Building an ADU in Ventura County costs $100,000–$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–$250,000 in property value and generates $1,800–$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 (400–600 sq ft) | $100K – $180K |
| Detached ADU (500–800 sq ft) | $180K – $280K |
| Detached ADU (800–1,200 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?
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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?
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