ADU Cost CalculatorSimi Valley & Ventura County — 2026 Prices
Answer a few quick questions and get a personalized cost estimate for your ADU. Free, instant, based on real Ventura County project data. Garage conversions, attached, and detached units.
ADU Costs in Ventura County (2026)
ADU construction costs in Simi Valley, Thousand Oaks, Moorpark, and surrounding Ventura County cities run $250–$450 per square foot depending on type and finishes. California labor rates, strict building codes, and Ventura County's notoriously slow permit process all impact the final price. Here's what to expect based on ADU type.
Garage Conversion
$80,000 – $150,000Convert an existing garage into a studio or 1-bed ADU. Includes foundation/floor prep, framing, electrical, plumbing, HVAC, drywall, flooring, and finishes. Parking is lost. Timeline: 3-6 months plus 6-12 weeks for permits. Fastest and most affordable ADU option. Works on virtually any lot. Best ROI per dollar invested.
Attached ADU
$150,000 – $250,000New 1-2 bed unit attached to the main house. Shares one wall with primary residence. Includes site prep, foundation, framing, all MEP systems, finishes, and permitting. Parking preserved. Timeline: 4-8 months plus 6-12 weeks for permits. Most popular ADU type in Simi Valley and Thousand Oaks. Balances cost and rental potential.
Detached ADU
$180,000 – $350,000+Standalone 1-2 bed unit. Includes site grading/drainage, foundation, complete framing, roof, full MEP systems, finishes, and yard improvements. Timeline: 6-12 months plus 8-12 weeks for permits. Highest upfront cost but best for long-term rental income, future sale flexibility, and maximizing property value. May require soil testing and grading reports.
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Get Your Free Estimate →What Does an ADU Include? Cost by Component
Understanding cost breakdown helps you prioritize and plan. Here's a typical component breakdown for an attached ADU in Ventura County:
| Component | Cost Range | % of Budget |
|---|---|---|
| Foundation & Site Work | $15,000 – $35,000 | 10-15% |
| Framing & Structure | $20,000 – $45,000 | 12-18% |
| Plumbing | $8,000 – $15,000 | 5-8% |
| Electrical | $8,000 – $15,000 | 5-8% |
| HVAC & Ventilation | $6,000 – $12,000 | 4-6% |
| Kitchen & Bathroom Finishes | $20,000 – $40,000 | 12-18% |
| Flooring & Wall Finishes | $12,000 – $25,000 | 8-12% |
| Labor (General) | $30,000 – $60,000 | 20-25% |
| Permits & Fees | $5,000 – $15,000 | 3-6% |
| Design & Architecture | $2,000 – $6,000 | 1-3% |
Labor and framing/structure represent the largest cost categories. Permit fees in Ventura County are particularly high due to the county's thorough review process and required inspections (soils, grading, electrical, plumbing, building, final).
Cost Per Square Foot by ADU Type
ADU construction in Ventura County typically ranges $250–$450 per square foot depending on type and finishes. Here's what that breaks down to:
Garage Conversion
$250–$350/sf
Uses existing structure, no site work
Attached ADU
$300–$400/sf
New construction, moderate site prep
Detached ADU
$350–$450/sf
Full build, extensive site work
ADU Permits & Regulations in Ventura County
California AB 2221 and SB 9 fundamentally changed ADU regulations, but Ventura County still has some of the strictest requirements in the state. All ADUs require building permits with no exceptions.
California State ADU Law (AB 2221 & SB 9)
- • Single-family R-1 lots can have up to 2 ADUs total (1 attached, 1 detached OR 2 attached)
- • No lot size minimums for garage conversions and junior ADUs (permanent living space in main home)
- • No deed restrictions can prevent ADUs
- • Ministerial approval required (no discretionary review)
- • Parking minimums eliminated for ADUs near transit
- • Density bonuses apply to developments with 50%+ affordable units
City-Specific Requirements
Each Ventura County city has interpreted state law differently. Here's what we typically encounter:
Simi Valley
Allows garage conversions and attached ADUs. Detached ADUs require Design Review. Permits: 8-10 weeks typical.
Thousand Oaks
More restrictive. Requires lot size minimums for detached ADUs. Design Guidelines strict. Permits: 10-12 weeks typical. Higher fee costs.
Moorpark
Most permissive in Ventura County. Approves garage conversions quickly. Attached/detached allowed on most lots. Permits: 6-8 weeks typical.
Newbury Park, Camarillo, Oxnard
Moderate to permissive. Most allow detached ADUs. Permits: 8-12 weeks typical. Check specific design guidelines.
ADU Permit Costs in Ventura County
ADU permits are expensive in Ventura County. Expect $5,000–$15,000 in fees depending on type and city:
Garage Conversion
$5,000–$8,000
Attached ADU
$7,000–$12,000
Detached ADU
$10,000–$15,000
Permit costs include building permit, electrical, plumbing, mechanical, and grading/drainage review. Inspections are separate (typically $1,000–$3,000 depending on scope). As a licensed contractor (Lic #1066117), we handle all permitting, inspections, and coordination with your city.
ADU Construction Timeline in Ventura County
Ventura County permit processing is notoriously slow. Here's a realistic timeline for an attached ADU from start to move-in:
Total typical timeline: 6-12 months from initial design to move-in. Garage conversions can compress this to 4-6 months (permits still take 6-8 weeks). Detached ADUs often stretch toward 12+ months if soil testing or grading reports are required.
ADU ROI & Rental Income in Ventura County
ADUs consistently deliver strong returns on investment. In Ventura County's tight rental market, ADU units rent quickly and command good rates.
$1,500–$2,800
Monthly rental income
20–30%
Property value increase
5–10 yrs
Payback period
Typical ADU Rental Rates in Ventura County (2026)
Studio ADU (250–350 sf)
$1,400–$1,800/month in Simi Valley; $1,600–$2,000 in Thousand Oaks
1-Bed ADU (450–550 sf)
$1,600–$2,200/month in Simi Valley; $1,900–$2,500 in Thousand Oaks
2-Bed ADU (600–700 sf)
$1,900–$2,500/month in Simi Valley; $2,300–$2,800 in Thousand Oaks
Property Value & Tax Impact
Adding an ADU typically increases your property value 20–30%. However, your property is reassessed for the ADU portion, which increases property taxes.
Example: A $200,000 ADU investment might increase property value by $40,000–$60,000. Annual property tax on the ADU portion would be roughly $4,000–$6,000 (approximately 1.25% of ADU value). If you rent the ADU for $2,000/month, annual rental income is $24,000. After property tax, insurance, and maintenance (~$6,000–$8,000/year), you net $10,000–$14,000 annually. Payback period: 5–10 years depending on construction cost and rental rates.
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ADU Resources & Guides
Explore in-depth guides on ADU laws, local permitting, costs by city, and financing options to help you make an informed decision about your ADU project.
2026 California ADU Law Changes
→Learn how AB 2221 and SB 9 affect ADU eligibility, regulations, and timelines in Ventura County.
Simi Valley ADU Costs
→Detailed cost breakdown for garage conversions, attached, and detached ADUs in Simi Valley for 2026.
Newbury Park ADU Guide
→Permits, regulations, costs, and timeline specifics for ADU projects in Newbury Park.
Oak Park ADU Guide
→Oak Park ADU regulations, permit costs, and local requirements for 2026.
Oxnard ADU Guide
→Complete guide to ADU regulations, permit timelines, and costs in Oxnard.
ADU Rental Income Guide
→Rental rates, income projections, and ROI analysis for Ventura County ADU investments.
ADU Financing Options
→Financing options, loans, and funding strategies for ADU construction in California.
ADU Construction Services
→Our full-service ADU design, permitting, and construction process from start to completion.