Cost to build accessory dwelling unit — Ontario, CA

Last updated: September 24, 2026

What affects the price

Building an ADU involves several moving parts that shape your total budget. The biggest factor is whether you choose a prefabricated unit or a custom build on-site. Site preparation is also significant; if your land requires extensive grading, utility trenching, or foundation work, costs will climb. Your choice of finishes—like kitchen cabinetry, flooring, and siding materials—also shifts the bottom line. We help you navigate these variables to find a setup that fits your property. Exact pricing confirmed free on the call.

About this service

When calculating the budget for a secondary unit, consider the costs for architectural design, permits, materials, and labor. Custom designs and high-end interior finishes will push your budget higher, while standardized layouts can keep costs more predictable. Exact pricing confirmed free on the call.

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: Julian Ranch, The Preserve, San Antonio Heights, Central Ontario, Chino, Jurupa Valley — and every surrounding area.