Fast, Reliable New Detached ADU Across Montclair
A new detached ADU in Montclair typically runs $220,000-$380,000 all-in, from permit set through final inspection, depending on whether you choose custom stick-build or a prefab system like Dvele or Mighty Buildings. Most projects take 8-14 months in this market, with permitting through Montclair’s building department adding 3-5 months to the construction timeline. Our crews serve Montclair from our Inland Empire office, and we coordinate site visits, utility trenching, and inspections without you driving to Fontana.
Call (909) 637-3715 to schedule your free estimate. Every project gets a written price before any work starts, per Haven Standard Clause 1.
Why Ellery ADU Studio Fontana Is Montclair’s Preferred New Detached ADU Company
We’ve designed, permitted, and built ADUs in Montclair since 2015, the year California’s ADU laws began their major shift. Our 900+ projects include dozens in the 91763 zip and surrounding Inland Empire tracts. Montclair homeowners find us because we name dollar ranges on the page, document every site visit with photos, and put our 365-Day Done Right Promise in writing before the first shovel hits dirt.
Our response time to Montclair is same-week for initial site visits. We know the city’s compact grid - the ranch tracts north of Mission Boulevard, the streets south of the 10 Freeway, the lots tucked against the Pomona border - and we know which plans Montclair’s building department has already seen and which ones trigger extra plan-check rounds.
Our license number appears on every page, and our crews are background-checked and uniformed. A live person answers calls 24/7. If you already have a written estimate from another firm, we offer a Free Second Opinion.
Our New Detached ADU Services in Montclair
Custom Built ADU
Custom stick-built detached ADUs are the right choice when your Montclair lot has constraints that prefab can’t accommodate - a tight side yard on a 6,000 sq ft parcel near Ramona Avenue, an existing mature tree you want to keep, or a slope that needs grading before anything levels up. We design the permit set in-house, build from that same set, and hold one contract for both phases. No designer-builder blame gap. A typical custom 1-bed detached ADU in Montclair runs $260,000-$340,000; a 2-bed unit, $320,000-$380,000. Those figures include the panel upgrade, mini-split, and trenching we almost always find necessary on these 1960s ranch lots.
Prefab ADU Installation
Prefab systems from Mighty Buildings and Dvele can cut 2-4 months off the Montclair schedule because the module arrives 80% complete. The tradeoff is site prep precision: your concrete pad, utility trench, and crane access path must be exact before the truck rolls in. On Montclair’s flat, compact lots, we often place prefab units in the back corner, running trench for power, water, and sewer around existing landscaping. Prefab 1-bed units in this market typically land at $220,000-$290,000 all-in, including the pad and connections. We coordinate the manufacturer’s delivery window with Montclair’s inspection schedule so you’re not paying for storage.
Modular ADU
Modular construction - factory-built to local code, delivered in sections - splits the difference between site-built and prefab. For Montclair’s standard 7,500 sq ft lot with a detached garage, a modular Studio ADU or 1-bed unit can tuck along the side yard while preserving the garage and most of the backyard. We specify Boxabl or Plant Prefab systems when the module dimensions match the setback math: 4 feet from property lines, 6 feet from the main house, and enough crane swing radius from the street. Modular projects here run $240,000-$310,000, with the factory queue often the longest variable.
Studio ADU
A Studio ADU - one open room plus bath and kitchenette - is the most common detached unit we permit in Montclair. It fits the side-yard strips and back corners that dominate these ranch lots, stays under the 850 sq ft threshold that triggers extra parking requirements, and rents at rates that pencil for homeowners adding long-term income. Studio detached ADUs in Montclair typically cost $220,000-$280,000. Every one gets the same dedicated cooling system, because 105°F July afternoons make natural ventilation a non-starter here.
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.
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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 Montclair
We specify and coordinate prefab and modular systems from Dvele, Mighty Buildings, Boxabl, and Plant Prefab, all within the same single contract that covers design, permitting, and installation. Our team maintains direct relationships with each manufacturer’s project coordinators, which means we can confirm lead times, crane requirements, and Montclair-specific foundation specs before you sign. We don’t drop-ship and disappear; we manage the interface between factory delivery and local inspection, including Montclair’s building department sign-offs on factory structural reports and electrical tie-ins. If a module arrives with a defect, we handle the warranty claim and schedule the correction - not you, not the factory’s general support line.
Common New Detached ADU Problems We See in Montclair Homes
- 100-amp panels that fail at plan check. Montclair’s original 1960s service almost always requires upgrading to 200 amps once the ADU adds HVAC, kitchen appliances, and GFCI outlets. We scope this on the first site visit, not after the permit gets kicked back. The $3,000-$6,000 upgrade goes into your written quote from day one.
- Setback math that kills first-draft site plans. Montclair requires 4-foot side and rear setbacks, plus 6 feet between structures. On a 6,000 sq ft lot, that leaves a narrow buildable band. We measure with a survey-grade sketch before design starts, so your permit set doesn’t get redrawn at plan check.
- Flat lots that pool water against new slabs. Montclair’s inland valley floor drains slowly. We grade and swale as standard scope, not an afterthought, because standing water voids foundation warranties and fails inspection.
- Missing cooling systems that trigger habitability rejections. San Bernardino County code, enforced through Montclair’s building department, requires mechanical cooling that can maintain 78°F indoors. We specify mini-splits on every detached ADU here; skipping it means a failed inspection and a return trip.
Pricing for New Detached ADU in Montclair, CA
Here is what new detached ADU projects cost in Montclair’s current market, based on the last 24 months of our permit sets and builds:
| Project Type | Typical All-In Range | Includes |
|---|---|---|
| Studio ADU, custom stick-build | $220,000-$280,000 | Permit set, foundation, structure, MEP, finishes, final inspection |
| 1-Bed ADU, custom stick-build | $260,000-$340,000 | Above plus full kitchen, dedicated mini-split, 200-amp service upgrade |
| 2-Bed ADU, custom stick-build | $320,000-$380,000 | Above plus additional bedroom, larger footprint, enhanced drainage |
| 1-Bed prefab (Mighty Buildings / Dvele) | $220,000-$290,000 | Module, foundation pad, utility trench, crane, connections, inspections |
| 1-Bed modular (Boxabl / Plant Prefab) | $240,000-$310,000 | Modules, foundation, assembly, MEP tie-in, final sign-off |
Every range above includes our standard scope: permit set, documented photo record on every visit, and the 365-Day Done Right Promise in writing. What moves you within the range: panel upgrade needed ($3,000-$6,000), grading and drainage complexity, utility trench length, and finish level. We do not quote without a site visit, and we do not change the price after signing - Haven Standard Clause 5, No Surprises.
Call (909) 637-3715 for your free estimate. Written price before any work starts.
We Also Serve Cities Near Montclair
Our crews work throughout the western Inland Empire, including New Detached ADU in Montclair as well as Ontario, Upland, Rancho Cucamonga, and Mira Loma. Dispatch and scheduling are coordinated from our Inland Empire office. If you’re on the border between cities, we’ll confirm which building department has jurisdiction before we quote - it affects permit timeline and fee schedule.
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 - New Detached ADU in Montclair
Yes, if your lot meets Montclair’s 4-foot side/rear setbacks and 6-foot separation from the main house. On a standard 50-by-120-foot Montclair ranch lot, that typically leaves a buildable side-yard strip or back corner of roughly 400-600 sq ft - enough for a studio or 1-bed detached unit up to 850 sq ft. We verify exact dimensions with a site survey before design starts. Call (909) 637-3715 and we’ll measure your specific lot.
Almost certainly yes, if your home has its original 100-amp service. San Bernardino County’s adopted electrical code, applied through Montclair’s building department, requires 200-amp service when an ADU adds HVAC, kitchen circuits, and modern outlets. This is a $3,000-$6,000 line item we include in every written quote. We check your panel on the first site visit. Call (909) 637-3715 to schedule that visit - estimates are free.
Yes. Montclair’s inland valley summers regularly hit 105-110°F, and the city’s building department enforces San Bernardino County’s habitability code requiring mechanical cooling to 78°F indoors. Natural ventilation, sometimes acceptable in coastal jurisdictions, does not satisfy Montclair inspectors. Every detached ADU we design here includes a dedicated mini-split system, sized to the unit’s square footage. The system and its electrical load are factored into your written quote and panel sizing from the start.
Custom stick-build projects typically take 10-14 months: 3-5 months for permit set and plan check through Montclair’s building department, 4-6 months for construction, and 2-3 weeks for final inspection and certificate of occupancy. Prefab and modular systems can reduce the construction phase to 2-4 months, but factory lead times vary. We provide a written schedule with your quote, updated monthly. Call (909) 637-3715 for a timeline based on your project type.
Yes, grading and drainage are standard in our scope, not an add-on. Montclair’s flat valley-floor lots drain slowly, and we’ve seen water pool against new slabs when the grade wasn’t adjusted. We regrade to shed water away from the structure and add surface swales where needed. On a recent project on the 4600 block of Monte Vista Avenue, we raised a back corner 6 inches and added a shallow swale to move roof runoff away from the new ADU slab. The work is documented with photos at each stage and included in your written price.
Call (909) 637-3715 to schedule your free estimate. Written price before any work starts, documented photo record on every visit, and our 365-Day Done Right Promise - all in writing, all before you sign.
Written by Nadia Ellery, Owner at Ellery ADU Studio, serving Montclair since 2015.