Costa Mesa, CA Accessory Structures: ADU Rules (2026)

Section 13-35 permits ADUs and Junior ADUs in single-family, multifamily, and mixed-use zones shown on the Citywide Land Use Matrix, and only on a legal lot with a proposed or existing residential dwelling that will remain on site. Applications meeting the section's standards are approved ministerially, with no public hearing, notice, or discretionary zoning review, per Government Code § 66317. On a single-family lot, one detached new-construction ADU, one conversion ADU, and one Junior ADU may be built; a detached ADU tops out at 1,200 square feet, an attached ADU at the greater of 50% of the existing home's floor area or 1,000 square feet.

On a multifamily lot, conversions within the existing structure are capped at 25% of the existing units (minimum one), and up to eight new detached ADUs are allowed, limited to the number of existing units on the lot. Standard setbacks are four feet from side and rear property lines, but no setback applies to ADUs converted within an existing structure's footprint or to units that keep an existing structure's dimensions. Height is capped at 27 feet and two stories, though an 800-square-foot detached ADU with four-foot side and rear setbacks and 18-foot height is allowed regardless of the underlying zone's standards.

Applications must be filed by the property owner on a city form with the fee set by council resolution, ADUs and JADUs cannot be sold separately from the primary residence, and each unit gets its own mailing address. ADUs over 800 square feet must also meet the 40% open space standard of section 13-32.