What does it cost to build an ADU in Ventura County?

A 400-600 sq ft detached ADU with mid-range finishes typically costs $150,000-$220,000 all-in in Ventura County as of 2026. An 800-1,200 sq ft ADU runs $280,000-$400,000. Garage conversions sit lower at $100,000-$180,000. The ranges include construction, permit and plan-check fees ($3K-$15K), utility connection charges ($5K-$30K), design and engineering ($8K-$25K), and 10-15% contingency. Calabasas pre-approved ADU plans can shorten the permit timeline. Hillside lots and long utility runs can add $30K-$100K+.

ADU Cost in Ventura County (2026)

ADU builds in Ventura County (and adjacent Calabasas in LA County) typically run $150,000 to $400,000 all-in as of 2026. The spread depends on size, finish level, the city's permit fees, and whether you have to extend utilities. Calabasas now offers pre-approved ADU plans that can cut permitting time meaningfully. I'm Brian Cooper, REALTOR(R) at eXp Realty. Below is the breakdown and what most homeowners miss.

Direct AnswerAn ADU build in Ventura County typically costs $150,000 to $400,000 all-in, depending on size (400-1,200 sq ft), finish level, city permit fees, and utility hookup needs. Calabasas (adjacent LA County) offers pre-approved plans that cut permitting time. Budget 10-15% contingency on top.

Data current as of May 2026.

Direct Answer

A small detached ADU around 400-600 sq ft with mid-range finishes typically lands $150K-$220K all-in. A larger 800-1,200 sq ft detached unit with full kitchen, separate utility meters, and higher finish runs $280K-$400K. Garage conversions sit at the low end - usually $100K-$180K.

These ranges assume a standard buildable lot with no unusual site work, soils issues, or hillside construction. Hillside builds, retaining walls, and long utility runs can add $30K-$100K+.

Why this question matters

ADUs are one of the few ways to add long-term housing income on a single-family lot in this market without buying a second property. At Ventura County rents, a completed 800-1,000 sq ft ADU can rent for $2,400-$3,200/month, which is a real return on a $250K-$300K build over a long hold.

But buyers and homeowners regularly underestimate the all-in cost by leaving out permit fees, utility connection charges, school impact fees, and contingency. The headline number from a contractor is rarely the actual number you'll pay.

The detail behind the answer

Here's the cost breakdown buyers usually need to see.

Cost category	Typical range
Construction (400-600 sq ft)	$120K-$200K
Construction (800-1,200 sq ft)	$220K-$340K
Permit + plan check fees	$3K-$15K
Utility connections (water/sewer/electric)	$5K-$30K
School / impact fees (if applicable)	$0-$10K
Design + engineering	$8K-$25K
Contingency (10-15%)	$15K-$50K

How to verify

Pull at least three bids from licensed contractors with ADU experience. Ask for the bid to include or itemize all permit fees, utility connection charges, and design costs. Then verify the jurisdiction's current fee schedule on the city or county planning website.

For Calabasas, check the city's pre-approved ADU plan program. The city has offered pre-approved plan sets that meaningfully shorten the plan-check timeline. Other Ventura County cities are at different stages of offering similar programs. Confirm current status on each city's planning page.

Step 1: Pull three contractor bids with full fee itemization.
Step 2: Verify city's permit fee schedule on the planning page.
Step 3: Check whether your city has a pre-approved plan program.

What I tell clients

Decide upfront whether the ADU is for income, multigenerational use, or future resale value. The right finish level depends. A rental-oriented ADU doesn't need premium finishes; a parents'-suite ADU usually does. Be honest about the use case before specifying the build.

And get the ADU added to your homeowner insurance policy and clarified for rental income reporting. These post-build items matter for both rental yield and resale, and they're easy to skip in the construction excitement.