Building an ADU in Costa Mesa

What you can build in Costa Mesa

ADUs are allowed in single-family, multifamily and mixed-use zones, and on approved master plan sites with homes, but only on a legal lot with a home that will stay. The rules are in Municipal Code section 13-35. The city says its ordinance "meets the minimum state requirements," and that what is unique to Costa Mesa is the 150 sq ft minimum size, optional parking and objective design standards such as second-story setbacks. The city's ADU page links to the code.

Single-family lots. You can have one new detached ADU, one ADU converted from the house or an existing accessory building, and one junior ADU (JADU). A converted accessory building can add up to 150 sq ft, but only for getting in and out.

Multifamily lots. On a lot with an existing apartment building you can convert non-living space (such as storage rooms or garages) into ADUs, up to 25% of the existing units (rounded down, but at least one). The code also allows up to eight detached ADUs, but no more than the number of existing units; one of them may be attached to the building. The code lists these two options with "or," but HCD's handbook says cities must let you combine them (at least the converted ADUs plus two detached ADUs), so confirm with the Planning Division. The code also allows one ADU, or up to two, with new construction that leaves at least two homes on the lot.

Recent changes. The City Council adopted Ordinance No. 2025-02 on April 1, 2025, amending section 13-35 "to conform to recent revisions to state law." Where state law is more generous than a city rule, state law controls. See our California ADU law page for the statewide rules. [1][2][4][5]

Size, height and setbacks

Size. Every ADU must be at least 150 sq ft. With a single-family home, a detached ADU can be up to 1,200 sq ft and an attached ADU up to 50% of the house or 1,000 sq ft, whichever is greater. With an existing apartment building, a new detached ADU can be up to 850 sq ft (1,000 sq ft with more than one bedroom), except as state law allows. ADUs made entirely from space inside existing buildings, or inside a proposed house, have no maximum size.

The state-protected ADU. Costa Mesa's development standards don't apply to converted ADUs, converted JADUs, or a new detached ADU of up to 800 sq ft with 4-foot side and rear setbacks and a height of up to 18 feet (2 more feet allowed to match the main home's roof pitch). Other ADUs must meet the city standards below, unless they would block an 800 sq ft ADU with 4-foot setbacks.

Setbacks.

• Side and rear: at least 4 feet for new ADUs. Mechanical equipment must meet the same setbacks.
• Front: the zone's front setback, except where state law requires otherwise. Second-floor ADUs follow the zone's setbacks.
• Conversions: no setback for converted living space or accessory buildings, or for a new ADU rebuilt in the same place and size, as long as fire safety setbacks are met.
• Between buildings: 6 feet between a detached ADU and the house or any other structure, unless the building code sets another distance.

Height. No new ADU can exceed 27 feet or two stories. Outside the exceptions in the next section, an ADU must be all on the first floor. State law says a city can't set a height limit below 25 feet (or the zone's height limit for the house, if lower) for an attached ADU, and HCD flagged the city's 2023 height rule, so confirm with the Planning Division if you plan an attached ADU. [1][5][6]

Second-story ADUs and design rules

On a lot with an existing legal home, an ADU can be partly or entirely on a second floor, or be two stories, only if all five of these are met:

• Every second-floor part is at least 25 feet from the front property line.
• Each stairway is inside, or meets the exterior stairway rules.
• Above a detached garage, the second floor doesn't go beyond the garage footprint.
• Second-floor side windows within 25 feet of a neighbor's home and facing it have sills at least 5 feet high.
• The second floor meets the zone's setbacks for additions.

An ADU built entirely inside a house, or in non-living space of an apartment building, is also allowed upstairs.

Other design rules. A new exterior stairway to a second-floor ADU can't be on a front facade within 20 feet of a property line on a street, and stairs and landings must be screened. Roof decks aren't allowed. Attached and detached ADUs must use the same materials, finishes and colors as the main home. The ADU entrance must match the house and can't be on the same side as the home's main entrance, unless the director approves another layout. ADUs over 800 sq ft must meet the zone's open space and landscaping rules.

These design rules don't apply to the state-protected ADU types described above. HCD flagged several of them in 2024 (see "How long it takes" below), so ask the Planning Division how they apply to your project. [1][6]

Parking and garage conversions

Parking is optional. Costa Mesa doesn't require parking for an ADU. If you choose to provide a space, it must be at least 8.5 by 18 feet and can be tandem on your driveway (without blocking the sidewalk) or in a setback area. A JADU needs no parking beyond what the house already needs.

Garage conversions. If you convert or demolish a garage or carport to build an ADU, you don't have to replace the lost parking. Your driveway keeps its legal status. This applies only if the ADU is built at least partly on the footprint of the demolished garage. Converted space needs no setbacks beyond fire safety, and a converted accessory building can add 150 sq ft for entry and exit. A JADU can go in an attached garage, but not a detached one.

• Driveway. If you remove the driveway to a converted garage, replace it with landscaping or open space, and replace the curb cut and apron with curb, gutter and parkway to city standards. A partly removed driveway must still fit a standard parking space.
• Garage door. The city recommends, but doesn't require, that a converted garage wall visible from the street be at least 50% covered with landscaping, or have openings on at least 10% of the wall with a door or a window that matches the house.

More in our garage conversion guide. [1][5]

Junior ADUs

A JADU is a unit of up to 500 sq ft entirely inside an existing or proposed single-family home, including an attached garage. It needs its own outside entrance and an efficiency kitchen (cooking appliance, room for a small refrigerator, counter and cabinets). If it shares a bathroom with the house, it also needs an inside door to the main living area. A JADU doesn't count as a separate unit for fire rules or utilities, and it doesn't need its own utility connection unless it is built with a new house.

Owner occupancy. Costa Mesa's code says the owner must live in either the house or the JADU, and before final inspection you must record a covenant that bars selling the JADU separately and limits its size. Under 2025 changes to state law (in effect by January 1, 2026), HCD says owner occupancy is required only if the JADU shares bathroom facilities with the house. The city's code text is older than that change, so confirm with the Planning Division what it will require. State law now measures JADU size in interior livable space, while the city code says 500 sq ft of "gross floor area."

See our junior ADU guide. [1][5]

Fees

Impact fees. State law bars impact fees on an ADU with 750 sq ft of interior livable space or less and on a JADU with 500 sq ft or less. Larger ADUs pay a share proportional to the size of the main home. The city's ADU standards table (updated April 2023) lists these formulas:

• Single-family ADU of 750 sq ft or more: (ADU sq ft / existing home sq ft) x $13,572.
• Multifamily ADU: (ADU sq ft / average existing unit sq ft) x $13,829.
• JADU: $5,000.

The table says the fee is never more than the fee for the main home. It predates the 2025 change in state law that exempts JADUs of 500 sq ft or less, so confirm current fees with the Planning Division.

School fees. The Building Division's ADU handout tells you to check fees with your school district (Newport-Mesa Unified or Santa Ana Unified); see the Newport-Mesa developer fees page. Under state law, ADUs and JADUs under 500 sq ft of interior livable space aren't subject to school impact fees.

Water and sewer. The city may require a new or separate utility connection for an ADU, but not for one created inside a single-family home (unless it's built with a new house). The city's site plan checklist says a sewer permit goes to the Costa Mesa Sanitary District and a water plan to Mesa Water District (the checklist also names the Irvine water district), and a new water service may be needed if you share a lateral with a neighbor. Under state law, converted ADUs and JADUs can't be treated as a new residential use for connection fees, and other connection fees must be proportionate. Building permit and plan check fees are set by city council resolution. For typical costs see our ADU cost guide. [1][5][7][8][9]

How long it takes, and HCD's review

Under state law, the city must tell you within 15 business days whether your application is complete, and approve or deny a complete application within 60 days when there is an existing home on the lot. If the city fails to make a completeness decision in time, the application is deemed complete. Costa Mesa's code says an ADU that meets its rules is approved ministerially, without a public hearing. See our ADU permit process guide.

State review. In a September 11, 2024 letter, California's housing department (HCD) found that Costa Mesa's 2023 ADU ordinance did not comply with state law in 21 ways. Among them were limiting single-family lots to one type of ADU, requiring owner occupancy and a deed restriction, the front setback and height rules, and subjective design terms such as "compatible." The city then adopted Ordinance No. 2025-02 in April 2025, and the current code now allows a detached ADU, a converted ADU and a JADU together and exempts state-protected ADUs from its design standards. If a city rule seems to block an ADU that state law protects, see our California page for how to contact HCD. [1][2][5][6]

Renting it out, selling it, and SB 9

You don't have to live on the property to build or rent out an ADU. ADUs and JADUs can't be rented for less than 31 days, so vacation rentals aren't allowed. Each ADU and JADU gets its own mailing address from the city.

Selling. City code says ADUs and JADUs may not be sold separately from the main home "except as otherwise provided by law." State law lets cities choose to allow ADU condominiums, and HCD noted in 2024 that Costa Mesa has that option.

SB 9 two-unit projects. Section 13-36 lets you build up to two primary homes on a single-family lot without a hearing, with 4-foot side and rear setbacks. The section defines the project as no more than four units in total, including ADUs and JADUs, but the affidavit you must record says the parcel may have at most two units, including ADUs and JADUs, and limits rentals to at least 31 days. Ask the Planning Division which limit applies to your lot. See our SB 9 guide. [1][5][6]

Pre-approved plans and legalizing an existing unit

Pre-Approved ADU Program. Costa Mesa has set up a pre-approved ADU program. Architects submit plans for review, and once approved, you pick a plan, contact its vendor and submit only a site plan for review in TESSA. As of October 2026, the city's page says approved plans are "COMING SOON" and will be posted when available. State law requires a city to approve or deny a detached ADU application that uses a preapproved plan within 30 days.

Safe ADU Legalization Program. If you have an ADU or JADU built without permits before January 1, 2020, the city's Safe ADU Legalization Program lets you legalize it under state health and safety standards (AB 2533). On request, a city inspector can check the unit before you apply. The city says it won't penalize you for the unpermitted unit if you pull the permits needed to fix safety problems. [5][9][10]

Permit steps in Costa Mesa

1. Check your lot and talk to a planner

   Confirm your zone and read Municipal Code section 13-35. The city recommends consulting a planner first: call the Planning Division at (714) 754-5245. [1][4][7]

2. Create a TESSA account

   Anyone submitting or following a project must have a profile in TESSA, the city's online permit system. The city posts TESSA help videos. [3][8]

3. Prepare plans

   You need a site plan, scope of work, plans on 24 x 36 inch sheets with the codes, occupancy and construction type listed, and current Title 24 energy forms. Precise grading plans, if needed, go in separately. A soils report isn't required for additions up to 800 sq ft on one story, or 1,200 sq ft on two stories with an 800 sq ft footprint. [8]

4. Submit an ADU plan check in TESSA

   Planning, Building, Public Works, Fire and outside agencies review the plans. For help with online submittals, book an appointment with a building technician or call the Building Division at (714) 754-5273. [3][9]

5. Clear outside agencies and pay fees

   Pay school district fees and get a sewer permit from the Costa Mesa Sanitary District and water approval from your water district as required. Upload any required rough grading certificate before the building permit is issued. [8][9]

6. Record any JADU covenant, build and pass final inspection

   For a JADU, the city's code requires a recorded covenant before final inspection. Inspections happen during construction and end with a final inspection. [1][9]

Who to call

• City of Costa Mesa Planning Division

  ADU zoning rules, pre-approved plans and legalization program (planninginfo@costamesaca.gov)

  (714) 754-5245

  77 Fair Drive, Costa Mesa, CA 92626

  [4][9]

• City of Costa Mesa Building Safety Division

  Building permits, plan check and TESSA help (buildingpermit@costamesaca.gov)

  (714) 754-5273

  77 Fair Drive, Costa Mesa, CA 92626

  [3][8]

Questions people ask