Fast, Reliable New Detached ADU Across San Leandro
A new detached ADU in San Leandro typically costs between $280,000 and $410,000 all-in, from the first site survey through final inspection. The single biggest variable on your lot isn’t square footage. It’s what Alameda County inspectors find when they open your existing garage foundation and check it against Hayward Fault requirements. Call (971) 497-2402 for a written price before any work starts.
We’ve worked San Leandro’s bayside neighborhoods long enough to know that salt air and seismic code converge here in ways they don’t eight miles inland. That’s not a sales pitch. It’s what the permit record shows.
Why Ellery ADU Studio Is San Leandro’s Preferred New Detached ADU Company
Ellery ADU Studio has designed, permitted, and built over 900 ADUs since 2015, every one with a written price before work started. Nadia Ellery founded the studio the year California’s ADU permitting landscape shifted, and she built the practice around one idea: the permit set is the product. Not an afterthought, not a line item to be farmed out. The drawings, the engineering, the construction, and the warranty live in a single contract at a single written price.
San Leandro homeowners call us after they’ve already talked to three other companies and heard three different stories about what their garage foundation will need. We hand them Clause 1 of The Haven Standard: a written price before any work begins. Then we show them the photo record our crews document on every visit. We don’t describe what we found. We show it.
We’re the team that tells you, before you sign anything, that your 1958 garage on Dutton Avenue needs mudsill anchor bolts and cripple-wall bracing because the Hayward Fault runs directly beneath this city. That’s not upselling. That’s the inspection you will not pass without it.
Our New Detached ADU Services in San Leandro
Custom Built ADU
A custom-built detached ADU in San Leandro is a stick-framed unit designed specifically for your lot’s setbacks, your soil report, and your neighborhood’s existing rooflines. Most of the ranch homes here were built between 1945 and 1965, which means modest lots, detached one-car garages, and rear yard setbacks that reward a design drawing rather than a stock floor plan. We name the pressure-treated lumber, the galvanized hardware, and the shear panel schedule in the written scope before construction begins.
Prefab ADU Installation
Prefab units from Cover and Dvele can shorten on-site framing time in San Leandro by weeks, but the foundation still has to meet Alameda County’s seismic standard. That’s the part prefab companies don’t handle. We do. We coordinate the factory-built module, the site prep, the utilities, and the permit set under one contract, so you’re not the person standing between a module manufacturer and a county inspector when the anchor bolt spacing gets flagged.
Modular ADU
Modular ADUs are built in sections off-site and assembled on your San Leandro lot. The material finishes get chosen by the factory, which means salt-air exposure needs to be specified upfront, not discovered when railings start rusting three years after move-in. We spec marine-grade fasteners and sealed joints at signing, so your modular unit doesn’t quietly corrode in the bayside air.
Studio ADU
A studio ADU is the smallest detached unit most San Leandro lots can accommodate, typically 400 to 600 square feet. It’s also the fastest path to rental income if your rear yard has the clearance. The same seismic retrofit rules apply whether the unit is 400 square feet or 1,200. Alameda County doesn’t grade foundation requirements on a curve.
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 San Leandro
We specify and coordinate prefab and modular systems from Cover, Dvele, and Mighty Buildings within the same single contract. We don’t sell these systems. We plan them, permit them, and build the foundation, utilities, and site work they sit on. For San Leandro homeowners, that’s the gap that matters. A factory-built ADU still needs a county-approved foundation on a fault line, and that’s the part we own. Your written price covers the module, the seismic hardware, the exterior finishes, and the final inspection.
Common New Detached ADU Problems We See in San Leandro Homes
- Untreated sill plates wick moisture and rot. In bayside neighborhoods like the Davis Tract and along Estudillo Avenue, the original redwood or fir sill plates under detached garages have been absorbing marine moisture for 60 years. By the time we open them for an ADU conversion, they’re often crumbling. We replace them with pressure-treated lumber and hot-dip galvanized anchor bolts.
- Zinc-plated nails and hangers rust through. Standard fasteners that work fine in Castro Valley fail early a mile from the San Leandro bayfront. Salt-laden air eats zinc plating. On ADU decks, ledgers, and shear walls, we spec stainless or hot-dip galvanized connectors, because a corroded ledger is a structural failure, not a cosmetic one.
- Missing shear panels and hold-downs crack drywall after small quakes. Detached garages built in the 1950s and 60s were never designed to live on the Hayward Fault. After even a minor tremor, walls without cripple-wall plywood shift, doors jam, and drywall cracks. Alameda County inspectors know this. They look for it. Your permit hinges on it.
- Older Craftsman homes near downtown carry knob-and-tube wiring and undersized panels. Pulling ADU permits on a 1920s bungalow near Davis Street often triggers an electrical service upgrade before the new unit can connect. That’s a real line item we name in the written price, not something we discover mid-project and bill you for later.
Pricing for New Detached ADU in San Leandro, CA
A custom-built detached ADU in San Leandro runs $300,000 to $410,000 all-in, drawings through final inspection. A prefab or modular unit from Cover or Dvele typically lands between $280,000 and $360,000, depending on the module, the foundation work, and site access. A studio ADU starts around $250,000.
The seismic retrofit line item, which Alameda County requires on nearly every detached garage conversion here, adds $8,000 to $18,000 depending on foundation condition. Coastal-spec materials, meaning stainless steel fasteners, marine-grade railings, and sealed paver joints, typically add $4,000 to $9,000 over inland-spec. The cheaper spec costs more within a decade, when you’re replacing rusted connectors and rotted sills. We quote the coastal-spec version in writing, every time. Call (971) 497-2402 to schedule your free estimate.
We Also Serve Cities Near San Leandro
Our crews work throughout the East Bay. If you’re in San Leandro, Castro Valley, Oakland, Alameda, or Union City, the same written-price guarantee applies. Our New Detached ADU team coordinates scheduling from our San Francisco Bay Area office and serves San Leandro locally.
Serving San Leandro, CA - Our Local Coverage Area
We’re based in the San Leandro 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 San Leandro
A seismic retrofit on a detached garage conversion in San Leandro typically adds $8,000 to $18,000 to the total project cost, depending on the foundation’s age, the condition of the sill plate, and whether the garage has any shear paneling at all. Alameda County requires mudsill anchor bolts and cripple-wall bracing on nearly every conversion here because the city sits directly on the Hayward Fault. That cost is named in our written price before work starts, never discovered later. Call (971) 497-2402 for an exact quote on your specific garage, estimates are free.
Use stainless steel or hot-dip galvanized roofing fasteners on any new detached ADU within a mile of the San Leandro bayfront. Standard zinc-plated fasteners corrode in the persistent marine moisture and salt-laden air that defines this stretch of the East Bay, and a rusted roofing fastener is a leak waiting to happen. We spec stainless on every detached ADU we build here, and it’s a specific line item in the written scope, not a preference left to the field crew.
Alameda County inspectors check mudsill anchor bolt spacing, cripple-wall plywood thickness and nailing pattern, and hold-down hardware on detached garage conversions in San Leandro before any framing can pass. They’re specifically alert to the Hayward Fault exposure, which means a missing anchor bolt or a toe-nailed sill that would slide through in a neighboring city gets flagged here every time. We install hot-dip galvanized anchor bolts and Simpson Strong-Tie hold-downs as a named line item in the written scope, and we pass on the first try.
The coastal-spec version of a new detached ADU in San Leandro costs $4,000 to $9,000 more than the inland-spec because marine-grade fasteners, sealed paver joints, and corrosion-resistant railings cost more up front. It’s worth it because standard zinc hardware in bayside air rusts through within a decade, and replacing a corroded deck ledger or rotted sill plate later means tearing into finished work. The cheap spec doesn’t save money. It defers it, with interest. Call (971) 497-2402 to see both numbers in writing.
Yes, you can build a new detached ADU in a San Leandro Craftsman neighborhood like the blocks around Davis Street without demolishing the existing garage, provided the new unit sits within the allowed rear or side yard setbacks and the existing garage’s electrical service can either be upgraded or isolated. Older Craftsman homes here often carry knob-and-tube wiring and undersized panels, which means the ADU permit may trigger an electrical service upgrade, but the garage structure itself can stay. We name every upgrade in the written scope at signing, per Haven Standard Clause 5, no door-side surprises.
Call Ellery ADU Studio for a Written Price
If you’re planning a new detached ADU in San Leandro, start with a number on paper, not a promise over the phone. We’ll walk your lot, open your garage foundation if needed, and hand you a written price that names the seismic hardware, the coastal-spec materials, and the permit path before you commit to anything. Call (971) 497-2402 to schedule your free estimate. If you already have an estimate from another company, bring it. Our Free Second Opinion will tell you what they missed.
Written by Nadia Ellery, Owner at Ellery ADU Studio, serving San Leandro since 2015.
New Detached ADU in Nearby Areas
