ADU Regulations in Costa Mesa, California (2026)

Regulations at a glance

Local rules & notes

Illegal Owner-Occupancy Requirement: The City attempted to reinstate owner-occupancy requirements for ADUs approved after Jan 1, 2025. HCD explicitly ruled this violates State Law. Subjective Design Standards Voided: The City's requirement for ADUs to be 'compatible' in exterior appearance is deemed subjective and unenforceable by HCD. Prohibition on Deed Restrictions: HCD argues that requiring a covenant or deed restriction as a condition of the permit is an impermissible 'additional standard'. 2nd Floor Setback Restrictions: The City requires 2nd floor ADUs to be set back 25 feet from the front and meet underlying zone side/rear setbacks. HCD states this is void if it prevents an 800 sq ft unit.

For the official ordinance and latest updates, check your city or county planning department.

Frequently asked questions about ADU regulations in Costa Mesa

• What is the maximum ADU size in Costa Mesa, California?

  In Costa Mesa, the maximum ADU size is typically 1200 square feet, depending on whether the unit is attached or detached. California state law allows up to 1,200 sq ft for detached ADUs and 800 sq ft for attached in many jurisdictions; Costa Mesa may follow state defaults or have local limits. Verify with Costa Mesa planning for your specific lot.

• What are the setback requirements for ADUs in Costa Mesa?

  In Costa Mesa, typical setback requirements are 4 feet from the side property line and 4 feet from the rear. These may be reduced to 4 feet under California Government Code § 65852.2 where applicable. Check with Costa Mesa planning for overlay zones or design standards that could affect your site.

• What is the maximum height for an ADU in Costa Mesa?

  In Costa Mesa, the maximum height for an ADU is generally 18 feet. State law often allows up to 16 feet for detached ADUs and up to 25 feet when within 4 feet of the property line in some cases. Confirm with Costa Mesa or county planning for your zone.

• Is ADU approval ministerial in Costa Mesa?

  In Costa Mesa, ADU approval is typically Ministerial (by-right) per Gov Code §65852.2. Ministerial (by-right) review means the city must approve qualifying applications without discretionary review, per California Gov Code § 65852.2. Your project must still meet objective standards; Costa Mesa planning can confirm the exact process.

• Are there parking requirements for ADUs in Costa Mesa?

  In Costa Mesa, parking for ADUs is generally Often waived per state law; check local ordinance. California law often waives parking for ADUs within a half-mile of transit or in certain historic districts. Check with Costa Mesa planning for your address.