ADU Cost Calculator: Ventura County 2026
Compare a garage conversion, junior ADU or detached unit. Get a preliminary range before a site review and written scope.
Step 1 of 4
Type & size
The biggest swing factor. New build costs 2 to 3× a conversion.
Within Safeway's core Ventura County service area.
School fees may apply; development impact-fee exemption by size
Factual update:
Understand what the estimate represents
A Simi Valley garage conversion and a detached ADU in Thousand Oaks can have very different scopes. This calculator lets you compare type, size, finish level and selected site conditions. Its numbers come from a pricing model, not a verified survey of 2026 local jobs.
The range is an early planning tool. It cannot inspect a slab, check a sewer route or determine what an agency will charge. Use it to identify the decisions that matter, then confirm the complete budget against plans and a site-specific written proposal.
Example budgets with the same selections
| Scenario | Rounded planning range |
|---|---|
| 400 sq ft garage conversion | $130,000–$180,000 |
| 600 sq ft detached ADU | $240,000–$330,000 |
| 1,000 sq ft detached ADU | $355,000–$485,000 |
Model examples as of September 16, 2026: Simi Valley ZIP 93065, standard finishes, one bedroom, one bathroom, laundry, private entrance, flat lot, no selected separate meter or septic work. The detached model includes a solar allowance. These are size comparisons, not recommended layouts or completed-project prices. Your selections change the result.
What needs a separate check
- Plans, engineering, surveys and any required soils or drainage work.
- The condition of an existing garage, including foundation, moisture, structure and residential code requirements.
- Utility capacity, provider requirements, trenching routes and actual connection charges.
- Agency fees and school district clearance for the specific address. A ZIP code is only an approximate location input.
- Any proportional development impact fees for an ADU above 750 square feet, which the calculator does not calculate.
- Unusual grading, retaining walls, major service upgrades, financing costs and costs outside the agreed construction scope.
Keep an allowance for unresolved conditions until the design and inspections answer those questions. When comparing bids, use the same drawings and list of inclusions. A lower number with missing scope is not a like-for-like comparison.
School fees and impact fees use different size rules
In 2026, the state development impact-fee exemption covers ADUs with 750 square feet of interior livable space or less. The specific statewide school-fee exclusion applies to units with less than 500 square feet. At exactly 500 square feet, obtain district clearance instead of assuming school fees are waived.
The calculator reserves a provisional school-fee allowance at 500 square feet and above. A district may offer a broader exemption or use a different rate. Building and plan-check charges remain separate; water and sewer charges have separate rules. Read Government Code section 66311.5 and request written assessments for the actual project.
Plan beyond the construction window
The displayed timeline is an indicative active-construction assumption. Feasibility, design, engineering and permit review come before it. Utility work and final occupancy approval also affect the date the unit can be used. Set those milestones with your project team.
A 60-day permit decision rule does not promise a complete project in two months. See our 2026 ADU law guide for the application stages, school-fee distinction and coastal considerations.
Evaluate family use, rental income and funding
An ADU may give family members independence or support a long-term rental plan. For a rental, obtain current comparable listings and estimate expenses before relying on an income figure. Construction cost does not establish resale value, occupancy or payback.
CalHFA’s current program information says its ADU grant funds were fully reserved as of December 28, 2023. Do not subtract a presumed grant from your budget. Confirm any new funding, eligibility and award directly with the program.
ADU calculator questions
How much does an ADU cost in Simi Valley?
Cost depends on the structure, size, finishes, utilities and site. Our September 16, 2026 standard-finish calculator scenarios produce roughly $130,000 to $180,000 for a 400-square-foot garage conversion and $240,000 to $330,000 for a 600-square-foot detached ADU. These are model outputs with stated assumptions, not verified market averages or contractor proposals.
Does the estimate include every project cost?
No. The model includes construction categories and provisional agency allowances. Architectural and engineering work, surveys, actual agency charges, major utility upgrades and unusual site work need separate verification. A written proposal should identify all inclusions, allowances and exclusions.
Are school fees waived for ADUs under 750 square feet?
That is not the statewide school-fee rule. In 2026 the development impact-fee exemption covers ADUs with 750 square feet of interior livable space or less. The specific school-fee exclusion applies below 500 square feet. A district may provide additional exemptions; obtain written clearance for the actual address.
How long does an ADU take?
The model uses indicative active-construction windows of 3 to 4 months for a JADU, 4 to 5 months for a garage conversion and 6 to 11 months for a detached unit, depending on size and scope. Design, engineering, permit review, utility scheduling and final occupancy approval are separate stages. The displayed construction range is not a complete project schedule.
Does the calculator confirm my lot is eligible?
No. It does not check parcel boundaries, easements, legal structures, utilities, coastal or hazard overlays, or the applicable ADU category. Confirm feasibility with the permitting agency and design team before committing to plans or financing.
Can I count on an ADU grant or a rental return?
Do not include an unconfirmed award or assume a particular rent. CalHFA currently reports that its ADU grant funds were fully reserved as of December 28, 2023. Obtain current program confirmation, local rent comparables and an expense budget for the actual unit.
Move from a range to a defined scope
Safeway Construction is based in Simi Valley, with 20+ years of California remodeling experience and license #1066117. Bring your address, intended unit size and preferred use to the first conversation. We can identify the project questions that a written proposal must answer.
See our ADU construction services, Simi Valley budget guide or Oxnard permit guide. For a property-specific discussion, call (805) 222-6544.
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