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 Montclair
Adding square footage to a Montclair home usually runs $45,000 to $130,000 for a room addition, while a full detached ADU typically lands between $90,000 and $220,000. Most homeowners in the 91763 zip code call us because their 1950s-70s slab foundation needs evaluation before anyone pours a single footing. We answer the phone at (877) 610-1318, and we’ll send Tom Barrett or one of his sons to walk your lot in a Montclair neighborhood within days, not weeks. You won’t get a call center, and you won’t get a quote that changes at the door. You’ll get a written scope, a real price, and a contractor who lives twenty minutes away.
Why Barrett & Sons Construction Is Montclair’s Preferred Home Additions & ADU Company
Montclair sits on an alluvial fan at the base of the San Gabriel Mountains, right next to the Cucamonga Fault system. That’s not a geology lecture. It’s the reason we pull a soils report on nearly every addition and ADU we permit in this city. Expansive clay-bearing soils under those post-WWII tract homes shift with every wet winter, and a foundation that was fine in 1968 may not carry a second story in 2026. Tom Barrett has been reading those soil conditions since he founded the company in 1998, and his sons run the crews that show up at your door with the Barrett name on the truck. We publish our license number, our address, and Tom’s name on every contract. Ask anyone on your street, or look it up yourself.
Montclair homeowners tell us the same story: a national company quoted one number on the phone, then handed them a different bill after demo started. That doesn’t happen here. Every change order arrives in writing before work proceeds. The permit inspector works for you, not for me, and we treat him that way. For over 29 years, our Home Additions & ADU team has pulled permits and signed inspections on every eligible job in Montclair. When you call, a live person answers, day or night. Not a voicemail box. Not an out-of-state call center.
Our Home Additions & ADU Services in Montclair
ADU Design & Permitting
Montclair’s lot sizes, mostly 6,000 to 8,000 square feet in the interior tracts, make ADU placement a real puzzle. Setback requirements, utility tie-ins, and that alluvial-fan soil report all feed into the design before a permit gets pulled. We handle the full package: site assessment, architectural drawings, structural engineering, and the permit submittal through the city. A typical ADU Construction runs $90,000 to $220,000, and the design and permitting phase is where surprises get caught early, on paper, before they become change orders in your driveway.
Garage Conversion ADU
Most Montclair garages from the 1960s and 70s were built as detached or attached vehicle storage, never as living space. That means the slab is usually uninsulated, the roof framing wasn’t designed for ceiling loads, and the walls lack the shear strength required for habitable space under current seismic code. A garage conversion in Montclair typically runs $30,000 to $80,000, and the smart money goes toward engineering the retrofit before drywall goes up. We’ve converted enough of these to know where the hidden costs hide.
New Detached ADU
A new detached ADU in Montclair gives you a clean foundation, engineered from the start for expansive soils and seismic loading. That’s the advantage over a conversion. We pour a slab with proper moisture barriers and rebar schedules sized for the Cucamonga Fault’s proximity, then frame with materials you can look up. A new detached ADU runs $90,000 to $220,000, and every dollar of that arrives in a written quote before the first trench is dug.
Junior ADU & Interior Conversion
A junior ADU, usually 500 square feet or less carved out of an existing home, works well in Montclair’s older ranch-style floor plans. Think enclosed patio conversions, under-used dens, or a split section of a long hallway bedroom wing. These projects run $45,000 to $130,000 depending on whether we’re adding a kitchenette, a separate entrance, or just framing walls and adding a door. The interior conversion route keeps you inside the existing footprint, which matters when your lot is already tight against a neighbor’s fence line.
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.
Trusted Brands We Service in Montclair
We install windows and doors from Simonton, Ply Gem, PGT, and CGI on Montclair ADUs and room additions. These are brands you can research before you sign, not mystery products off a van. PGT’s impact-rated windows make sense for homeowners who watched a Santa Ana event throw embers across the Pomona Valley, and Simonton’s vinyl lines hold up to the thermal cycling that cracks cheaper frames in our 100-degree summers. We order direct for each job, which means fewer surcharge games and clearer lead times for your project.
Common Home Additions & ADU Problems We See in Montclair Homes
- Slab heave and diagonal cracking after wet winters. Montclair’s expansive adobe clay reactivates with winter rain, and 1960s-70s homes show diagonal cracks at corners and above windows. Any remodel or addition permit pulled here should trigger a soils report before foundation work begins.
- Undersized footings on pre-1980 foundations. Many Montclair homes were poured before modern seismic and expansive-soil engineering requirements. Adding a second story or a heavy ADU without evaluating the footing size is how you get differential settlement.
- Stucco cracking from diurnal temperature swings. Summers in the Pomona Valley bowl routinely exceed 100°F and drop sharply at night. That cycling accelerates stucco cracks and opens pathways for moisture into framing, which becomes a real problem when you’re tying an addition into an existing wall.
- Roof thermal cycling failures on flat and low-slope structures. Older Montclair homes with flat or low-slope roofs take a beating from the heat. An addition or ADU attached to that old roofline needs a full roof condition assessment before you tie in.
Pricing for Home Additions & ADU in Montclair, CA
A room addition in Montclair runs $45,000 to $130,000. A detached ADU runs $90,000 to $220,000. A garage conversion lands between $30,000 and $80,000. Those ranges include engineering, permits, materials, and labor, but they shift based on your foundation condition, whether you need a soils report, and how much seismic retrofit your existing structure requires. When your Montclair home has slab damage from expansive clay, the foundation remediation gets priced as a line item before framing starts, and it’s in writing. Call (877) 610-1318 and we’ll walk the job with you, give you a written line-item quote, and never surprise you at the end. Estimates are free.
We Also Serve Cities Near Montclair
Our crews cover Home Additions & ADU in Montclair plus the surrounding communities: Claremont, Ontario, Upland, and La Verne. If your project sits close to the city line or you live in one of the neighboring towns, we’ll coordinate scheduling from our Inland Empire office without the runaround. And while we’re on your street, we also handle Kitchen Remodeling in Montclair and Bathroom Remodeling in Montclair if your addition is part of a broader home update.
Serving Montclair, CA - Our Local Coverage Area
We’re based in the Montclair 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 Montclair
A room addition in Montclair typically costs $45,000 to $130,000, depending on square footage, foundation condition, and whether you’re adding a second story. Detached ADUs run $90,000 to $220,000, and garage conversions land between $30,000 and $80,000. Every quote we give is a written line-item scope, so you’ll see exactly where the money goes before you sign. Call (877) 610-1318 for a free estimate on your specific project.
Yes, we work throughout the entire 91763 zip code and every Montclair neighborhood, from the interior tract homes off Monte Vista Avenue to the streets near Montclair Place and the 10 Freeway corridor. We’re a local crew, not a franchise dispatching strangers from another county. Tom Barrett and his sons are on-site most days.
Most Montclair ADU projects can start within 2 to 4 weeks after the written scope and deposit are signed, with design and permitting happening in parallel. The city’s permit timeline varies, and we factor that into your schedule honestly from day one. We don’t promise same-day anything on a construction project, but we do promise you’ll know the schedule before we dig. Call (877) 610-1318 to get a realistic timeline for your address.
Only if you change the scope, and even then, the new number arrives as a written change order before any extra work proceeds. We don’t play the allowance game: every allowance is on paper from the start. An allowance that isn’t on paper is a surprise waiting for a date. That’s why Tom Barrett writes it all down and gives you a copy.
Yes, we pull permits and sign inspections on every eligible job in Montclair. The permit inspector works for you, not for us, and we like it that way. If a contractor tells you permits are optional on an addition or ADU, walk away. That’s the advice we give our own families, and it’s the same advice we give every Montclair customer. Call (877) 610-1318 and we’ll walk you through what your project needs, starting with a free estimate.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving Montclair since 1998.