How United Build Partners helps Ontario homeowners
We're not a construction company. United Build Partners is an ADU advisory and contractor-matching service: we start with a free review of your property and goals, help you understand what's realistic for your lot under Ontario's rules, and — when it's a good fit — connect you with up to three contractors in our network suited to your project type and neighborhood. From there, you compare proposals and decide who to work with; the licensed contractor you choose handles the actual design and construction.
ADU rules and permitting in Ontario
Like every California city, Ontario's ADU rules build on the statewide floor — ministerial review and a 60-day clock on complete applications (our California ADU laws guide covers the framework). Locally, the rules live in Chapter 5 of the Ontario Development Code (§5.03.010), and the Building Department publishes the process end to end.
What's specific to Ontario
- Preapproved ADU plan program. The city maintains a preapproved ADU plans pathway with its own published approval-process document — using one can bypass much of design review. Ask the Building Department what's currently in the catalog.
- All-electronic submittal. ADU plans are submitted electronically through the city's Citizen Portal Access site — no counter-only bottleneck — using the published ADU Plan Submittal Requirements checklist (rev. October 2024).
- Five reviewers, including the city utility. Plans may be reviewed by Building, Planning, Engineering, Fire, and OMUC — the Ontario Municipal Utilities Company. A city-run utility in the chain is a timeline factor most investor-owned-utility cities don't have.
- ADUs on single-family and multifamily lots. Ontario permits ADUs on lots with existing or proposed single-family or multifamily dwellings; JADUs (up to 500 sq ft, efficiency kitchen) are limited to lots with one single-family home.
- Standard setbacks. Four-foot minimum setbacks to property lines for the ADU, with front-yard setbacks following the underlying zone.
- Published FAQ handout. The city's ADU FAQ handout (revised January 21, 2025) is the best first read — it answers the size, height and landscape questions homeowners actually ask.
Property and lot considerations
Ontario's residential stock spans older neighborhoods near downtown with mature lots and alleys, and newer master-planned areas — including the fast-growing Ontario Ranch to the south, where HOAs and newer utility infrastructure are common. Older central lots with existing garages are natural conversion candidates; newer tracts more often pencil for detached units where yard geometry allows. Flat terrain keeps site work simpler than in foothill cities — utility routing and lot coverage are usually the questions that matter here.
Common ADU types for Ontario homeowners
A detached ADU — potentially from the preapproved catalog — fits Ontario's many conventional lots, and a garage conversion suits the older core. A Junior ADU works within larger single-family homes, and multifamily property owners have their own path under state multifamily ADU law.
Cost factors in Ontario
Inland Empire construction pricing generally runs below the coastal counties, and Ontario's flat lots help keep site work predictable. Budget the OMUC review and any utility work early — utility line items belong in the bid, not the change orders. Anchor against our cost guide and soft-cost categories, then get exact fees from the city.
Financing your Ontario ADU
Most homeowners fund ADU construction through a HELOC, cash-out refinance, a construction loan, or some combination — see our ADU financing overview and loan payment calculator for how each works. Financing options aren't generally city-specific, but a lender may consider your neighborhood and local rents when evaluating projected rental income as qualifying income.
How our process works
Frequently asked questions
Yes — the city maintains a preapproved ADU plans pathway with a published approval process. Using a preapproved plan can bypass much of design review, and under AB 1332 the city must act on a complete preapproved-plan application within 30 days. Ask the Building Department what's currently in the catalog.
Electronically, through the city's Citizen Portal Access site, following the ADU Plan Submittal Requirements checklist. Review can involve Building, Planning, Engineering, Fire, and OMUC — the city's own utility — so complete first submissions save real time.
The Planning Department at 909-395-2036 or PlanningCounter@ontarioca.gov for zoning-side questions (size, setbacks, height), and the Building Department for permit-process questions. The city's ADU FAQ handout, revised January 2025, answers most of the common ones.