining a detached ADU with a conversion ADU and a JADU on one lot was a specific point state regulators had to correct in the city's ordinance - get current confirmation before you design around an assumption.
Ask about the pre-approved ADU design program. Approved in March 2025, it can cut design time and cost compared to a fully custom plan.
Have an unpermitted unit built before 2020? Look into the Safe ADU Legalization Program. It offers a path to bring it up to code under AB 2533 rather than starting over.
Plan your rental strategy around the 31-day minimum. It applies to both ADUs and JADUs, so design and furnish with a longer-term tenant in mind rather than a short-term rental.
Budget for plan-check and permit fees separately from construction. They're billed on a different schedule than your contractor's invoice and easy to underestimate when comparing bids.
FAQ
Common questions
Do I need to live on the property to build an ADU in Costa Mesa?
No, not for a standard ADU - California law (AB 976, effective January 1, 2024) permanently bars cities from requiring owner-occupancy. A Junior ADU (JADU) still requires the owner to occupy either the primary home or the JADU, unless the owner is a government agency, land trust, or qualifying housing organization.
How many ADUs can I build on one lot?
On a single-family lot, one ADU (detached, attached, or conversion) plus one JADU. Qualifying multifamily lots can add more, including up to eight detached ADUs under statewide SB 1211 (2024).
Do I need extra parking for an ADU?
Generally not. No additional parking is required beyond what already applies to the primary home in most cases, and demolished parking doesn't have to be replaced when it's removed to build the ADU.
How long does the permit process take?
State law caps ministerial ADU review at 60 days from a complete application.
What's the maximum size for a detached ADU?
Up to 1,200 sq ft. Height can run up to 27 ft or two stories under specific siting conditions, though at least 18 ft is always allowed, and an 800 sq ft version with 4 ft setbacks and an 18 ft cap is guaranteed regardless of other local standards.
How much does an ADU cost to build in Costa Mesa?
Garage conversions tend to run roughly $70,000-$185,000, and a new detached unit commonly runs $150,000-$350,000 or more, based on local contractor estimates and regional reporting, with Orange County costs pushing many projects toward the higher end. Add roughly $5,000-$12,000 in permit and plan-check fees.
Was Costa Mesa's ADU ordinance actually found noncompliant with state law?
Yes. In a September 11, 2024 letter, HCD found the city's 2023 ADU ordinance out of compliance with state law on 21 separate points. The city adopted a comprehensive fix, Ordinance No. 2025-02, effective May 2025, that addressed nearly all of them - with the exception of a 31-day minimum rental rule for JADUs, which the city kept despite HCD's objection.
Can I rent my ADU out on Airbnb or a short-term rental platform?
No. Both ADUs and JADUs in Costa Mesa may only be rented for terms of 31 days or longer.
Can I add multiple ADUs if I own a multifamily property?
Yes. Under statewide SB 1211 (2024), owners of existing multifamily properties can add detached ADUs up to the number of existing units on the lot, capped at eight, plus conversion ADUs within a share of the structure's non-livable interior space.
