New Detached ADU in Fontana - Plans, Permits & Build, One Fixed Price

Fast, Reliable New Detached ADU Across Fontana

A new detached ADU in Fontana typically runs $220,000-$380,000 all-in, from permit set through final inspection, with most projects completing in 8-14 months depending on utility complexity. We’re Ellery ADU Studio San Bernardino, and our New Detached ADU team coordinates every phase - design, permitting, construction - from our Inland Empire office with crews serving Fontana directly. Call (909) 359-6463 to schedule your free estimate; every project starts with a written price before any work begins, per Haven Standard Clause 1.

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Why Ellery ADU Studio San Bernardino Is Fontana’s Preferred New Detached ADU Company

We’ve designed, permitted, and built ADUs across Fontana since 2015 - the same year California’s ADU laws opened the floodgates. That timing matters: we learned Fontana’s building department workflows while they were still forming, not after they’d hardened into backlogs. Our permit sets for Fontana projects include the high-wind framing details the city now requires in north and east Fontana, so we don’t eat a revision cycle that costs clients six weeks.

Fontana homeowners receive the same documentation every Ellery client gets: a written price before signing, a photo record from every site visit, and the 365-Day Done Right Promise in writing. No designer blaming the builder, no builder blaming the designer. One contract, one written price, one phone number that a live person answers 24/7. Our license number appears on every page - verified, not just claimed.

We’re not headquartered in LA or Orange County pretending to serve the Inland Empire. Dispatch and scheduling run from our Riverside-San Bernardino office, and our crews know the difference between a south Fontana slab-on-grade ranch and a north Fontana hillside cut. That local knowledge saves money. Out-of-area contractors who frame with coastal nailing schedules routinely fail Fontana’s Cajon Pass wind-corridor inspection, then bill the homeowner for the $3,000-$5,000 in hold-downs and roof-to-wall connectors they should have priced from day one.

Our New Detached ADU Services in Fontana

Custom Built ADU

Custom built ADUs in Fontana start around $280,000 and scale past $380,000 for two-bedroom units with full kitchens and separate utility meters. The custom route makes sense on Fontana’s oversized R-1 lots - typically 8,000-12,000 square feet from the Kaiser Steel-era build-out - where homeowners want the ADU to mirror the existing ranch architecture or need specific room counts for multigenerational households. We design to Fontana’s specific setback and height limits, then build from that permit set. No handoff to a separate contractor who claims he never saw the drawings.

Prefab ADU Installation

Prefab ADU installation in Fontana runs $220,000-$320,000 depending on manufacturer and site prep scope. We specify and coordinate systems from Dvele, Mighty Buildings, and Plant Prefab - brands whose factory-built shells we integrate under the same single contract as site work and utility tie-ins. Fontana’s hard water and high-wind corridor mean we don’t accept the manufacturer’s standard fastener package; we upgrade to hot-dip galvanized and stainless hardware during install, documented in your permit set. The factory builds the box. We make it survive Fontana.

Modular ADU

Modular ADUs in Fontana typically fall between prefab and custom pricing, around $250,000-$340,000, with faster close-in times than stick-built once the modules arrive. The catch is transport: modules coming through the Cajon Pass corridor face weight and width restrictions that don’t affect coastal deliveries. We coordinate Fontana-specific trucking permits and crane placement before the modules leave the factory. Our modular projects include the same high-wind hold-downs and 200A service upgrades as our custom work - no corner-cutting because the walls showed up pre-built.

Studio ADU

Studio ADUs are the most common detached unit we build in Fontana, typically $220,000-$290,000 all-in. They fit the city’s ADU demand pattern: in-law casitas for extended family, not market-rate rentals. Fontana’s multigenerational household rate is among the highest in San Bernardino County, and a 400-550 square foot studio with kitchenette and three-quarter bath serves that need without the cost of a full one-bedroom. We size mini-splits at 12,000-18,000 BTU for Fontana’s 105°F+ summers - not the 9,000 BTU coastal spec that fails here.

1-Bed & 2-Bed ADU

One-bedroom detached ADUs in Fontana run $260,000-$330,000; two-bedroom units start around $310,000 and reach $380,000 with upgraded finishes and separate metering. The 1-bed layout suits adult children or aging parents who need distinct sleeping space; the 2-bed works for full multigenerational households or caregiver arrangements. Both trigger Fontana’s standard ADU parking exemption if within a half-mile of transit, but we verify that exemption in writing before construction starts. Under Haven Standard Clause 5 (No Surprises), the scope is locked at signing.

Trusted Brands We Service in Fontana

We specify and coordinate prefab and modular systems from Dvele, Mighty Buildings, and Plant Prefab - factory-built ADU shells integrated into our single-contract design-build model. For site-built work, we source James Hardie fiber-cement siding rated for Fontana’s UV exposure and thermal cycling, with stainless or hot-dip galvanized fasteners that won’t corrode in hard water conditions. Our Fontana clients don’t receive a materials list to shop themselves; we specify, procure, and install under the same written price, with every component documented in the permit set and photo record.

Common New Detached ADU Problems We See in Fontana Homes

• Failed high-wind inspections from coastal nailing schedules. Out-of-area contractors frame detached ADUs with standard nailing patterns and miss Fontana’s Cajon Pass corridor requirements for hold-downs, roof-to-wall connectors, and diaphragm nailing. The correction adds $3,000-$5,000 and six weeks. We price it from the start.
• Corroded fasteners and fence posts within three summers. Fontana Water Company’s hard water accelerates galvanic corrosion on zinc-plated hardware and untreated wood. We specify hot-dip galvanized or stainless fasteners and sealed paver joints rated for thermal cycling - not the cheap spec that looks identical on day one.
• Undersized mini-splits that can’t hold setpoint past 105°F. A 9,000 BTU system adequate in coastal LA fails in Fontana’s Inland Empire heat pocket. We size 12,000-18,000 BTU units with Title 24-compliant insulation, or the ADU is unusable for extended family in July and August.
• 100-125A panels that can’t support an ADU load. Fontana’s 1960s-1980s housing stock typically has 100-125A service; an ADU with electric range, dryer, and mini-split requires 200A. We pull the permit for the panel upgrade as part of the same project, not a surprise add-on after rough-in.

Pricing for New Detached ADU in Fontana, CA

We Also Serve Cities Near Fontana

Our crews build New Detached ADU in Fontana and across the surrounding Inland Empire, including Rialto, Rancho Cucamonga, Riverside, and Grand Terrace. Each city has distinct building department workflows and climate considerations; we adjust the permit set and specifications accordingly, never copy-pasting from one jurisdiction to another.

FAQs - New Detached ADU in Fontana