Check your property's zoning, browse designs & get contractor estimates for your ADU project:
Last Updated 09/19/21
City Hint: The City of Costa Mesa permitted a total of 27 ADU projects in the past year of 2020.
Number of ADUs allowed:
One JADU and attached ADU or a detached ADU is permitted per single-family lot
At least one unit and no more than 25% of the existing unit count in a multi-family building (interior conversion)
One interior conversion ADU and One detached ADU
Two detached ADU
Types of ADUs allowed:
Detached ADU, Attached ADU, Junior ADU are allowed. ADUs may be new construction or converted from existing space. JADUs must be converted from existing space.
Accessory Dwelling Unit projects on single family or multi-family residential properties will not be subject to the minimum lot size of the underlying local zoning code or by special ordinance. ~ According To AB68
JADU Maximum Size: 500 square feet
Conversion ADU: No Maximum
Detached New Construction ADU: 1,200 square feet
Attached New Construction ADU: 1,000 square feet or no more than the greater of 50% of the square footage of the existing single family dwelling
Multiple-Family ADU Maximum Size: No Maximum
Multiple-Family ADU Maximum Size: 800 square feet
JADU and Conversion Setbacks: Maintain setbacks of existing structure
Detached and Conversion New Construction ADU Front Setback: 20 feet for single-story; 25 feet for second floor / two-story
Detached and Conversion New Construction ADU Side Setback: 4 feet for single-story; 5 feet for second floor / two-story
Detached and Conversion New Construction ADU Street Side Setback: 10 feet for single-story; 10 feet for second floor / two-story
Detached and Conversion New Construction ADU Rear Setback: 4 feet for single-story; 20 feet for second floor / two-story; 5 feet for second floor / two-story abutting a Public Right of Way
Detached Multiple-Family ADU Front: 20 feet
Detached Multiple-Family ADU Side: 4 feet
Detached Multiple-Family ADU Street Side: 20 feet
Detached Multiple-Family ADU Rear: 4 feet
May provide parking but not required.
For a detached ADU, fire sprinklers are required; attached ADU, required only if it was required for the principal residence.
Shape, materials and style:
An ADU is subject to the City’s Residential Design Guidelines. An ADU must contain an independent exterior access with an optional interior access. All residential guidelines apply to the construction of the ADU, with the exception of the required setbacks for the conversion of legal non-conforming structures and an ADU addition above an existing garage.
Conversion and JADU height limited to that of existing structure.
New Detached Single-story: 16 feet or height of any dwelling on the property, whichever is greater
New Detached Second floor / Two-story: 27 feet with a sloped-roof or 22 feet with a flat roof
Multiple-Family Detached ADU: Single-story: 16 feet or height of any dwelling on the property, whichever is greater
An application shall be considered ministerial and go through the standard plan check process. However, discretionary applications (Design Review, MCUP, CUP) may be required if an ADU deviates from the residential development standards and/or design guidelines. Note: A second story ADU addition or conversion to a habitable space requires a second story notification.
Number of Bedrooms:
Not specified by State Standards.
Required in either the primary dwelling or JADU.
All accessory dwelling units and junior accessory dwelling units must be connected to public utilities (or their equivalent), including water, electric, and sewer services. b. Except as provided in subsection iii below, the City may require the installation of a new or separate utility connection between the accessory dwelling unit, junior accessory dwelling unit and the utility. The connection fee or capacity charge shall be proportionate to the burden of the proposed accessory dwelling unit based on either its square feet or number of drainage fixture unit values. c. No separate connection between the accessory dwelling unit and the utility shall be required for units created within a single-family dwelling, unless the accessory dwelling unit is being constructed in connection with a new single-family dwelling.
*** Beginning in 2020, according to state law, all residential zoning districts, including single-family and multi-family, are allowed to build Accessory Dwelling Units. ***
An ADU is permitted in Single Family Residential Zone (R-1) and Medium Density Residential Zone (R-2MD) where one single-family dwelling unit exist on the property (see lot size requirements).
Conversions of Existing Structures:
If the accessory structure or garage received final inspections prior to February 17, 2018, when the local ordinance was effective, it may be converted to an ADU.
Code / Manual:
Residential Building Code:
The California Building Code (CBC) 2019 is adopted.
This estimate is based on recently completed projects in the area and is inclusive of all typical soft costs such as permitting, design and engineering fees.