A free site visit, a check of what your lot allows and a written, line-item price.
Get your free estimateADU Company in Ontario, CA
Ontario, CA is one of the fastest-moving ADU markets in the Inland Empire, and for good reason. The city’s older single-family lots off Euclid Avenue, combined with a new generation of buyers looking for multigenerational housing, has pushed accessory dwelling units from a niche idea to a standard home improvement. We’re Fielding ADU Builders, the ADU and room-addition team inside the Garrison Partners family. We build to a written price, we build by the book, and we stand behind the work for a full year. Call (361) 315-4177 for a free estimate.
Why Ontario Homes Are Different
Ontario’s housing stock runs older than what you’d find in the newer master-planned communities east of the 15. The original neighborhoods along Holt Boulevard and around Chaffey High School were built from the 1920s through the 1970s, which means shorter setbacks, detached garages set at the rear lot line, and electrical panels that were never sized for a second dwelling. That changes how we approach an ADU here. Ontario also sits at the base of the San Gabriel foothills, so we get real summer heat. Roof loads, insulation choices, and HVAC sizing for an attached ADU all get picked with triple-digit July weeks in mind. The city’s planning department reviews ADU applications under California state law, which keeps the process moving, but Ontario still requires site plans, structural drawings, and title-24 energy compliance. We pull the permits, schedule the inspections, and hand you a copy of the final sign-off before we call the job done.
What you’ll notice most is the lot geometry. Many Ontario parcels are deep and narrow, which means the best ADU solution is often a detached unit at the rear, converting the old garage, or a junior ADU carved out of an oversized primary bedroom. The right answer depends on where your sewer lateral runs and where the city easement sits, and we’ll map both before we recommend a plan.
Our Main Ontario Services
Most Ontario homeowners call us for one of three things: a permitted room addition, a garage conversion that actually gets finished, or a backyard ADU. Each service has its own page below.
Room Addition Contractor in Ontario
A room addition in Ontario usually means bumping out toward the rear lot line or capturing a covered patio and turning it into conditioned square footage. The older homes here need a close look at the foundation and existing roofline before we commit to a price, because tying a new slab into a 1960s footing on expansive soil takes more than a standard detail. We handle the structural plans, the title-24 energy calc, and the city inspection from first dig to final sign-off. Read the full write-up on our Room Addition Contractor in Ontario page.
Garage Conversion Contractor in Ontario
Ontario has thousands of detached garages sitting at the end of long driveways, and converting one into a rental unit or an in-law suite is often the cheapest path to new living space. The catch is that many of these garages were built on a thin slab with no vapor barrier, and the electrical runs are minimal. We handle the slab inspection, the panel upgrade if needed, insulation to current code, and the change from garage door to a real exterior wall. Details are on our Garage Conversion Contractor in Ontario page.
ADU Builders in Ontario
A detached ADU in Ontario gives you a full second home in the backyard: kitchen, bath, bedroom, and its own address. We work with both conventional framing and prefab platforms, matching the method to your lot access and budget. Ontario’s setback rules and utility connection requirements mean the site plan drives the price more than anything else, and we put that price in writing before you approve anything. See our ADU Builders in Ontario page for full scope and pricing.
What happens when you call
- 1
A real person answersNo phone trees - you reach a local pro.
- 2
You get an upfront price rangeHonest numbers before anyone is dispatched.
- 3
A background-checked tech heads outLicensed & insured, dispatched right away.
- 4
You approve before work beginsNothing starts until you say go.
Brands We Service in Ontario
We build with conventional materials and with prefab platforms that Ontario homeowners are already researching. We’re experienced with Abodu, Cover, and Dvele systems, and we also run conventional builds with James Hardie siding. Parts are sourced through our Inland Empire supply chain, not shipped from out of state, which keeps lead times shorter. See our brands for the full list.
Pricing for ADU Company in Ontario, CA
| Service | Typical Range in Ontario |
|---|---|
| Garage conversion (standard, with permits) | $85,000 – $135,000 |
| Detached ADU (conventional build) | $180,000 – $320,000 |
| Prefab ADU (platforms like Abodu or Cover) | $160,000 – $290,000 |
| Junior ADU (interior conversion) | $60,000 – $110,000 |
| Room addition (primary suite, 250 – 400 sq ft) | $95,000 – $170,000 |
| Room addition (great room / kitchen bump-out) | $130,000 – $220,000 |
| Panel upgrade to 200A (if needed) | $3,500 – $6,500 |
| Title-24 energy documentation | $1,200 – $2,500 |
| Site plan and structural engineering | $4,000 – $9,000 |
In Ontario, the main price swing comes from site conditions: how far the utility connections are from the proposed ADU, whether the existing panel needs upgrading, and whether the soil under the new foundation requires deeper footings. Every quote we hand you is a written price before any work starts. The number you approve is the number on the final invoice.
Neighborhoods We Serve in Ontario
We run crews throughout Ontario and the surrounding area, from the older grid neighborhoods near downtown to the newer developments south of the 60. Response is same-week for estimates across the city.
- Downtown Ontario and the Euclid Avenue corridor
- Old Town / Historic District
- Mountain Avenue area
- South Ontario near Ontario Mills
- Archibald Ranch
Service Area – Cities Near Ontario
While Ontario is our core market, we also schedule estimates and builds in the neighboring cities that share the same building departments and permit patterns. Homeowners in Montclair, CA, Claremont, CA, Upland, CA, and Rancho Cucamonga, CA call us for the same written-price, warrantied work we do in Ontario.
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 About ADU Company in Ontario
We typically schedule estimates within the same week for Ontario homeowners. The visit takes about an hour: we walk the lot, check the panel, look at the sewer lateral location, and talk through which build method fits your budget. Then the written price follows, usually within a few days. Call (361) 315-4177 to set a time.
Yes. Every ADU, garage conversion, and room addition we build in Ontario goes through the city’s permit process. We submit the drawings, schedule the inspections, and confirm the final sign-off is in your hand before the job closes. We don’t do unpermitted work, and we won’t ask you to sign off on a job that skipped a required inspection.
No. The Garrison Standard means the price goes in writing before any work starts. If you approve that number, that’s the number on the final invoice. The only exception is if you ask us to change the scope after approvals, and we’ll write that change order down and get your signature before we proceed. Call (361) 315-4177 for a free estimate and a written price.
What happens when you call
- 1A real person answersNo phone trees - talk to a local adu pro.
- 2You get an upfront price rangeHonest numbers before anyone is dispatched. No surprises.
- 3We come out and measureLicensed & insured, at a time that suits you - the visit and the quote are both free.
- 4You get the price in writingLine-itemed, yours to keep, and nothing starts until you say go.
What Ontario Customers Say
"Honest feedback on what the lot could fit. Told me up front what would fit on the lot and what the setbacks would allow."
- Rita C."Clear price breakdown. Change orders were all in writing."
- Hal P."Solid job. Cabinet delivery was a week behind but they worked around it."
- Ben G.
