ADU Constructionin Costa Mesa, CA
ADU construction in Costa Mesa runs under Municipal Code 13-35, which the City rewrote in 2025 after the state housing department found the earlier ordinance out of compliance. Approval is ministerial with no hearing, a detached ADU can reach 1,200 square feet, and a garage conversion needs no replacement parking.
Costa Mesa was incorporated in 1953 as a semi-rural farming community of 16,840 people and nearly quintupled within three decades, after the war years brought thousands of trainees to the Santa Ana Army Air Base. The Census Bureau's 2024 estimates count 15,526 single-family detached homes, 35.0 percent of 44,365 housing units, and 62.4 percent of all units date from before 1980. Those postwar lots are where most backyard units get built. The median owner-occupied home value is $1,166,800, and 62.6 percent of occupied homes are rented.
This page covers the 2025 rewrite of the ordinance, size and setback limits, how many units one lot may hold, garage conversions, a 1950s to 1970s lot, fees and utilities, and the permit path. The City's pages blocked our tools, so the ADU rules come from the GoCodebook mirror of Municipal Code 13-35 and from Internet Archive copies of City pages, and the City's ADU submittal sheet was not archived. Confirm each point with the Building Safety Division and Planning. Price ranges and phases are on our ADU construction page.
Costa Mesa Planning and Building rules that apply to ADU construction
Where adu construction permits come from in Costa Mesa: City of Costa Mesa Economic and Development Services Department, Building Safety Division.
- Costa Mesa's ADU rules are Municipal Code 13-35, amended by Ordinance 2025-02 (adopted April 1, 2025 as Code Amendment PCTY-24-0002 to conform to recent state law revisions) and by Ordinance 2025-07 (cited in the code history as October 21, 2025). The code requires ministerial approval without a public hearing, and a lot with a single-family home may have one new-construction ADU, one conversion ADU and one junior ADU.
- Under 13-35 a detached ADU on a single-family lot is capped at 1,200 square feet and an attached ADU at the greater of 50 percent of the existing home or 1,000 square feet, with a 150 square foot minimum and no size cap for interior conversions. A junior ADU is capped at 500 square feet, must sit within the home (including an attached garage) and carries an owner-occupancy requirement.
- 13-35(d) sets 4 foot side and rear setbacks for new ADUs (none for conversions or same-footprint rebuilds), a 6 foot gap to other buildings, a 27 foot or two-story height limit, and for a second-floor ADU a location at least 25 feet from the front property line with no side windows facing a neighbor within 25 feet unless the sill is 5 feet high. Exterior stairs may not sit on a front facade within 20 feet of a property line on the street, and new roof decks are prohibited.
- 13-35 says ADU parking does not have to be replaced when a garage is converted or demolished for an ADU, but an abandoned driveway must be replaced with landscaping and the curb cut and apron removed and rebuilt as curb, gutter and parkway to City standards. ADUs may not be sold separately (except as state law provides) and may not be rented for fewer than 31 days, and fire sprinklers are not required for an ADU unless the main home needs them.
- HCD's September 11, 2024 letter to the City found that Costa Mesa's earlier ADU ordinance (No. 2023-03, adopted February 2023) did not comply with state ADU law, citing among other things the deed-restriction and owner-occupancy requirements, front setback and height limits, rental-term limits and the one-category-per-lot rule. The City's Ordinance 2025-02 followed and the mirrored 13-35 text reflects it, but the City's reply to HCD was not read.
- The Building Information handouts page lists an ADUs and Jr. ADUs Submittal Requirements sheet, and the City says plans for building permits are submitted through TESSA. The sheet itself was not archived, so its checklist items are not stated here.
- California Government Code 66317 (quoted in HCD's letter) requires the City to approve or deny a complete ADU application within 60 days when there is an existing home on the lot, and to return written comments listing defects within that period if it denies.
Costa Mesa rewrote its ADU ordinance in 2025: what HCD's letter said and what changed
On September 11, 2024, California's housing department wrote to the City that its earlier ADU ordinance, No. 2023-03 from February 2023, did not comply with state law. The letter cited the deed restriction and owner-occupancy requirements, front setback and height limits, rental-term limits and a one-category-per-lot rule. The City answered with Ordinance 2025-02, adopted April 1, 2025 as Code Amendment PCTY-24-0002 to conform to recent state revisions, and a second amendment, Ordinance 2025-07, cited in the code history as October 21, 2025. We did not read the City's reply to the state, so the mirrored text is our source.
Any older Costa Mesa ADU guide, including the City's own April 2023 standards and August 2021 FAQ listed in search results, can be out of date. Section 13-35 now requires ministerial approval without a public hearing. For a lot with an existing home, state law gives the City 60 days to approve or deny a complete application and 15 business days to say whether it is complete, and the statute lists the full application defects if it denies. The text is in Government Code 66317.
How big can an ADU be in Costa Mesa, and how close to the fence can it sit?
Under 13-35 a detached ADU on a single-family lot is capped at 1,200 square feet, an attached ADU at the greater of 50 percent of the existing home or 1,000 square feet, with a 150 square foot minimum, and an interior conversion has no size cap. New ADUs need 4 foot side and rear setbacks and a 6 foot gap to other buildings, and the height limit is 27 feet or two stories. Conversions and same-footprint rebuilds need no setback.
A second-floor unit must sit at least 25 feet from the front property line and may have no side windows facing a neighbor within 25 feet unless the sill is 5 feet high. Exterior stairs may not sit on a front facade within 20 feet of a street property line, and new roof decks are prohibited. State law caps how low a city may set limits, with 800 square feet at 4 foot setbacks always allowed, and the City's code cannot override that, since a conflict goes to the state rule.
Can you have two ADUs in Costa Mesa? One of each type on a single-family lot
Yes, with limits. The code lets a lot with a single-family home hold one new-construction ADU, one conversion ADU and one junior ADU, and those categories can be combined. A junior unit is capped at 500 square feet, has to sit inside the home, including an attached garage, and carries an owner-occupancy requirement in the City's text. State law narrowed that rule in 2026 to cases where the junior unit shares a bathroom with the main house, so ask Planning which version it applies.
Units cannot be sold separately except as state law provides, and they cannot be rented for fewer than 31 days. Fire sprinklers are not required in an ADU unless the main house needs them. If you are choosing between a unit and a bigger house, our room addition vs ADU guide sets the two side by side, and the ADU vs JADU guide explains the differences in kitchens, entrances and rent rules.
Garage conversion ADU in Costa Mesa: no replacement parking, but the driveway comes out
Section 13-35 says parking does not have to be replaced when a garage is converted or demolished for an ADU. That is where the City's rule gets specific: an abandoned driveway must be replaced with landscaping, and the curb cut and apron must be removed and rebuilt as curb, gutter and parkway to City standards. That is paving, drainage and Public Works work, and it belongs in the quote as its own line, since a garage conversion price that ends at the garage door misses part of the job.
The City's list of services also names driveway and sidewalk permits and permits for work in the public right-of-way, so a curb rebuild brings a second approval. Inside the garage, a converted unit needs what any bedroom suite needs: a bathroom, a kitchen or kitchenette, and sleeping-room escape openings that meet the City's handout dimensions. Our garage conversion permit steps walk through the sequence, though its inspection items come from Los Angeles and not from Costa Mesa.
- Does the plan show the curb cut and apron removal, and who pulls the Public Works permit?
- Where does the water heater go once the garage is a bedroom suite?
- Is the main house's panel big enough for a new unit?
Building an ADU behind a 1950s to 1970s Costa Mesa house: lot history and legalizing old units
Fairview, Harper and the farm roads around them gave way to tract streets after 1945, and the Census table counts 5,758 homes from the 1950s, 10,248 from the 1960s and 10,685 from the 1970s. A lot from those years is usually flat and wide with an older garage at the back, which is why conversion and detached units dominate. We found no Costa Mesa source naming which neighborhoods have the largest lots or oldest garages, so ask for a survey before design starts.
Some older garages were turned into rooms without permits. State law protects a unit built before January 1, 2020 from denial over building-code or ordinance violations unless correction is needed for health and safety under Health and Safety Code 17920.3, and bars most impact and connection fees on such units. That path is in Government Code 66311.7. Whether Costa Mesa has its own legalization program is something a contractor directory claims and we could not confirm, so ask Building Safety. Lead matters here too, since remodeling a pre-1978 structure for a paid unit can bring EPA's lead-safe rule into play.
ADU impact fees, utility connections and solar on a new Costa Mesa unit
State law, Government Code 66311.5, bars impact fees on an ADU of 750 square feet or less and requires fees on larger units to be proportional to the main house's size. For school fees, a unit under 500 square feet is treated as adding no assessable space. Utility connection and capacity charges are separate and are not covered by the 750 square foot exemption, but a unit converted from existing space cannot be forced to get a new connection. We did not read the City's fee schedule, and we did not confirm which water or sewer district serves your lot, so ask both before budgeting.
HCD's handbook says the Energy Code requires solar panels on a newly built detached ADU, but not on a conversion or an attached addition. Plan the roof and the panel size for it. For payment, the state's CalHFA ADU grant had run out of money when HCD last wrote, so do not count on it. The ADU cost guide explains the budget lines.
From TESSA filing to final inspection: how an ADU permit runs in Costa Mesa
Plans for an ADU go through TESSA, and the Building Information handouts page lists an ADUs and Jr. ADUs Submittal Requirements sheet whose checklist we could not read. The City's Permit Types handout gives standard plan check as typically 5 to 30 working days depending on permit type, with accelerated review at 5 to 10 working days for certain types through an outside consultant billed to the applicant. Whether an ADU qualifies for the accelerated track is not stated, so ask. The 2025 California codes with Costa Mesa amendments took effect January 1, 2026.
Costa Mesa Fire and Rescue is a City department and the city is not an OCFA member, so fire review comes from the City. The City adopted fire hazard severity zones on June 3, 2025, and a state fire official quoted by ABC7 said the city has no Very High tier, so a mapped parcel is Moderate or High, an inference. A bluff-top lot needs a minor conditional use permit to build within 10 feet of the crest. We make no claim about flood, liquefaction or coastal zone maps, which we did not read. Work hours are 7 a.m. to 7 p.m. weekdays and 9 a.m. to 6 p.m. Saturdays.
ADU photos from Costa Mesa and nearby homes
ADU scenes tied to Costa Mesa and the Orange County communities around it, from garage conversions and slabs to finished studios. Tap a photo to enlarge it.
What adu construction costs around Costa Mesa
Our ADU construction page sources a garage conversion to a studio or one bedroom at roughly $100,000 to $200,000, and a new detached unit of 400 to 800 square feet at roughly $200,000 to $400,000. Three independent reads sit behind those numbers: a UC Berkeley survey of Los Angeles County ADUs finished in 2018 and 2019, with a $100,000 median, $90,000 for garage units and $180,000 for detached ones; the Terner Center's 2020 Los Angeles estimate of $148,000; and Remodeling magazine's 2025 Cost vs. Value report, which lists a $178,536 job and about 39.9 percent recouped at resale. None is a Costa Mesa quote, and we found no Orange County ADU survey. The curb and parkway rebuild, utility connections and the City's fees each show up as separate lines in a written proposal. Neighboring Orange County cities apply their own versions of state law, so compare ADU construction in Irvine, Santa Ana, Huntington Beach and Anaheim. A bath for the unit is on the Costa Mesa bathroom page, a bigger main house on the Costa Mesa room addition page, and the Costa Mesa area page lists every trade we cover in the city.
| What it covers | Typical range |
|---|---|
| Garage conversion to a studio or one bedroom | $100,000 to $200,000 |
| New detached ADU, 400 to 800 sq ft | $200,000 to $400,000 |
| UC Berkeley survey, Los Angeles County median, 2018 and 2019 projects | $100,000 overall. $90,000 garage. $180,000 detached. |
| Terner Center and Berkeley CCI, 2020 | $148,000 Los Angeles estimate. $167,000 statewide average. |
| Remodeling Cost vs. Value 2025, Los Angeles | $178,536 job cost |
| Electrical and plumbing, Angi Los Angeles, January 2026 | Electrical $2,600 to $7,800. plumbing $3,900 to $10,400. |
| City permit and plan check fees | Not priced here |
Costa Mesa ADU questions, answered
Can I build an ADU in Costa Mesa?
Yes, on a lot with a single-family home, and the City must approve without a public hearing. Municipal Code 13-35 allows one new ADU, one conversion ADU and one junior ADU per lot, a detached unit up to 1,200 square feet and 4 foot side and rear setbacks. Confirm your lot with Planning at (714) 754-5245.
How do you build and permit an ADU?
You design the unit to the city's size and setback rules, file plans through the City's TESSA portal, and answer plan check comments, then build under inspection. State law gives a city 60 days to rule on a complete application. A general contractor can handle the filing and the licensed trades from start to final.
How big can an ADU be in California?
State law lets a detached ADU reach 1,200 square feet and an attached one up to 50 percent of the existing house, while cities must always allow 800 square feet at 4 foot setbacks. Costa Mesa's code sets a 1,200 square foot detached cap and a junior unit cap of 500 square feet.
How much does it cost to build an ADU in California?
Sourced Los Angeles area ranges run roughly $100,000 to $200,000 for a garage conversion and $200,000 to $400,000 for a new detached unit of 400 to 800 square feet. They come from a 2018 and 2019 survey and 2025 report data, so a written proposal sets the real price.
Do I need to provide parking for my ADU?
Not when a garage is converted or demolished for the unit, because Costa Mesa's section 13-35 does not require replacement parking. State law also waives parking near transit, in historic districts and inside the main home. The catch is the old driveway, which the City requires you to replace with landscaping and a rebuilt curb.
What is the difference between an ADU and a JADU?
A junior ADU is capped at 500 square feet and must sit inside the existing home, including an attached garage, while a regular ADU can be detached or attached and far larger. Costa Mesa's code adds an owner-occupancy rule for junior units, which state law narrowed in 2026, so ask Planning which applies.
Can I convert my garage to an ADU in Costa Mesa?
Yes, a garage conversion is one of the three ADU types the code allows on a single-family lot, with no setback and no replacement parking. You must remove the driveway curb cut and apron, rebuild the curb, gutter and parkway to City standards and landscape the old driveway, so price that work with the garage.
Can I sell my ADU separately from the main house?
No in most cases. Costa Mesa's code bars separate sale of an ADU except as state law provides, and state law allows it only through a qualified low-income buyer arranged by a qualifying nonprofit or a city condominium ordinance. Rentals must run at least 31 days, so short-term vacation use is not allowed.
Can my HOA stop me from building an ADU?
Not outright. Under state law, covenants that effectively prohibit or unreasonably restrict an ADU on a single-family lot are void, and 2025 changes bar HOA rules that add fees or financial requirements. An association can still apply reasonable rules, so read your documents and ask the board early.
Tell us about your Costa Mesa ADU plans
Tell us whether you are weighing the garage or the back yard, roughly when the house was built and whether the driveway is a straight run, and the first visit will start from the lot rules and the curb question.