How Much Does an ADU Cost in Ventura County in 2026?

Broad 2026 planning ranges in Ventura County run about $100,000 to $220,000 or more for a garage conversion, $150,000 to $250,000 or more for a small detached ADU, $200,000 to $350,000 or more mid-size, and $300,000 to $450,000 or more for a larger detached unit. These are planning ranges, not quotes, and site conditions move them substantially.

Why the range is that wide

Because an ADU is a complete small house, and small houses have a fixed cost floor. One kitchen, one bathroom, one electrical service, one set of utility connections and one permit cost roughly the same whether the unit is 400 square feet or 750. Everything above that floor scales with size and complexity.

What actually moves the number

• Utility runs. Trenching sewer, water and electrical from the house or street to a new building at the back of a deep lot. Frequently the largest single surprise on the job.
• Electrical service capacity. If the existing panel cannot carry a second dwelling, a service upgrade comes first.
• Converting versus building new. A usable existing slab, walls and roof removes real construction.
• Site conditions. Slope, poor soils, retaining and tight access all add cost.
• Fire requirements. Ignition-resistant construction in wildland-urban interface areas is a genuine line item, and much of the unincorporated county is affected.
• Jurisdiction. Camarillo requires a solar electric system on new detached ADUs. Oxnard coastal parcels need a Coastal Development Permit. Both cost money and time.

The 750 square foot line

An ADU with 750 square feet of interior livable space or less is generally exempt from certain local agency, special district and water corporation impact fees. Above that, impact fees are generally charged in proportion to the primary dwelling. Different rules can apply to other fees, including school-related fees and utility charges.

What we would tell you on the phone

A range, honestly, in about five minutes. What we cannot responsibly do is quote your project before anyone has looked at where your sewer lateral runs, what your panel can carry and what your jurisdiction requires.

Read the full ADU cost guide for the detailed breakdown.

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.