ADU rules in Costa Mesa

Detached ADUs are allowed on any legal lot with a proposed or existing home in zones that allow single-family or multifamily dwellings, and at approved master plan sites with residential uses in other zones; the Citywide Land Use Matrix (Table 13-30, row 22.2) shows ADUs as permitted in R1, R2-MD, R2-HD, R3, TC, PDR-LD, PDR-MD, PDR-HD, PDR-NCM, PDC, PDI and LT, and § 13-35(b)(1) controls if the matrix conflicts. A single-family lot may combine one detached new-construction ADU, one conversion ADU and one JADU, the combination HCD's 9-11-2024 findings required and Ord. 2025-02 added. Lots with an existing multifamily building may add up to eight detached ADUs (never more than the number of existing units), and new multifamily construction may include up to two. A new detached unit of up to 800 sq ft with 4 ft side and rear setbacks and an 18 ft height (plus 2 ft for a roof pitch matching the house) is exempt from the local development standards under Gov. Code 66323(a).

Attached ADUs, ADUs converted from space inside an existing home or a lawfully built accessory structure (up to 150 sq ft may be added to an accessory structure for access), and ADUs built entirely inside a proposed home are all allowed, and a conversion ADU and a JADU may share the same single-family dwelling. A JADU of up to 500 sq ft must be entirely within an existing or proposed single-family dwelling, which includes an attached garage, with its own exterior entrance and an efficiency kitchen. On lots with an existing multifamily building, ADUs may be created in non-livable space (storage rooms, attics, basements, garages) for up to 25 percent of the existing units, and at least one. Local-track ADUs must stay on the first floor unless built entirely within a home or unless they meet all five second-story conditions in (d)(9)(a): second-floor portions at least 25 ft from the front property line, interior or compliant exterior stairs, no larger than the garage footprint when built over a detached garage, 5 ft sill heights for side windows within 25 ft of a neighboring dwelling, and the setbacks that apply to additions.

On a lot with a single-family home a detached ADU may not exceed 1,200 sq ft, and an attached ADU may reach the greater of 50 percent of the existing home or 1,000 sq ft. On lots with an existing multifamily building a detached ADU is capped at 850 sq ft, or 1,000 sq ft with more than one bedroom (state-exempt units excepted), and an attached ADU at the greater of 1,000 sq ft or 50 percent of the average existing unit. ADUs converted entirely from existing space, or built entirely within a proposed single-family home, have no size cap; JADUs are limited to 500 sq ft of gross floor area; and an ADU may be as small as 150 sq ft. No new-construction ADU may exceed 27 ft or two stories, and local-track units stay on one floor unless they meet the second-story conditions, while the state-exempt detached unit is limited to 800 sq ft and 18 ft (plus 2 ft for a matching roof pitch) with 4 ft side and rear setbacks. The code does not say how ADU floor area is measured.

No owner-occupancy requirement applies to ADUs: Ord. 2025-02 deleted the clause that would have required owner occupancy for ADUs approved on or after 1-1-2025 (HCD's 9-11-2024 finding 13), and Gov. Code 66315 forbids one. For JADUs the codified text still requires the owner to live in the main home or the JADU in every case ((c)(3)) and to record a covenant saying so ((d)(5)(b)), but since 1-1-2026 Gov. Code 66333(b) allows that requirement only when the JADU shares sanitation facilities with the main home, and the covenant applies only 'unless subsequently prohibited by an amendment to state law.' A JADU with its own bathroom therefore carries no owner-occupancy duty, and the pending Neighborhoods Where We All Belong amendment would rewrite (c)(3) to the shared-sanitation rule. Government agencies, land trusts and housing organizations are exempt in any case.

Costa Mesa requires no off-street parking for an ADU: § 13-35(d)(15) says only that the owner 'may provide' a space of at least 8.5 by 18 ft, as tandem parking on an existing driveway or within a setback, and the city ADU page calls parking optional. JADUs need no parking beyond what the primary dwelling already requires. Garages or spaces converted or demolished for an ADU need not be replaced, and the driveway keeps its status as a lawful driveway, though a demolition counts only if the ADU is built within the old garage footprint. The residential parking section defers to § 13-35 (§ 13-85(d) as restated by Ord. 2026-03, adopted 4-7-2026 and not yet codified on eCode360).

An ADU application that meets § 13-35 is approved ministerially through a building permit, without a public hearing, notice of decision or zoning approval, under Gov. Code 66317 (completeness check within 15 business days and a decision within 60 days). The local development standards in (d) do not apply to the state-exempt units in Gov. Code 66323(a), and they yield wherever they would preclude an 800 sq ft ADU with 4 ft side and rear setbacks. The application must be filed by the owner of the parcel on the city form with the council-set fee. The pending Neighborhoods Where We All Belong draft would add the 66317 completeness and written-denial procedure that HCD's 2024 finding 2 asked for, without changing the ministerial path.

An ADU or JADU may be rented long term but not for periods of less than 31 days, and the city's short-term rental page says ADUs cannot be used as short-term rentals. Urgency Ord. 2021-17 (adopted 11-2-2021, effective immediately and deliberately left uncodified) separately prohibits every rental of 30 days or fewer in all zones except hosted home-sharing by a natural person who was already home-sharing a primary residence, or an ADU or JADU where short-term rentals were not otherwise prohibited, by 12-1-2020; on 1-28-2025 the council voted 5-2 to receive and file, rather than adopt, a proposed suspension for wildfire evacuees. The 31-day floor runs one day past Gov. Code 66315, which lets a city require rental terms of 30 days or longer for ordinary ADUs, but matches the longer-than-30-day terms Gov. Code 66323(e) and 66333(g) require for state-exempt units and JADUs. ADUs and JADUs may not be sold or conveyed apart from the primary residence except as state law provides (Gov. Code 66341 qualified nonprofits); § 13-35 contains no Gov. Code 66342 condominium opt-in.