In short
- 399 ADU permits on record, nearly all since 2020.
- 99 in 2025, up 2% from 2024.
- Typical permit time: 195 days from application to permit (median of 329 permits with both dates recorded).
About this data
- Source: California HCD Housing Element Annual Progress Reports. The state data does not say whether an ADU is detached, attached, a garage conversion or a JADU, so most records show as 'unknown'. Permit valuations are not published in this source. Timelines use the application date the city reported to the state, matched to the permit where possible; some cities report their planning application date, so timelines can include planning review. Each project is counted once, in its building-permit year (or completion year when no permit date was reported).
Permits by year
Permits by year and type (table)
| Year | All ADU permits | Garage conversions | Detached |
|---|---|---|---|
| 2018 | 6 | — | — |
| 2019 | 9 | — | — |
| 2020 | 19 | — | — |
| 2021 | 45 | — | — |
| 2022 | 56 | 0 | 0 |
| 2023 | 68 | — | — |
| 2024 | 97 | — | — |
| 2025 | 99 | — | — |
“—” means the source for that year does not report ADU type.
Read more: ADU types explained
ADU rules in Costa Mesa
State law check: HCD found the city's 2023 ADU ordinance out of line with state law (September 11, 2024); the city amended its ADU rules April 1 and October 21, 2025, and no HCD letter on the new version is on file.
| Rule | Costa Mesa's code | State law guarantees |
|---|---|---|
| Maximum size | Detached: 1,200 sq ft. Attached: 50% of the house or 1,000 sq ft, whichever is greater. No cap for conversions. Minimum 150 sq ft. Junior ADU 500 sq ft. | At least 850 sq ft (1,000 sq ft with 2+ bedrooms) must be allowed; an 800 sq ft detached ADU always is. Conversions of existing space have no size cap. Junior ADU: up to 500 sq ft. |
| Height | Up to 27 ft and 2 stories, but only on the ground floor unless second-story rules are met (25 ft from the front lot line, interior or screened stairs, window limits). State-law detached ADU: 18 ft, plus 2 ft for roof pitch. | Detached: at least 16 ft (18 ft within ½ mile of transit, plus 2 ft for a matching roof). Attached: 25 ft or the zone's limit, whichever is lower. |
| Setbacks | 4 ft side and rear; front setback matches the zone. 6 ft between a detached ADU and other buildings. None for conversions. | No more than 4 ft side and rear; none for conversions of existing space. |
| Parking | None required: parking is optional, and a space an owner adds can be tandem on the driveway or in a setback. No replacement parking for converted garages. | At most 1 space; none within ½ mile of transit and in several other cases. No replacement parking when a garage is converted. |
| Owner living on site | None for ADUs. Junior ADU: the owner lives in the house or the junior ADU, with a recorded covenant. | None for ADUs. A junior ADU that shares a bathroom with the house requires the owner to live in one of them. |
| How many on a single-family lot | One converted ADU, one new detached ADU and one junior ADU on a single-family lot. | One ADU converted from existing space, one junior ADU and one new detached ADU (up to 800 sq ft). From January 1, 2027: two new detached ADUs (a city then needn't allow a junior ADU on that lot). |
| Short-term rentals | 31 days or more, for ADUs and junior ADUs. | No guarantee: cities may require rentals of more than 30 days, and junior ADUs can't be short-term rentals. |
- No roof decks; balconies and exterior stairs to a second-floor ADU have setback and screening rules.
- ADUs over 800 sq ft must meet open space and landscaping rules.
- New ADUs use the house's materials, finishes and colors.
- The city offers pre-engineered ADU plans and a Safe ADU Legalization Program for existing units.
Where local rules conflict with state law, state law controls. Sources: Costa Mesa Municipal Code 13-35 · HCD findings (September 11, 2024) · City of Costa Mesa ADU page; HCD's list of review letters; California ADU rules. Verified Oct 5, 2026. Rules change and every lot is different: confirm with the city before you design.
How long permits take
| Type | Median days, application to permit | Middle half | Permits |
|---|---|---|---|
| Junior ADU (JADU) | not reported | ||
| Unclassified | 195 | 105 – 334 | 329 |
Middle half: the range covering the middle 50% of permits (a quarter were lower, a quarter higher).
Read more: How long ADUs take to build
From permit to completion
Of the 185 ADU permits Costa Mesa reported issuing from 2020 through 2023, 92 (50%) have a completion reported to the state. For those, the median time from permit to completion was 12 months.
The state reports don't record expired or cancelled permits, and a completion the city hasn't reported looks the same as an unfinished project, so the share is a floor. Years with incomplete reports are left out.
What neighbors declared as cost
Cost data (declared permit valuations) is not published for this area in these sources.
Read more: How to choose an ADU builder
An ADU builder serving Costa Mesa? Feature your business on this page.
Questions about ADUs in Costa Mesa
How many ADUs have been permitted in Costa Mesa?
Public records show 399 ADU permits in Costa Mesa through 2025, nearly all since 2020. 99 in 2025, up 2% from 2024. These counts come from the city's reports to the state, which place a project in its completion year when no permit date was reported.
How long does an ADU permit take in Costa Mesa?
Among the 329 ADU permits in Costa Mesa with both dates recorded, half were issued within 195 days of the application; the middle half took 105 to 334 days. That includes plan-check corrections and resubmittals, not just the review itself.
Can I build an ADU on my property in Costa Mesa?
California law allows ADUs on most lots with a house, including in Costa Mesa, but what you can build depends on your lot, local rules and utilities. This site shows permits, not eligibility. See California ADU rules.
Where do I apply for an ADU permit in Costa Mesa?
Apply with the City of Costa Mesa through a building permit application; its Planning Division also offers pre-engineered ADU plans and a program to legalize existing ADUs. Official ADU page.
How current is this?
- State annual progress reports: through 2025 (published once a year)
How our data works · Download the data · Free to reuse with credit to SoCal ADU Tracker (socaladutracker.com), under CC BY 4.0.