How Big Can an ADU Be in Ventura County?

Detached ADUs up to 1,200 square feet are widely allowed across Ventura County jurisdictions, and state law protects a baseline of 800 square feet at 16 feet in height meeting four-foot side and rear setbacks. Junior ADUs are capped at 500 square feet. Several cities also tie attached unit size to a percentage of your existing house.

The numbers that matter

• 500 square feet. The ceiling for a Junior ADU, which must sit entirely within an existing or proposed single-family home.
• 750 square feet of interior livable space. At or below, an ADU is generally exempt from certain local agency, special district and water corporation impact fees.
• 800 square feet at 16 feet. A baseline local lot coverage standards generally cannot be used to block, where four-foot side and rear setbacks are met.
• 1,000 to 1,200 square feet. Common local ceilings, varying by city and by attached versus detached.

Where local rules differ

Santa Paula caps an ADU at 50 percent of the existing living area or 1,200 square feet, whichever is smaller.

Fillmore allows detached units up to 1,200 square feet but limits attached units to 50 percent of the primary dwelling's floor area or 1,000 square feet, whichever is smaller.

Oxnard allows ADUs from an efficiency unit up to 1,200 square feet under its 2026 ordinance.

Maximum allowed is not the same as maximum sensible

Crossing 750 square feet of interior livable space changes which fees apply. More area also means more of every system, for space that in a great many family situations does not get used.

Try our ADU size calculator.

ADU laws and local requirements can change. Information on this page is general and is not a guarantee that a specific project will be approved. Confirm current requirements for the property and jurisdiction before making project or financial decisions.