Costa Mesa Zoning and ADU Guide for Homeowners and Sellers

Last updated November 13, 2026. Reviewed by the Cash Home Buyers CA team. Information comes from the City of Costa Mesa; sources are listed at the end.

Costa Mesa says its ADU rules meet minimum state requirements, and lists a 150 square foot minimum size, optional parking and objective design standards such as second-floor setbacks as its own local provisions. The city also runs a Safe ADU Legalization Program page.

Costa Mesa ADU rules at a glance

What if my Costa Mesa ADU or garage conversion has no permit?

Start with the city’s Safe ADU Legalization Program page and the Planning Division at (714) 754-5245 to learn what legalizing involves for your property. The city enforces land-use rules through code enforcement, so see our Costa Mesa code enforcement guide.

How an unpermitted unit affects a sale

Sellers generally have to disclose known unpermitted work. On a traditional sale, buyers may ask for the unit to be legalized, removed or priced out, and lenders can be cautious about living space missing from city records. A cash buyer can purchase the property with the unit unresolved and price the risk into the offer.

How to sell a house in Costa Mesa with an unpermitted ADU

1. Check city records. Look at permit records for the address.
2. Ask Planning. Find out what legalizing would take.
3. Compare options. Legalize, sell with a credit, or sell as-is.
4. Disclose honestly. A cash offer does not change what you must disclose.

See selling a house as-is in Costa Mesa or our cash-offer process.

Questions about your unit? Call (424) 435-2326 or request a cash offer. No cost, no obligation.

What the city’s ADU page points you to

The Costa Mesa ADU page defines ADUs and junior ADUs as independent living facilities on the same parcel as the main house, which may be occupied by extended family or tenants.

It then links to three resources that matter to a seller: a Pre-Engineered ADU page, the Safe ADU Legalization Program page and the ADU municipal code on the city’s online code site.

The city describes its ordinance as meeting the minimum state requirements while keeping the existing urban fabric, and names three Costa Mesa specifics: a 150 square foot allowable minimum size, optional parking, and objective design standards such as second-floor or two-story setbacks.

Read plainly, a lawful ADU in Costa Mesa is a unit that went through the city’s process and meets those standards. A unit that was built without that process is the one the Safe ADU Legalization Program page is meant for, so start there before you decide how to sell.

Is my backyard unit, garage or spare room an ADU?

• A detached cottage or studio: usually an ADU if it has its own living facilities. Check permit records for ADU new construction.
• A converted garage: usually an ADU conversion. The city lists ADU conversion as its own permit type in the TESSA portal.
• A room inside the house with its own entrance and kitchen: may be treated as a junior ADU, which the city lists separately.
• A bedroom or bonus room with no separate facilities: normally a residential addition or alteration, not an ADU.

If you are not sure which category applies, the Planning Division at (714) 754-5245 is the contact the city lists for questions.

Selling with an unpermitted unit: what changes for each type of buyer

A buyer who needs a mortgage will usually face an appraiser who counts only permitted living space and a lender who wants clean records, so an unpermitted unit can cut the value they can finance.

An investor buying for cash looks at the cost and risk of legalizing, removing or leaving the unit as is, then prices the house accordingly. A buyer who intends to keep the unit as a rental also has to think about tenants and local rules.

None of these paths is closed to you; they differ in speed and price. Our California as-is and unpermitted work guide covers disclosure, and our tenant-occupied home sale guide applies if someone lives in the unit.

A step-by-step plan if you want to sell a Costa Mesa house with an unpermitted unit

1. Photograph and measure the space. Keep a record of what exists today.
2. Check the permit history. Our Costa Mesa building permits guide shows how to ask Building Safety what is on file.
3. Read the Safe ADU Legalization Program page. It tells you what the city offers for units built without permits.
4. Ask for a rough legalization scope and cost. Contractors can price the work, and the number is what you weigh against selling as-is.
5. Compare three routes. Legalize first, sell with a credit, or sell as-is. See seller closing costs if you are weighing a traditional sale.
6. Disclose what you know. The disclosure duty does not go away in an as-is sale.

Mistakes to avoid with an unpermitted unit

• Advertising the unit as a legal rental when the city has no record of it.
• Collecting rent without checking how that affects a sale, especially if a tenant is in place.
• Starting legalization work without confirming the process with Planning first.
• Assuming the unit is fine because it has been there for decades.

More Costa Mesa ADU questions

Does Costa Mesa have a program for unpermitted ADUs?

The city’s ADU page links to a Safe ADU Legalization Program page. Contact the Planning Division at (714) 754-5245 for details.

Does Costa Mesa require parking for an ADU?

The city lists optional parking as one of its local provisions.

Where is the ADU code?

The city links the ADU municipal code on its ADU page.

Costa Mesa zoning and ADU FAQ

Does Costa Mesa allow ADUs?

Yes. The city says its ADU ordinance meets state minimums and allows ADUs and JADUs in all residential districts.

What is the minimum ADU size in Costa Mesa?

The city lists 150 square feet as its allowable minimum size.

Who do I call about ADUs in Costa Mesa?

The Planning Division at (714) 754-5245.

Can I sell a Costa Mesa house with an unpermitted ADU?

Yes, with disclosure. A cash buyer can purchase it as-is and price the risk into the offer.

Sources

• City of Costa Mesa: Accessory Dwelling Units
• City of Costa Mesa: Building Permits

This guide is general information, not legal advice. Rules change; confirm requirements with the City of Costa Mesa.