Costa Mesa, CA Accessory Structures: Garage Conversions (2026)

Garage conversions in Costa Mesa are primarily governed by California's ADU laws, which preempt local parking replacement requirements. Under Government Code Section 65852.2, converting an existing garage, carport, or covered parking structure to an ADU is permitted without requiring the property owner to provide replacement parking spaces. This removal of the parking replacement mandate was a key feature of California's 2020 ADU reforms designed to encourage ADU production. To convert a garage to a habitable ADU, the property owner must obtain a building permit from Costa Mesa's Building Division. The conversion must bring the space into compliance with California Building Code standards for habitable rooms, including minimum ceiling height of 7 feet, natural light and ventilation through operable windows, adequate electrical outlets, smoke and carbon monoxide detectors, and proper insulation. If the ADU includes a kitchen or bathroom, plumbing permits are required, and the space must connect to the sewer system. The garage door may be replaced with a solid wall, window, or sliding door. The converted unit must have independent exterior access and cannot share a common entrance with the main dwelling unless the conversion is for a junior ADU (JADU) under 500 square feet. Impact fees are waived for garage conversions to ADUs under 750 square feet per state law. The Planning Division at (714) 754-5245 reviews ADU applications for zoning compliance, and the Building Division issues the construction permits.