Accessory Dwelling Units (ADUs) in Ventura County have moved from edge-case investments to mainstream options for homeowners with side yard, backyard, or detached garage capacity. California's 2020-2024 ADU law overhauls (SB 9, AB 671, AB 2221, AB 1033) have pushed each of the four main Ventura County cities — Simi Valley, Thousand Oaks, Camarillo, Moorpark — to streamlined permit pathways. The question for homeowners is whether the math actually works: does the rental income plus the equity addition justify the build cost and the permit hassle? This page has a calculator that runs the numbers and walks the city-by-city differences in permit fees, allowed size, and typical rent comps.
Use the ADU ROI calculator
Pick your city, enter your size and cost inputs, and the calculator outputs cash yield, equity yield, payback, and city permit fee range. Estimate only — build costs and rent comps vary by parcel, design, and contractor. Verify with a licensed contractor, your appraiser, and a tax professional before relying on the result.
Estimate only. Build cost, rent comps, and equity addition vary widely by parcel and design. Verify with a contractor, appraiser, and tax pro before relying on the result.
The math behind the calculator
Total project cost is ADU square footage times build-cost-per-square-foot plus permit and design soft costs. For an 800 sqft detached ADU at $425/sqft plus $35K permit/design, total project is $375,000.
Net annual rent is monthly rent times 12 minus vacancy and management. For $2,800/month rent with 8% vacancy/mgmt, gross annual is $33,600 and net annual is $30,912.
Cash-on-cost yield is net annual rent divided by total project cost. For the example above: $30,912 / $375,000 = 8.2%. Equity addition yield is the appraiser's estimated added market value to the parcel divided by total project cost minus 1. For a $250K equity addition on a $375K project: ($250K/$375K) - 1 = -33%, meaning the project added less in equity than it cost — but the rental cash flow continues, so the combined return is still positive.
Rental payback years is total project cost divided by net annual rent. For the example: $375K / $30,912 = 12.1 years to recover the build cost from rent alone, ignoring equity addition and appreciation.
Simi Valley ADU specifics
Simi Valley permits ADUs up to 1,200 sqft under state law and local code, with up to 1,000 sqft for detached new construction and up to 1,200 sqft for attached. Garage-conversion ADUs are allowed. JADUs (Junior ADUs) up to 500 sqft within the existing home footprint are also allowed.
Permit fees in Simi Valley typically run $8,500-$14,000 total including building permit, planning fees, school district fee, and utility connection fees. Plan-check timeline currently runs 4-8 weeks for streamlined ADU permits.
Typical rent comps in Simi Valley as of May 2026: 600-800 sqft 1BR detached ADU $2,400-$2,900/month; 800-1,000 sqft 2BR $2,700-$3,300/month. Rent assumes long-term tenant; short-term (STR) rules are restrictive in most Simi neighborhoods and STR is not a reliable assumption.
Thousand Oaks ADU specifics
Thousand Oaks similarly permits ADUs up to 1,200 sqft under state law, with the city's local code aligning to state minimums. The city's ADU portal at the Planning Department has standardized plans available for streamlined approval.
Permit fees typically run $10,000-$18,000 total — slightly higher than Simi Valley due to broader fee schedule. Plan-check timeline currently runs 4-10 weeks.
Typical rent comps in Thousand Oaks as of May 2026: 600-800 sqft 1BR ADU $2,700-$3,300/month; 800-1,000 sqft 2BR $3,100-$3,800/month. Higher than Simi reflecting the broader Conejo Valley rental market.
Camarillo ADU specifics
Camarillo permits ADUs under state law with local code aligned. The city's Planning Division has a dedicated ADU streamlined process. Camarillo has been one of the more proactive Ventura County cities on ADU streamlining.
Permit fees typically run $9,000-$15,000 total. Plan-check timeline currently runs 4-8 weeks for streamlined ADU permits.
Typical rent comps in Camarillo as of May 2026: 600-800 sqft 1BR ADU $2,500-$3,000/month; 800-1,000 sqft 2BR $2,900-$3,500/month. Comparable to Simi Valley with slight coastal-influence premium.
Moorpark ADU specifics
Moorpark permits ADUs under state law. The city's smaller footprint and tighter staff capacity sometimes means longer plan-check timelines than the larger Ventura County cities — currently running 6-10 weeks.
Permit fees typically run $8,000-$13,500 total — the lowest of the four cities reflecting Moorpark's smaller fee schedule.
Typical rent comps in Moorpark as of May 2026: 600-800 sqft 1BR ADU $2,300-$2,800/month; 800-1,000 sqft 2BR $2,600-$3,200/month. Lowest of the four cities reflecting Moorpark's smaller rental market and slightly lower demand.
City comparison table
Below is a side-by-side of the four cities for a typical 800 sqft detached ADU build in 2026.
| City | Permit fees | Plan check | Typ 800 sqft rent |
|---|---|---|---|
| Simi Valley | $8,500-$14,000 | 4-8 weeks | $2,700-$3,300 |
| Thousand Oaks | $10,000-$18,000 | 4-10 weeks | $3,100-$3,800 |
| Camarillo | $9,000-$15,000 | 4-8 weeks | $2,900-$3,500 |
| Moorpark | $8,000-$13,500 | 6-10 weeks | $2,600-$3,200 |
The equity addition question
Most ADU projects do not add equity equal to their build cost. A $375K ADU project typically adds $200K-$300K in appraised market value to the parent property. This is a known gap and the reason the IRS does not treat ADU build cost as a wash on the sale — the cost basis adjustment is the actual build cost, but the market value addition is usually less.
Equity addition varies by neighborhood. In high-value zip codes (Westlake, Calabasas, Hidden Hills), the addition can be closer to 70-90% of build cost. In lower-tier zip codes, it can be 50-70%. The appraiser's adjustment depends on rental income capitalization plus the comparable sales data, which is still thin in some areas.
The combined return — rental cash flow plus equity addition plus eventual appreciation — usually makes ADUs financially positive over a 10-20 year hold. The pure flip-the-ADU-for-cash math rarely works in year one.
Financing the ADU
Common financing structures: cash-out refinance of the parent property to fund the ADU build; HELOC (home equity line of credit) drawn during construction; ADU-specific construction loans (a few credit unions and lenders specialize); or cash from savings.
Cash-out refinance works when current mortgage rate is at or above current refi rate. With rates in the 6.5-7% range in 2026, many existing mortgages from 2020-2022 at 3-4% should not be refinanced — the rate increase on the existing balance offsets the benefit. HELOC is often the better structure for those owners.
Construction loans for ADUs typically require a draw schedule tied to inspection milestones. Interest accrues on draws as funds are released. After completion, the construction loan converts to permanent financing or is rolled into a refi.
Tax treatment basics (verify with CPA)
Rental income from an ADU is taxable income. Expenses (mortgage interest allocable to the ADU, property tax allocable, utilities, repairs, depreciation) are deductible against rental income on Schedule E.
Depreciation: residential rental property depreciates over 27.5 years on a straight-line basis. For a $375K ADU project, that's roughly $13,600/year of depreciation deduction. This often makes the rental show a paper loss for tax purposes even when cash flow is positive.
When you sell the property, the ADU build cost is added to your cost basis. Depreciation recapture applies on the ADU portion at sale. Section 121 primary-residence exclusion only applies to the main dwelling, not the ADU rental portion.
Talk to your CPA before starting an ADU project, particularly if you plan to convert the ADU between rental and family use over time, or if the parent property may be sold in the near future.
Common ADU pitfalls in 2026
Cost overruns. The single most common pitfall is starting construction with an incomplete cost estimate. Site conditions (slope, soil, sewer lateral capacity), utility upgrades (electrical service may need to be upgraded from 100A to 200A; water meter may need upsize), and design changes during build can push final cost 15-30% above initial budget. Build a 20% contingency into your financing.
Rent assumption miss. Many homeowners assume top-of-range rent comps without verifying. The actual achievable rent depends on layout, parking allocation, separate utility metering, and the specific micro-location. Pull comparable rentals (Zillow, RentCafe, Apartments.com) for the specific ZIP and unit size before committing to a rent assumption in your ROI calculation.
Property tax reassessment. ADU construction is a 'new construction' event that may trigger reassessment of the added value (not the whole property — California Prop 13 protects the existing structure's basis). The ADU portion gets assessed at completed market value. This adds typically $2,000-$5,000/year to property tax on a $375K ADU project. Factor this into the ROI.
Insurance gap. Your existing homeowners policy may not cover the new ADU automatically. Notify your carrier when construction starts and again at completion. Coverage limits and premium may change. Some carriers will not insure a property with a rental ADU on the same policy and require a separate landlord policy on the ADU.
What I tell clients considering an ADU
What I tell clients: an ADU rarely makes sense as a pure investment with 1-3 year horizon. It makes sense for owners with a long-term hold horizon (10+ years) who want either the rental income, a family member living separately, or both.
The other common scenario: multi-generational housing. Adult children moving home, aging parents moving in. ADUs work very well for this when the design and finish level match the eventual occupant's needs.
If you're considering an ADU, get three contractor bids before relying on any cost estimate. The 2026 build-cost range is wide because contractor capacity, site conditions, and finish level all vary significantly. The calculator gives a planning estimate; the real number comes from contractor bids on your specific site.
Finally, talk to your CPA about the tax treatment before starting design. Depreciation, basis adjustment, and the eventual sale treatment (Section 121 vs Section 1031 vs recapture) can shift the after-tax economics meaningfully. The pre-build conversation with your tax advisor is much cheaper than the post-build cleanup.
Frequently Asked Questions
What is a realistic ADU build cost per square foot in 2026?
Detached new construction $375-$525/sqft. Garage conversion $250-$400/sqft. Attached addition $350-$475/sqft. Add $25,000-$50,000 for permit, design, and soft costs.
Which Ventura County city has the lowest ADU permit fees?
Moorpark typically runs $8,000-$13,500 — lowest of the four. Simi Valley $8,500-$14,000. Camarillo $9,000-$15,000. Thousand Oaks $10,000-$18,000.
Will an ADU add equity equal to build cost?
Usually not in the short term. Most ADU projects add 50-90% of build cost in appraised market value. The combined return — rental cash flow plus equity addition plus eventual appreciation — usually makes ADUs financially positive over a 10-20 year hold.
What's a typical ADU rent in Ventura County?
May 2026: 600-800 sqft 1BR $2,300-$3,300/month depending on city; 800-1,000 sqft 2BR $2,600-$3,800/month. Thousand Oaks and Camarillo are higher; Moorpark and Simi Valley are lower.
How long does the ADU permit process take?
Simi Valley 4-8 weeks plan check. Thousand Oaks 4-10 weeks. Camarillo 4-8 weeks. Moorpark 6-10 weeks. Construction adds another 5-9 months for detached new construction, 3-5 months for garage conversion.
Can I short-term-rent (Airbnb) the ADU?
Most Ventura County cities restrict short-term rentals significantly. Long-term rental (30+ day lease) is broadly allowed; STR rules vary by city and HOA. Verify before assuming STR income — the calculator assumes long-term rental which is the safer planning basis.
