Building an ADU in Ventura County costs $100,000 to $380,000 depending on type and size in 2026. Garage conversions start at $100K, $180K, while detached ADUs run $180K, $380K. A well built ADU adds $150,000 to $250,000 in property value and generates $1,800 to $3,200/month in rental income across Simi Valley, Thousand Oaks, and Moorpark.
ADUs (Accessory Dwelling Units) are the fastest growing housing type in California. New state laws have made it easier than ever to add a second unit to your property, whether that is a detached backyard cottage, a garage conversion, or an attached addition. In Ventura County, a well built ADU adds $150,000 to $250,000 in property value and can generate $1,800 to $3,200 per month in rental income.
What ADUs Cost to Build
| Type | Cost Range |
|---|---|
| Garage conversion (400 to 600 sq ft) | $100K, $180K |
| Detached ADU (500 to 800 sq ft) | $180K, $280K |
| Detached ADU (800 to 1,200 sq ft) | $250K, $380K |
| JADU (within existing home, up to 500 sq ft) | $50K, $100K |
California ADU Laws: What You Need to Know
California has some of the most ADU friendly laws in the country. Key provisions: most single family lots can build one ADU plus one JADU. Cities cannot require owner occupancy for ADUs (through 2030). Setback requirements have been reduced to 4 feet. Parking requirements are eliminated for ADUs near transit or within half a mile of a bus stop.
Each city implements these laws slightly differently. Moorpark has a unique 3 agency permit process including a school developer fee. Simi Valley offers streamlined permitting for ADUs under 800 square feet. We know the permit process in every Ventura County jurisdiction.
ADU Timeline: How Long It Takes
From design to move in, expect 6 to 12 months for a detached ADU and 4 to 8 months for a garage conversion. The permit process alone takes 4 to 12 weeks depending on your city. Construction runs 3 to 6 months for most projects.
Rental Income Potential
ADU rental rates in Ventura County (as of early 2026): Simi Valley 1BR ADUs rent for $1,800 to $2,200/month. Thousand Oaks and Moorpark command $2,000 to $2,800. Calabasas and Woodland Hills can reach $2,500 to $3,200+. At the mid range, an ADU generating $2,200/month pays back its construction cost in 7 to 10 years, while also adding six figures to your property value.
What Would Your ADU Cost?
Get a free instant estimate for your ADU project. Garage conversion, detached new build, or addition, answer 5 questions and get a real cost range.
Get Instant EstimateDeep Dives: ADU Resources
ADU Costs in California 2026
Realistic cost ranges for detached ADUs, garage conversions, and JADUs.
Read guide →ProcessADU Building Guide: Ventura County
Complete building process from design to move-in for Ventura County ADUs.
Read guide →RegulationsCalifornia ADU Laws 2026
Current state and local regulations, setbacks, size limits, and owner-occupancy rules.
Read guide →FinancingADU Financing Options
How to pay for an ADU: construction loans, HELOCs, cash-out refinance, and state programs.
Read guide →InvestmentADU Rental Income in Ventura County
What ADUs rent for in Simi Valley, Thousand Oaks, and other Ventura County cities.
Read guide →ConversionGarage Conversion ADU Guide
Converting your garage to an ADU: costs, permits, and what to expect.
Read guide →PermitsMoorpark ADU Permit Process
Moorpark's unique 3-agency permit process and the school fee nobody tells you about.
Read guide →ComparisonADU vs Room Addition
Which makes more sense for your property: an ADU or a room addition?
Read guide →Ready to learn about our ADU construction services?
View ADU Construction Services