Free written scope consultation - allowances, timeline, and change-order rules on paper before a single trade is scheduled
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AfterFast, Reliable Home Additions & ADU Across Ontario
Adding square footage or building an ADU in Ontario, CA typically runs $45,000 to $130,000 for an addition, and $90,000 to $220,000 for a standalone ADU, with windows & doors - Ontario projects and garage conversions landing between $30,000 and $80,000. Those are the numbers we quote on paper, in writing, before the first nail moves. Barrett & Sons Construction has been owner-operated since 1998, and when you call (877) 610-1318 about a project in Ontario, you’ll get a live person who knows the difference between a 1920s bungalow off Euclid Avenue and a post-2010 tract home in Ontario Ranch. That local knowledge shapes everything from foundation strategy to which permit window will actually get your plans through the city.
What to Look For When Hiring an Ontario Home Additions & ADU Contractor
Every contractor in the Inland Empire will tell you they can build. Far fewer will tell you, in writing, what happens when something goes wrong. Here’s what we’d ask any company before signing:
- Does the name on the contract match the person on-site? Tom Barrett founded this company in 1998, and his sons run crews. The person who signs your paperwork is the person you’ll see at the hardware store in Ontario. We publish our name, address, and license number. Look it up.
- Are change orders written, or spoken? An allowance that isn’t on paper is a surprise waiting for a date. Every scope adjustment on our jobs arrives as a written change order before work continues. No end-of-job invoices that make your jaw drop.
- Who pulls the permit? We do, on every eligible job. The inspector works for you, not for us. That’s why we like having them on-site. Permits pulled and inspections signed isn’t a slogan here; it’s Tuesday.
- What happens after the final check clears? We back every job with our 90-Day Done Right Promise. If something isn’t right, one call brings Tom back. No dispatcher, no call center, no “that’s outside our warranty window.”
Why Barrett & Sons Construction Is Ontario’s Preferred Home Additions & ADU Company
Ontario is our backyard. We’ve worked on craftsman bungalows in the 91762 ZIP near downtown, where original plaster walls and 1940s framing make every opening a negotiation with the building code. We’ve also framed additions in the 91764 neighborhoods east of the airport, where mid-century ranch homes need structural upgrades before you can think about a second story. Our crews know that a summer pour on a concrete footing in Ontario means a pre-dawn start, because July temperatures here routinely hit 108°F and rapid curing failures aren’t hypothetical. That’s the kind of local knowledge you only get from a contractor who’s been doing this work in the Inland Empire since 1998, not from a national outfit reading a script.
Our Ontario customers call us back because we don’t treat their home like a line item. A kitchen addition in a 1930s house near Chaffey High School needs different structural thinking than a sunroom off a 2015 tract home south of the 60. We don’t pretend otherwise. And when the Santa Ana winds rip through the Cajon Pass in October, we brace our active framing sites accordingly. You can’t learn that from a franchise manual.
Our Home Additions & ADU Services in Ontario
We’re the Home Additions & ADU team that handles everything from a 300-square-foot junior ADU to a full second-story addition. Here’s what Ontario homeowners call us for most often:
ADU Design & Permitting
Ontario’s ADU rules have shifted over the years, and the city’s planning department applies them differently to a 1920s lot near Euclid than to a 2018 lot in Ontario Ranch, which our Ontario Abodu service team navigates regularly. We handle the design, the plans, the permit pull, and the inspections. You handle picking the flooring. California’s streamlined ADU laws help, but local setback and height limits still vary by zone, and our team knows which questions to ask before you pay an architect for drawings the city won’t approve.
Garage Conversion ADU
Converting an attached or detached garage is the most cost-effective way to add a rental unit or in-law suite in Ontario, typically running $30,000 to $80,000 depending on condition and scope. Older garages in the 91762 and 91764 neighborhoods often need slab leveling and electrical service upgrades before conversion can start. Newer garages in the 91761 growth zone sometimes sit on compacted fill that has already settled unevenly. Either way, we address the foundation truth before we frame a single interior wall.
New Detached ADU
A ground-up detached ADU in Ontario generally runs $90,000 to $220,000. The spread comes down to size, site prep, utility runs, and finishes. If your lot sits on expansive clay soil, which is common across former agricultural flatland in southern Ontario, your foundation and drainage design matter more than your countertop choice. We plan for the soil we’re actually building on, not the soil a generic plan set assumes.
Junior ADU & Interior Conversion
A junior ADU inside your existing footprint can be the fastest path to rental income or multigenerational living. These conversions often mean reworking a primary bedroom suite, a large laundry room, or a bonus room into a self-contained unit with its own entrance, kitchenette, and bath. Ontario’s older homes sometimes surprise us here. Wall cavities in 1920s bungalows aren’t consistently framed, and plumbing stacks from the 1950s weren’t built for modern fixtures. We plan for what we’ll find, and our change order process means you’ll know the cost of any discovery before we proceed.
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.
Room Additions, Second Stories, and Sunrooms in Ontario
If you need more living space without adding a separate unit, we build room additions that feel like they were always there. A family room bump-out off a 1960s ranch in the 91764 ZIP, a primary suite addition on a 1940s home near downtown, or a dedicated home office that finally gets the desk out of the dining room. These projects typically land between $45,000 and $130,000 in Ontario, depending on square footage and structural work.
Second-story additions are where our engineering depth shows. Many of Ontario’s older single-story homes were never framed to carry a second floor. We assess the foundation, the existing walls, and the roof structure before we tell you what’s possible. Sometimes the answer is a full second story. Sometimes it’s a partial addition over a single room. Either way, you’ll get an honest engineering assessment, not a sales pitch.
Sunrooms and enclosed patios make sense in Ontario’s climate about nine months of the year. A well-built sunroom with proper insulation and tempered glass gives you extra square footage without a full addition’s foundation cost. We build these to code, with permits, because an unpermitted patio enclosure in Ontario becomes a headache when you sell.
Trusted Brands We Install in Ontario
We don’t install mystery materials off a van. On Ontario roofing and addition projects, we commonly spec Schluter waterproofing systems in wet areas, Sterling fixtures where budget-conscious durability matters, and GAF roofing products when an addition ties into a new roof plane. These are brands you can look up before you sign. We also know which products hold up when Ontario hits 112°F in August, because we’ve watched cheaper materials fail here. If you want fixtures or finishes outside our standard lines, we’ll write that choice into your scope so there’s no confusion about what’s included.
Common Home Additions & ADU Problems We See in Ontario Homes
- Differential settlement on cut-and-filled lots. Ontario Ranch and much of the southern growth zone was graded aggressively from former vineyard and dairy land. We regularly see slabs that have settled unevenly within five to eight years of original construction. Any addition or ADU built on top of that without proper soil compaction and foundation design inherits the problem.
- Heat-stressed framing and stucco on west-facing walls. Ontario’s July and August temperatures exceed 108°F regularly. Older homes with unshaded west walls show accelerated stucco cracking and framing stress. When we add onto these homes, we check the existing envelope before tying in new construction.
- Outdated electrical panels that can’t carry an ADU. Many 1950s and 1960s homes in the 91762 and 91764 ZIPs still have 60-amp or 100-amp panels. A new ADU with a kitchenette, mini-split, and laundry will overload that panel. We include panel upgrades in our written scope when they’re needed. No surprises at rough-in.
- Santa Ana wind damage on active framing sites. The Cajon Pass funnels serious wind through Ontario in the fall. Bracing standards that work in Chino Hills may not survive a Santa Ana event on an exposed Ontario lot. We over-brace during framing season and schedule inspections around wind forecasts when we can.
Pricing for Home Additions & ADU in Ontario, CA
We publish our ranges because you shouldn’t have to wade through three sales meetings to get a ballpark. A room addition in Ontario runs $45,000 to $130,000. A garage conversion ADU runs $30,000 to $80,000. A new detached ADU runs $90,000 to $220,000. What moves the number? Square footage, site prep, soil conditions on your specific lot, utility runs, and finish level. A junior ADU inside existing walls costs less than a ground-up unit because the envelope already exists. A second story costs more than a ground-floor bump-out because the structure below has to carry it. Every estimate we write is free, line-itemed, and fixed until you change the scope. And if you want a second opinion on another contractor’s written estimate, we’ll review it at no charge. Call (877) 610-1318 and we’ll walk your property with you.
We Also Serve Cities Near Ontario
If you’re in Montclair, Upland, Chino, or Claremont, you’re inside our regular service area. We coordinate scheduling from our Inland Empire office and keep crews local to each job. If you need Kitchen Remodeling in Ontario as part of a larger addition project, our team handles that scope under the same contract. The same goes for Bathroom Remodeling in Ontario, which often pairs with a primary suite addition. One team, one warranty, one name on the truck.
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 – Home Additions & ADU in Ontario
How fast can you start an ADU project in Ontario?
Most Ontario ADU projects can start design and permitting within two to four weeks of a signed contract, assuming no lot-specific complications like heritage status or unusual setback variances. The permit review timeline depends on the city’s current workload, not on us. Call (877) 610-1318 and we’ll give you an honest current wait estimate.
Do you cover all Ontario ZIP codes?
Yes. We work throughout 91762, 91764, 91758, and 91798, plus the 91761 Ontario Ranch growth zone. We know the difference between a downtown bungalow lot and a new-tract parcel, and we scope each job accordingly.
Is building an ADU in Ontario more expensive than in nearby cities?
Not meaningfully. Permit fees in Ontario are competitive with Montclair, Upland, Chino, and Claremont. The bigger cost drivers are site conditions, utility runs, and your finish choices. We keep our pricing transparent so you can compare line items, not just bottom lines.
Will the price change once work starts on my Ontario addition?
Only if you change the scope. Our contract includes fixed pricing for the work described, and any change arrives as a written change order before work continues. If we discover a hidden condition, like a settled slab or outdated wiring, we’ll show you the issue and the cost before we touch it. No allowance surprises, no end-of-job invoices you never saw coming.
What does the warranty cover on an Ontario ADU?
Every job carries our written 90-Day Done Right Promise. If something isn’t done right, one call brings Tom Barrett back to make it right. Beyond that, our workmanship is backed by the same accountability that’s kept our name in the Inland Empire since 1998. Ask anyone on your street, or look up the license number yourself.
If you’re thinking about an addition or ADU on your Ontario property, start with a conversation. Call (877) 610-1318 to schedule a free estimate. We’ll walk the lot with you, talk soil and sun exposure and Santa Ana bracing, and hand you a written number before we leave. That’s how we’ve done it since 1998, and that’s how we’ll do it on your project.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving Ontario, CA and the surrounding Inland Empire since 1998.