Fast, Reliable New Detached ADU Across Ontario
A new detached ADU in Ontario typically runs $185,000-$340,000 all-in, from the first site survey through final inspection, with custom builds at the higher end and modular or prefab systems toward the lower. We’re Nadia Ellery’s team at Ellery ADU Studio, coordinating design, permitting, and construction from our Inland Empire office so Ontario homeowners get a single contract and a single written price. If you’re on a county parcel outside city limits, on a flight path near Ontario International Airport, or working with a post-WWII ranch lot in the older core, the permitting path and build specs change materially. Call us at (909) 637-3715 - a live person answers, and we’ll schedule your free estimate with a written quote before any work starts.
Why Ellery ADU Studio Fontana Is Ontario’s Preferred New Detached ADU Company
We’ve designed, permitted, and built over 900 ADUs since 2015, the year California’s ADU laws shifted. Many of those sit in Ontario and the surrounding Inland Empire corridor. Our crews know the difference between a city of Ontario permit set and a San Bernardino County submission, and we scope that correctly before quoting so the price you sign is the price you pay.
Ontario homeowners leave documented feedback. We don’t summarize how they felt; we show you the photo record of what was found and what was built. Under Haven Standard Clause 1, every project gets a written price before any work begins. No “call for a quote” language appears anywhere in our process. Clause 5 locks the scope at signing - no door-side upsells, no scope creep.
Dispatch and scheduling run from our Riverside-San Bernardino metro office, with crews serving Ontario directly. Response to Ontario inquiries is same-day during business hours, and a live person answers calls 24/7. We don’t promise callbacks that don’t materialize.
Our license number appears on every page, verified, not just claimed. Background-checked, uniformed crews handle every site visit, with a documented photo record delivered to you after each one.
Our New Detached ADU Services in Ontario
Custom Built ADU
Custom detached ADUs in Ontario’s older residential core - the neighborhoods west of Euclid Ave and around the historic downtown grid - often start with a post-WWII ranch on a 7,000-10,000 sq ft lot. These homes frequently carry 100-amp electrical panels and original galvanized supply lines that must be upgraded before permits clear. A custom build lets you match the existing roofline, specify the exact bedroom count, and integrate with your main home’s landscaping. In Ontario, we regularly see custom builds run $220,000-$340,000 all-in, depending on whether we’re pulling city permits or navigating county well and septic setbacks on acreage outside the city limits.
Prefab ADU Installation
Prefab systems from brands like Plant Prefab and Boxabl can compress the construction timeline for Ontario homeowners who need workforce housing renting fast to the ONT logistics corridor. The catch: prefab still requires a full permit set, foundation, and utility tie-ins. On county parcels around Ontario, we coordinate well and septic setback surveys before the module ever ships. Prefab installations in this market typically land between $185,000-$260,000, with the factory build quality controlled and the site work scoped separately in your written quote.
Modular ADU
Modular construction - Dvele and Mighty Buildings are the systems we specify most often - delivers a fully finished box that crane-sets onto your prepared foundation. For Ontario’s open lots with long material hauls and limited equipment access, modular reduces on-site labor days and weather exposure. The inland thermal basin’s 105°F peaks and 30°F+ night swings mean we specify radiant-barrier roof sheathing and correctly sized Mini-split capacity as standard, not upgrades. Modular ADUs in Ontario generally range $195,000-$280,000, with the AIA sound-attenuation package adding $8,000-$15,000 if you’re south of the 60 freeway.
Studio ADU and 1-Bed & 2-Bed ADU
Studio ADUs work well on tighter Ontario lots where setback variance - the extra distance the city requires between your new ADU and the property line - eats into buildable footprint. One-bedroom and two-bedroom configurations dominate our Ontario pipeline because they match the rental demand from Ontario International Airport’s freight and logistics workforce. A 1-bed detached unit in Ontario typically runs $200,000-$275,000; 2-bed units with full kitchens and stacked laundry land at $245,000-$340,000. We size the Mini-split and insulation package for the thermal basin, not coastal LA specs.
What happens when you call
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A real person answersNo phone trees - you reach a local pro.
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You get an upfront price rangeHonest numbers before anyone is dispatched.
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A background-checked tech heads outLicensed & insured, dispatched right away.
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You approve before work beginsNothing starts until you say go.
Before
AfterTrusted Brands We Service in Ontario
We specify and coordinate prefab and modular systems from Dvele, Mighty Buildings, Boxabl, and Plant Prefab within the same single contract that covers your design, permit set, and site work. For Ontario’s climate, we frequently pair modular shells with James Hardie exterior cladding and radiant-barrier roof sheathing - materials that handle the aggressive thermal cycling our inland basin delivers. Because we maintain relationships with these manufacturers’ West Coast distribution networks, material lead times for Ontario projects stay predictable, and we build that schedule into your written quote upfront.
Common New Detached ADU Problems We See in Ontario Homes
- County versus city permitting confusion. Homeowners on acreage around Ontario assume the city will issue their ADU permit. On county parcels, San Bernardino County controls the process, and well and septic setback distances plus a separate county plan check add weeks and real cost. We verify jurisdiction before quoting.
- Inadequate insulation and cooling specs for the thermal basin. Ontario’s 105°F summer highs and extreme diurnal temperature swings crack concrete slabs and stucco on ADU shells built to coastal standards. We specify radiant-barrier roof sheathing and Mini-split capacity for inland conditions, not LA beach specs.
- Missing the Airport Influence Area review. South of the 60 freeway, ADUs in noise-affected zones trigger sound-attenuation requirements - STC-rated windows, upgraded insulation, mechanical ventilation in lieu of operable windows. Contractors who miss this see their ADU fail plan check after materials are ordered.
- Electrical and plumbing upgrades on post-WWII stock. Ontario’s 1950s-1970s ranch homes frequently have 100-amp panels and original galvanized supply lines. Both must be upgraded before the ADU permit clears, and we scope that work into your initial quote rather than discovering it mid-project.
Pricing for New Detached ADU in Ontario, CA
Here’s what Ontario homeowners can expect, all-in, drawings through final inspection:
| ADU Type | Typical Range in Ontario |
|---|---|
| Studio ADU (custom) | $185,000 - $240,000 |
| 1-Bed ADU (custom or modular) | $200,000 - $275,000 |
| 2-Bed ADU (custom or modular) | $245,000 - $340,000 |
| Modular ADU (Dvele, Mighty Buildings) | $195,000 - $280,000 |
| Prefab ADU (Plant Prefab, Boxabl) | $185,000 - $260,000 |
| AIA Sound-Attenuation Package (south of 60 freeway) | Add $8,000 - $15,000 |
| County well/septic setback survey | Add $2,500 - $4,500 |
These ranges reflect Ontario’s specific conditions: county permitting complexity, thermal-basin climate specs, and the AIA overlay. Under Haven Standard Clause 1, your written quote locks these line items before any work begins. No change orders, no door-side upsells. Call (909) 637-3715 for your free estimate.
We Also Serve Cities Near Ontario
Our crews work throughout the western Inland Empire, including New Detached ADU in Ontario and neighboring Montclair, Upland, Rancho Cucamonga, and Mira Loma. Each city carries its own permitting authority and climate considerations, and we adjust the permit set and build spec accordingly.
Serving Ontario, CA - Our Local Coverage Area
We’re based in the Ontario area and know this community well. Use the map below to see our service coverage - if you’re nearby, we can almost certainly help.
FAQs - New Detached ADU in Ontario
It depends on your parcel’s jurisdiction. Lots inside Ontario city limits use city planning and building departments. Parcels on acreage outside the city limits fall to San Bernardino County, which runs a separate plan check and enforces well and septic setback distances that city permits don’t address. We verify jurisdiction during your free site survey and build the correct permitting path into your written quote. Call (909) 637-3715 to schedule.
Yes, typically $8,000-$15,000 more. The Ontario International Airport Influence Area triggers sound-attenuation construction requirements: STC-rated windows, upgraded wall insulation, and mechanical ventilation in lieu of operable windows. These are plan-check requirements, not optional upgrades, and missing them means your ADU fails review after materials are ordered. We flag AIA status before quoting and include the package in your written scope. Call (909) 637-3715 for an exact quote - estimates are free.
Because the existing systems often can’t support the additional load. Ontario’s 1950s-1970s ranch stock frequently has 100-amp electrical panels and original galvanized or early copper supply lines. San Bernardino County and the city of Ontario both require these upgrades to current code before they’ll clear an ADU permit tied to the same meter or water service. We inspect and scope this work into your initial quote so it doesn’t surface as a surprise. Call (909) 637-3715 for a free assessment.
We stage concrete pumps from the street when rear access doesn’t exist, grade the pad precisely for drainage away from the foundation, and coordinate crane or flatbed delivery for modular units based on the haul route from the nearest freeway. On a recent 1-bed detached ADU near Euclid Ave and the old downtown grid, our crew pumped concrete over a graded pad and hauled a James Hardie and radiant-barrier shell package in from the street because the lot sat open with no rear access. Every access constraint gets a line item in your written quote. Call (909) 637-3715 to walk your site.
Ontario’s inland thermal basin demands more than coastal specs. We specify radiant-barrier roof sheathing, R-19 minimum wall insulation, and Mini-split capacity sized for 105°F peak loads with a buffer for the extreme diurnal swing. The 30°F+ night temperature drop cracks concrete slabs and stucco on shells built to milder standards. Our permit set includes these specs as standard, not upgrades, because a detached ADU that can’t maintain 78°F in August is not a rentable unit. Call (909) 637-3715 for a written quote that names every line item.
Ready to build? Call (909) 637-3715 to schedule your free estimate. A live person will answer, and you’ll receive a written, line-item quote before any work begins - that’s Haven Standard Clause 1, and it’s how we’ve operated since 2015.
Written by Nadia Ellery, Owner at Ellery ADU Studio, serving Ontario and the Inland Empire since 2015.