Costa Mesa council updates ADU rules to match state law, approves preapproved design program

Costa Mesa City Council on Tuesday voted 7-0 to introduce an ordinance revising the city's accessory dwelling unit rules to reflect recent state law changes and guidance from the California Department of Housing and Community Development (HCD).

City Senior Planner Chris Yeager, who presented the item, said the revisions were required after several state statutes reorganized ADU provisions. "The changes to the number of allowed ADUs ... the new ordinance would allow for up to 8 detached ADUs on a multifamily site provided that the number of ADUs does not exceed the number of existing units," Yeager said.

The ordinance reworks allowed ADU counts and objective development standards. Among the changes: single-family properties may have a conversion ADU, a junior ADU and an additional detached or attached ADU; multifamily sites can add more detached ADUs in certain circumstances; maximum sizes for multifamily ADUs were adjusted to the state standard; and some previously subjective design rules were removed or rewritten as objective standards.

Yeager also described a legalization path required by AB 2533 for unpermitted ADUs built before Jan. 1, 2020: owners may apply to bring those units into compliance with building code requirements and thereby legalize them. "This allows unpermitted ADUs that were built prior to 01/01/2020 ... to seek legalization," Yeager said.

The council also approved a new preapproved ADU design program. Under the program, architects and builders may submit standardized ADU plans for staff plan-check review; approved designs will be posted on the city website so homeowners can select a ready-made plan. "The goal here is to allow for faster processing time and cost reductions for the residents," Yeager said.

Councilmembers asked for clarifications about how ADU affordability is calculated and the reliability of counts that cities may report to HCD. Anna McGill, the city's advanced planning manager, said staff used a county-provided affordability chart and a survey of recent ADU permit applicants to estimate how many ADUs met very low- or low-income thresholds. "This year ... our staff sent out a survey to any applicant who had applied for and either still in the process or received a building permit in 2024. And so we extrapolated the data ... to make an estimate," McGill said.

Councilmember Arliss Reynolds, who moved the measure, said she welcomed the preapproved plans and encouraged staff to explore incentives that could encourage deed-restricted affordable ADUs in future work. "I think it's a really popular low-impact way to add affordable units to our community," Reynolds said.

The ordinance was introduced for first reading; councilmembers agreed to staff follow-ups to clarify some permissive vs. mandatory language and to tighten wording in sections about driveway removal and façade treatments. The vote to introduce the ordinance was 7-0.

The council also directed staff to post the first set of preapproved ADU designs on the city's web site in the coming weeks and to include additional data collection on ADU applications so future progress reports to HCD will have firmer evidence about affordability levels.

The council's action was taken during a public hearing and comes as the state continues to revise ADU law and HCD presses cities to ensure local codes do not add discretionary or subjective barriers to construction.