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ADU Builders & Contractors in Ontario, CA

Ontario, CA homeowners looking to build an ADU have 329 licensed contractors to choose from, ready to help. Whether you're planning a garage conversion, detached ADU, or junior ADU, finding the right contractor in Ontario starts here.

📊 ADU Quick Facts for Ontario

Avg. Cost/sqft

$209-$384

Permit Timeline

60-120 weeks

Max Size

1200 sqft

Setbacks

4ft sides

Top 329 ADU Contractors in Ontario

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#1WEST UNITED BUILDER

Ontario, CA

West United Builder — licensed ADU contractor in Ontario, CA. Detached ADU construction. 7+ years experience.

View Profile📞 (909) 676 7720
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#2CANO ARCHITECTURE CONCRETE REPAIR

Ontario, CA

Cano Architecture Concrete Repair — licensed ADU contractor in Ontario, CA. Detached ADU construction. 44+ years experience.

View Profile📞 (909) 627 5477
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#3DOMINGUEZ JOD

Ontario, CA

Dominguez Jod — licensed ADU contractor in Ontario, CA. Detached ADU construction.

View Profile📞 (909) 938 0881
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#4PROJECT X DEVELOPER INC

Ontario, CA

Project X Developer Inc — licensed ADU contractor in Ontario, CA. Detached ADU construction.

View Profile📞 (626) 548 8678
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#5ADAMS JOHN I CONSTRUCTION

Ontario, CA

Adams John I Construction — licensed ADU contractor in Ontario, CA. Detached ADU construction. 10+ years experience.

View Profile📞 (909) 208 4358
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#6CHAVA'S MARBLE & TILE

Ontario, CA

Chava'S Marble & Tile — licensed ADU contractor in Ontario, CA. Detached ADU construction. 31+ years experience.

View Profile📞 (909) 827 4304
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#7DAVID LEUNG

Ontario, CA

David Leung — licensed ADU contractor in Ontario, CA. Detached ADU construction. 29+ years experience.

View Profile📞 (626) 926 7612
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#8STANGER CONSTRUCTION AND REMODEL

Ontario, CA

Stanger Construction And Remodel — licensed ADU contractor in Ontario, CA. Detached ADU construction.

View Profile📞 (909) 267 5888
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#9ALLIANCE CONSTRUCTION

Ontario, CA

Alliance Construction — licensed ADU contractor in Ontario, CA. Detached ADU construction. 17+ years experience.

View Profile📞 (909) 578 1698
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#10MARLU CONSTRUCTION

Ontario, CA

Marlu Construction — licensed ADU contractor in Ontario, CA. Detached ADU construction. 14+ years experience.

View Profile📞 (909) 841 2810
View All 329 Contractors →

💰 How Much Does an ADU Cost in Ontario?

Garage Conversion

$74,500 - $134,500

Converting existing garage to living space

Detached ADU

$159,000 - $334,000

New standalone unit (600-1,200 sqft)

Permit & Fees

$1,110 - $5,110

Plan check, permits, utility connections

* Costs vary based on size, finishes, site conditions, and contractor. Get quotes from multiple Ontario contractors to compare pricing.

❓ Frequently Asked Questions About ADUs in Ontario

How much does it cost to build an ADU in Ontario?▼
The average cost to build an ADU in Ontario ranges from $159,000 to $334,000, or approximately $209 to $384 per square foot. Garage conversions typically cost $74,500 to $134,500.
How long does it take to get an ADU permit in Ontario?▼
The ADU permit process in Ontario typically takes 60 to 120 weeks. California law requires cities to approve or deny ADU permits within 60 days of receiving a complete application.
What are the ADU size limits in Ontario?▼
In Ontario, detached ADUs can be up to 1200 square feet. Attached ADUs are limited to 50% of the primary dwelling or 1,200 sqft, whichever is less. Junior ADUs (JADUs) are limited to 500 sqft and must be within the existing home.
What is the setback requirement for ADUs in Ontario?▼
Ontario requires 4-foot setbacks from side property lines and 4-foot setbacks from rear property lines for new ADU construction. Converted structures (like existing garages) may maintain their current setbacks.
Do I need to provide parking for my ADU in Ontario?▼
No parking is required for ADUs in Ontario if your property is within half a mile of public transit, in a historic district, part of an existing multi-family building, or if the ADU is a garage conversion. Otherwise, one space may be required.

Also See ADU Contractors In

Los Angeles (3215)San Diego (2582)San Francisco (2170)Sacramento (1551)Riverside (925)Fresno (904)Anaheim (784)Irvine (760)

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