Best ADU Builders in Ontario, CA
Compare vetted, CSLB license-verified ADU builders and ADU contractors in Ontario. The average cost to build an ADU in Ontario is $250–$400 per square foot. Top-rated builders include Acton ADU, Villa Homes, Nestadu.
Every builder below is CSLB license-verified, bonded, and insured — vetted before they ever reach a homeowner. Browse ratings and real reviews, compare starting prices, or let us match you with the best fit for your lot and budget. It's free; we're paid by the builders, not you.
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What kind of ADU are you planning?
ADU activity in Ontario
ADU projects in Ontario commonly budget $250-$400 per sq ft, or about $200,000-$320,000 for an 800 sq ft ADU, before site conditions, utility upgrades, and finish choices.
Showing 4 of 4 builders in Ontario, CA
Last updated July 16, 2026
License VerifiedActon ADU
Serving Ontario, CA · Campbell, CA
“We had a great experience working with Acton ADU on our ADU project, and I want to give a special shoutout to Sandro. Sandro was fantastic from start to finish—…”— N K
Pricing on request
A California general building contractor licensed since 1992, serving the LA market statewide.
License VerifiedVilla Homes
Serving Ontario, CA · San Francisco, CA
“Excellent job! The Villa crew from start to finish nothing short of professional. Foundation, home delivery, finish work etc top notch. I would recommend Vill…”— M Ferreira
Pricing on request
Tech-enabled prefab ADU builder delivering turnkey backyard homes across California.
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License VerifiedNestadu
Serving Ontario, CA · Mill Valley, CA
“Nestadu did a great job on my backyard ADU. Their team was professional, and the final result looks fantastic—well-built and blends in nicely with my home. Pric…”— Stephen Reid
Pricing on request
Custom and prefab ADUs for California homeowners — design, permit, and build, turnkey.
License VerifiedBuild ADU Home Remodel
Serving Ontario, CA · Ontario, CA
“We hired Jessica and her team to help us remodel our kitchen and bathroom at our house here in Upland. Instantly, from the moment we met her, you can tell she w…”— Cesar Martinez
Pricing on request
CSLB-verified B | C54 contractor in Ontario, CA.
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Frequently Asked Questions
- How much does it cost to build an ADU in Ontario, CA?
- In Ontario, ADUs typically run $250–$400 per square foot all-in. A detached 800 sq ft unit commonly lands between $230k and $340k depending on finishes, site conditions, and utility connections. Garage conversions are the most affordable path, often under $180k.
- How long does it take to build an ADU in Ontario?
- Plan to budget 6–12 months end to end. California law caps ADU plan check at 60 days for compliant designs, and construction usually takes 4–8 months after permits are issued.
- Do I need a permit to build an ADU in Ontario?
- Yes. All ADUs require a building permit, but California mandates ministerial (non-discretionary) approval for compliant units, so there's no public hearing or design review. Your builder typically pulls the permit on your behalf.
- How do I choose a builder in Ontario?
- Confirm the contractor holds an active CSLB license, is bonded and insured, and has built ADUs in Ontario specifically. Every builder in this directory is license-verified. Use "Get a Quote" to be matched with the best fit for your lot and budget.
ADU costs in Ontario: city vs. metro vs. state
Building an ADU in Ontario runs roughly $250–$400 per square foot all-in. Here's how the city benchmarks against the broader metro and statewide averages.
| Area | Avg. cost / sq ft | Typical 800 sq ft ADU |
|---|---|---|
| Ontario | $250–$400 | $200,000–$320,000 |
| Ontario metro | ~$320 | ~$256,000 |
| California (CA) | ~$330 | ~$264,000 |
Cost by ADU type in Ontario
| ADU type | Typical size | Project cost | Timeline |
|---|---|---|---|
| Detached ADU | 600–1,200 sq ft | $180,000–$420,000 | 8–12 months |
| Attached ADU | 400–800 sq ft | $140,000–$300,000 | 6–10 months |
| Garage Conversion | 400–600 sq ft | $90,000–$180,000 | 4–7 months |
| JADU (Junior ADU) | ≤ 500 sq ft | $60,000–$140,000 | 3–6 months |
ADU rules & permits in Ontario, CA
Local ADU permitting in Ontario follows California state ADU law, which caps plan-check at 60 days for compliant units. Confirm setbacks, height, and parking rules with the local building department.
| Jurisdiction | Permit timeline | Permit fees | Notes |
|---|---|---|---|
| Ontario (independent city) | 60 days max by CA statute | Set by local ordinance | Permits are issued by the Ontario building department, not the City of Los Angeles |
| Statewide (CA) | 60 days max by statute | Varies | Ministerial approval; owner-occupancy requirement waived through 2030 |
Financing your ADU in Ontario
Common ADU funding paths include renovation and construction loans, a HELOC, or a cash-out refinance. ADU-specific lenders underwrite against after-build value, which often unlocks more than a standard HELOC.
Construction / renovation loan
Funds released in draws as the build hits milestones; underwritten against after-build value.
HELOC / home equity
Borrow against existing equity. Fast to set up if you have 20%+ equity.
Cash-out refinance
Replace your mortgage and pull equity as cash — best when rates are favorable.
ADU-specific lenders
RenoFi and similar lenders underwrite the projected value the ADU adds, often unlocking more.
Pre-ADU Checkup — $499 — Get finalized permits, a pre-ADU checklist, and an on-site feasibility visit. Homeowners save $1,000s by doing this before engaging a contractor.