Home Additions & ADU in Ontario - One Fixed Price, Permits Handled

Fast, Reliable Home Additions & ADU Across Ontario

Adding square footage to your Ontario home, whether it’s a second story, a room addition, or a full detached ADU, typically costs between $45,000 and $130,000 depending on scope, and most projects take 4 to 9 months from permit to final inspection. Ontario’s mix of 1920s bungalows in the historic core and brand-new tract homes near Ontario Ranch means no two addition projects look the same, but the fundamentals don’t change: permits pulled, change orders in writing, and a crew led by people whose names are on the truck. Call (877) 610-1318 for a free estimate on your addition or ADU, and we’ll walk the property with you before any numbers are discussed.

Call (877) 610-1318

Why Barrett & Sons Construction Is Ontario’s Preferred Home Additions & ADU Company

Barrett & Sons Construction has been owner-operated since 1998, and Tom Barrett puts his name, address, and license number on every contract. That matters in a city like Ontario, where homeowners have watched out-of-town crews roll through, finish a job, and disappear. Our Home Additions & ADU team works in Ontario neighborhoods from the historic Euclid Avenue corridor down to the new builds south of the 60, and when you call, a live person answers. Not a voicemail box. Not an out-of-state call center.

We’ve built additions near Chaffey High School, worked around mature trees in older Ontario Ranch-era lots off Grove Avenue, and pulled permits from the City of Ontario’s Building Department more times than we can count. That local reps-and-inspections familiarity means fewer surprises when the plan checker asks for a soils report or an updated Title 24 calc. Ask anyone on your street, or look up the license number yourself. Tom lives close enough to Ontario that if something’s not right on a job, he hears about it at the hardware store.

Every project comes with the 90-Day Done Right Promise in writing. If an addition or ADU isn’t done right, one call brings Tom back. No fine print, no arguing. That’s how we’ve kept Ontario homeowners calling us back for 29 years.

Our Home Additions & ADU Services in Ontario

ADU Design & Permitting

We’ve lived California’s ADU code changes for years now, and in Ontario that means navigating the city’s specific setback and parking standards alongside state mandates. We handle architectural drawings, structural engineering, Title 24 energy compliance, and the full permit package with Ontario’s Building and Safety Division, so you don’t sit at a counter wondering what form comes next. The permit inspector works for you, not for me, though we like to say anyway. Every change is on paper before a single nail moves.

Garage Conversion ADU

Ontario’s older core, especially around ZIPs 91762 and 91764, is full of detached garages from the 1920s through the 1950s that make ideal ADU conversions. Those structures often have aging foundations and original framing that need reinforcement before drywall goes up, and we factor that into the estimate from day one. A garage conversion in Ontario generally lands between $30,000 and $80,000, depending on whether you’re adding a full kitchen and bath or keeping it studio-simple.

New Detached ADU

On larger Ontario lots, particularly in the ranch-era neighborhoods north of I-10, a new detached ADU gives you rentable space without touching the main house. Ontario’s summer heat sits between 108°F and 112°F in July and August, so we size insulation, window shading, and HVAC runs for real sustained heat, not a mild coastal day. Detached ADU construction in Ontario typically runs $90,000 to $220,000, with site prep and utility trenching accounting for a healthy slice of that. We pull every permit and sign every inspection.

Junior ADU & Interior Conversion

If you’ve got a spare bedroom or underused interior space, a Junior ADU can create a separate living area with its own efficiency kitchen and exterior entry. These projects often cost less than a full addition and move faster through the city’s review process. In Ontario’s historic craftsman homes, we’ve converted enclosed porches and back bedrooms into nicely finished JADUs that respect the original trim and scale. We also handle Kitchen Remodeling in Ontario when an addition connects to a kitchen that needs reworking to flow right, and Bathroom Remodeling in Ontario when the addition includes a new bath or you’re updating an existing one alongside.

Trusted Brands We Service in Ontario

We don’t show up with mystery materials off the back of a van. On Ontario addition and ADU projects we install Schluter waterproofing systems in wet areas, Sterling bath fixtures where budget calls for solid value, and GAF roofing on any new or tied-in rooflines. You can look every one of those names up before you sign. We schedule material deliveries around Ontario’s early-morning calm, because staging lumber and drywall on a residential street at noon in August is asking for trouble.

Common Home Additions & ADU Problems We See in Ontario Homes

After years of estimating additions across Ontario, here’s what we see most often. The goal is to catch these before a project starts, not after the concrete has cured.

• Expansive clay soil under new foundations. Ontario Ranch sits on former vineyard and dairy flatland that was cut and filled aggressively. Within a few years of original construction, we regularly see differential settlement. Pre-build soil testing matters more here than it does ten miles north.
• Unpermitted additions from decades ago. Ontario’s old core has plenty of enclosed porches and converted garages that never saw a building inspector. When we add on, those illegal spaces can trigger code corrections before our permit gets approved, so we walk the property early and flag what’s on the table.
• Santa Ana wind pressure on open framing. The Cajon Pass funnels serious fall wind events straight into Ontario. Framing and temporary bracing have to be engineered for gusts that hit an exposed second-story wall with real force, or you’re chasing studs after the first big blow.
• Heat-affected concrete and exterior work. Summer pours in Ontario need pre-dawn starts and proper curing blankets or you get rapid surface cracking. Cal/OSHA’s outdoor heat standard also means our crews stop and hydrate, which keeps everyone sharp and the job on schedule.

Pricing for Home Additions & ADU in Ontario, CA

We Also Serve Cities Near Ontario

If you’re just outside the Ontario city limits, we work in Ontario and the surrounding communities of Montclair, Upland, Chino, and Claremont. Same owner-operated crews, same written pricing, same 90-Day Done Right Promise. Dispatch and scheduling run out of our Inland Empire office, so we can get to most nearby neighborhoods the same week you call.

FAQs - Home Additions & ADU in Ontario