Accessory dwelling unit contractor — Ontario, CA

Last updated: September 24, 2026

What affects the price

Building an ADU involves several moving parts that shape your total investment. The final figure depends heavily on whether you choose a prefab unit or a custom build from the ground up. Site conditions are another major factor; if your property requires extensive grading, utility trenching, or foundation work, costs will climb. You also have to consider your choice of finishes, like high-end appliances or custom cabinetry. Navigating local permit fees and impact assessments also adds to the total.

About this service

A contractor oversees the physical build, managing the subcontractors, materials, and daily construction progress. They are responsible for making sure the structure is built according to the approved plans and local safety standards. You need a contractor when you have your designs finalized and are ready for the actual building phase to begin. They act as the primary point of contact for all site activity.

Frequently asked in Ontario

Also serving nearby: Chino · Rancho Cucamonga · Jurupa Valley · Pomona · Fontana

All cities in California →

More on this: HUD ADU resources — ADU policy and financing research.

Service area details

Serving all of Ontario — population 182,457; about 3,650 residents per square mile across 50.0 sq mi. ZIP codes covered include 91758, 91761, 91762, 91764.

Neighborhoods served in Ontario: Downtown Ontario, Central Park, San Antonio Heights, Montclair, Upland — and every surrounding area.