California ADU rules · Orange County
Can I Build an ADU in Costa Mesa?
Yes — you can build an ADU in Costa Mesa. California's statewide ADU law requires every city, including Costa Mesa, to approve a qualifying accessory dwelling unit by right, plus a junior ADU (JADU) on a single-family lot. What varies city to city are the local specifics below.
Can you build one?
ADUs allowed
CA state lawYes — by right
ADUs and JADUs are allowed by right in all residential and mixed-use zones; ministerial approval, no hearing.
Units on a single-family lot
CA state law1 ADU + 1 JADU
At least one ADU and one JADU per single-family lot (a conversion ADU may also be possible).
Units on a multifamily lot
CA state lawConversion ADUs (≥1, up to 25% of units) + up to 8 detached
Conversions of non-livable space (at least 1, or up to 25% of existing units) plus up to 8 detached ADUs on a lot with existing multifamily — never exceeding the number of existing units. JADUs are single-family only.
Junior ADU (JADU)
CA state lawAllowed
Up to 500 sq ft, within the walls of an existing single-family home (single-family lots only).
Size & height
Max detached ADU size
Local rule1,200 sq ft
On a single-family lot a detached ADU may not exceed 1,200 sq ft; on a multifamily lot the cap is 850 sq ft (studio/1-bed) or 1,000 sq ft (2+ bedrooms) unless state law allows more for a conversion.
Max attached ADU size
Local ruleGreater of 50% of the existing single-family dwelling or 1,000 sq ft (on a multifamily lot: greater of 1,000 sq ft or 50% of the average existing unit floor area)
Guaranteed minimum size
CA state law800 sq ft
An 800 sq ft ADU must be allowed regardless of FAR/lot-coverage caps; a city also cannot cap an attached ADU below 850 sq ft (1,000 for 2+ bedrooms).
Max JADU size
CA state law500 sq ft
Max height
Local rule18 ft
18 ft (plus 2 ft for matching roof pitch) for a new detached ADU, with the state transit/multifamily allowances; a second-story or attached ADU may reach up to 27 ft / 2 stories, subject to extra requirements (e.g. 25 ft from the front line, window restrictions).
Setbacks
Side setback
Local rule4 ft
4-ft side/rear setback for new-construction ADUs; conversions of an existing structure need no setback except as required for fire/life safety.
Rear setback
Local rule4 ft
4-ft side/rear setback for new-construction ADUs; conversions need none.
Front setback
CA state lawNo front setback may block the mandatory 800 sq ft / 16-ft / 4-ft-setback ADU; otherwise per base zone
Separation from main house
Local rule6 ft
Costa Mesa requires at least 6 ft between a detached ADU and the primary dwelling and all other structures (including garages), unless the California Building Code sets otherwise — state law sets none.
Parking
Parking required
CA state lawUp to 1 space
At most 1 space per ADU (or per bedroom, whichever is less) — and waived in the common cases below.
Transit parking exemption
CA state lawYes — no parking required near transit & for conversions
No parking may be required within ½ mi walking distance of transit, in a historic district, for an ADU within an existing structure, within one block of car-share, or in certain permit-parking areas.
Garage-conversion replacement parking
CA state lawNot required
When a garage/carport is demolished or converted to build an ADU, replacement parking cannot be required.
Approval, timeline & cost
Approval path
CA state lawMinisterial (by right)
No public hearing or discretionary design review for a qualifying ADU.
Approval timeline
CA state law60 days
The agency must act within 60 days of a complete application — and the ADU is deemed approved if it does not.
Owner-occupancy
CA state lawOnly for a JADU
No owner-occupancy requirement for ADUs (the prior sunset was removed). A JADU may require owner-occupancy, principally where it shares sanitation with the main house.
Impact-fee waiver
CA state lawWaived under 750 sq ft
An ADU of 750 sq ft or less pays no impact fees (the 750 sq ft threshold itself is exempt under § 66311.5); larger ADUs pay fees proportional to the primary dwelling. JADUs (≤ 500 sq ft) are exempt.
Permits required
CA state lawBuilding permit
Only a ministerial building permit — no conditional use permit or discretionary design review for a qualifying ADU.
Costa Mesa caps detached ADUs at 1,200 sq ft on single-family lots (smaller on multifamily lots), holds new detached ADUs to 18 ft while allowing certain attached/second-story ADUs up to 27 ft, and imposes a 6-ft separation between a detached ADU and the main house and other structures. It records no local fire, coastal, historic, or hillside ADU overlays, and owner-occupancy, parking, and impact-fee rules track the state floor.
Frequently asked questions
Can I build an ADU in Costa Mesa?
Yes. California's statewide ADU law requires Costa Mesa to approve a qualifying accessory dwelling unit ministerial (by right), plus a junior ADU (JADU) on a single-family lot.
How big can an ADU be in Costa Mesa?
A detached ADU of at least 1,200 sq ft must be allowed, and an ADU of 800 sq ft can never be zoned out by floor-area or lot-coverage limits. A JADU can be up to 500 sq ft.
What are the setback requirements for an ADU in Costa Mesa?
Side and rear setbacks are limited to 4 ft. No front setback may block the mandatory 800 sq ft / 16-ft / 4-ft-setback ADU; otherwise per base zone.
Is parking required for an ADU in Costa Mesa?
Up to 1 space. No parking can be required within ½ mile of transit, for conversions, or for units inside the primary home. When a garage is converted, replacement parking is not required.
How long does ADU approval take in Costa Mesa?
A complete application must be acted on within 60 days, with ministerial (by right) — no public hearing or discretionary design review for a qualifying ADU.
Do I have to live on the property to build an ADU in Costa Mesa?
Owner-occupancy: only for a jadu. ADUs under 750 sq ft are also exempt from impact fees.
Sources
- Costa Mesa Mun. Code § 13-35 (Accessory Dwelling Units)
- § 13-35(b)(5)a.1, b.1
- § 13-35(b)(5)a.2, b.2
- § 13-35(b)(9), (d)
- § 13-35(b)(8)b
- § 13-35(b)(8)c
- Cal. Gov. Code §§ 66310–66342 (ADU Law)
- HCD 2025 Accessory Dwelling Unit Handbook
- Cal. Gov. Code § 66317 (ministerial approval, 60-day deadline)
- Cal. Gov. Code § 66323 (by-right ADU/JADU, 800 sq ft, 4-ft setbacks)
- Cal. Gov. Code § 66333 (junior ADUs)
- Cal. Gov. Code § 66321 (size minimums & height)
- Cal. Gov. Code § 66322 (ADU parking; § 66334 covers JADU parking)
- Cal. Gov. Code § 66314 (local ordinance standards)
- Cal. Gov. Code § 66315 (owner-occupancy)
- Cal. Gov. Code § 66311.5 (impact fees: none for an ADU ≤ 750 sq ft or JADU ≤ 500 sq ft)
Last updated 2026-06-25. This is an AI-assisted summary of Costa Mesa's ADU rules, drawn from its published ordinance and California ADU law — general information, not legal advice, and it may be incomplete or out of date. Verify the controlling rule for your specific parcel with Costa Mesa Planning before relying on it.
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