ADU Basics in Ventura County: Rules, Permits and Planning Steps

Updated October 23, 2026 · General information, not legal or professional advice.

Accessory dwelling units, usually called ADUs, are one of the most common home projects in California. They can house a family member, add rental income or create a home office. California has passed a series of laws since 2017 that make ADUs easier to permit, and those state rules apply in every city in Ventura County and in the unincorporated county. This guide explains the basics so you can start a conversation with your building department and contractor with confidence. It is general information, not legal or design advice. Rules change often, so confirm details with the agency that will issue your permit.

What counts as an ADU

An ADU is a secondary home on the same lot as a single-family or multifamily home. It has its own kitchen, bathroom and sleeping area. California recognizes several types:

Detached ADU: a separate building, such as a backyard cottage.
Attached ADU: an addition connected to the main house.
Conversion ADU: space inside the existing home or an accessory building, such as a garage, converted into a unit.
Junior ADU (JADU): a smaller unit created within the walls of a single-family home, which may share a bathroom with the main house and must have an efficiency kitchen.

The California Department of Housing and Community Development (HCD) publishes an ADU Handbook that explains these types and the state rules in detail. It is the single most useful document to read before you start.

Key state rules that apply in Ventura County

State ADU law, found in the California Government Code, sets a floor that local governments must follow. Local ordinances can be more permissive but generally cannot be stricter than the state on the core standards. Some of the most important points from HCD's guidance are:

Permit timeline. Local agencies must approve or deny a complete ADU application within 60 days. This is ministerial review, meaning no public hearing is required for a project that meets the objective standards.
Setbacks. Local agencies cannot require side and rear setbacks greater than four feet for ADUs, and no setback is required when converting an existing legal structure.
Size. State law allows ADUs of at least 800 square feet in many situations, even where local rules on lot coverage or floor area would otherwise block them. Maximum sizes depend on the type of unit and the number of bedrooms. JADUs are limited to 500 square feet.
Owner occupancy. State law currently does not allow cities to require the owner to live on the property for an ADU. JADUs have different rules. Check HCD's handbook for the current position.
Parking. Many ADUs need no extra parking, for example those within half a mile walking distance of public transit, and replacement parking is generally not required when a garage is converted.
Impact fees. State law limits impact fees on smaller ADUs. Utility connection and school fees may still apply, so ask early.
Short-term rentals. Local agencies may prohibit renting ADUs for terms of 30 days or less.

City or county: who reviews your ADU

If your home is inside a city, apply to that city. Cities such as Ventura, Oxnard, Camarillo, Thousand Oaks, Simi Valley, Moorpark and Santa Paula each have their own ADU ordinance, application checklist and, in some cases, pre-approved plans. If you live in unincorporated areas such as Oak View, Somis, Saticoy or Piru, the County of Ventura Resource Management Agency (Planning and Building and Safety divisions) handles the application. Several jurisdictions in California now offer pre-approved ADU plan sets, which can shorten design time. Ask your planning counter whether one is available.

Special site conditions to check first

Coastal zone. Parts of Ventura, Oxnard, Port Hueneme and unincorporated coast fall within the coastal zone. ADUs there may need a coastal development permit, though state law also streamlines that process. The California Coastal Commission and your local agency can explain the steps.
Fire hazard severity zones. Much of the county, including hillside parts of Thousand Oaks, Simi Valley, Ojai and Ventura, is mapped by CAL FIRE. New buildings in these areas must meet wildland-urban interface construction standards, and access and water supply for fire fighting can affect the design.
Septic systems. Rural lots on septic need confirmation that the system can handle the added unit. County Environmental Health reviews septic capacity.
Easements, slopes and flood zones. A survey or title report can reveal easements, and FEMA flood maps show flood zones that change the construction requirements.
HOAs. State law limits how homeowners associations can block ADUs, but HOAs may still apply reasonable design rules.

Step-by-step: from idea to move-in

Research. Read the HCD handbook and your city or county ADU page. Pull your parcel information.
Feasibility. Have a designer or design-build contractor check the lot, utilities, setbacks and access. Ask whether water, sewer and electrical service need upgrades.
Design. Choose the unit type and layout. Plans usually include site plan, floor plans, elevations, structural drawings and Title 24 energy compliance documents. New detached ADUs generally must include solar under the California Energy Code, with some exceptions.
Submit and respond to comments. The 60-day review clock starts with a complete application. Plan reviewers often send correction comments; quick, complete responses keep things moving.
Hire and build. Use a licensed contractor with ADU experience. A general contractor (B license) normally coordinates the trades.
Inspections and final. The unit needs a final inspection, and usually utility sign-offs, before it can be occupied. Your county assessor may reassess the value added by the new construction.

Choosing an ADU builder

ADUs combine site work, foundations, framing, utilities and finishes, so experience matters. Ask how many ADUs a contractor has completed in your city, whether they handle permits and utility applications, and who is responsible for design corrections. Check every license on the CSLB website using our license check guide, and review our 25 questions to ask a remodeler. Be wary of companies that offer to build without a permit or ask for large payments before work starts; see contractor red flags.

Common mistakes to avoid

Designing before confirming utility capacity and sewer connection locations.
Assuming a garage conversion is simple when the slab, framing or electrical service needs major upgrades.
Forgetting that a rental ADU has landlord obligations under California law.
Not budgeting time for plan check corrections and utility company schedules.

Find help near you

Browse licensed general contractors in Ventura County cities, or use the form on this page to describe your ADU project. Our Ventura County permit guide covers related permit questions.

ADU questions homeowners ask most

Can I build both an ADU and a JADU? State law allows many single-family lots to have one ADU and one JADU, and in some cases a detached new-construction ADU plus a JADU. HCD's handbook explains the combinations.

Can I sell the ADU separately? Generally ADUs cannot be sold separately from the main home, though state law allows limited exceptions where a city has adopted an ordinance permitting separate sale under specific conditions. Ask your city.

Will my property taxes go up? New construction is typically assessed for the value it adds, while the existing home keeps its assessed value. The County of Ventura Assessor can explain how this works for your property.

What about legalizing an existing unpermitted unit? State law includes provisions that help owners bring unpermitted ADUs built before certain dates into compliance, unless there are health and safety issues. Your building department can explain the process.

How long will it take? The permit review clock is 60 days from a complete application, but design, corrections, utility work and construction take longer. Plan for the full process, not just the permit.

Official sources: HCD Accessory Dwelling Units and ADU Handbook; County of Ventura Resource Management Agency; California Coastal Commission; California Energy Commission; CSLB.

Ready for the next step? Get matched with a pro using the short form on this page, or search the directory.

← All guides