A free site visit, a check of what your lot allows and a written, line-item price.
Get your free estimateFast, Reliable ADU Builders Across Union City
Adding an ADU in Union City moves faster than almost anywhere else in the Bay Area. The city waives all impact fees, allows up to 1,200 square feet on any residential lot, and the planning department’s ADU Express counter routinely issues permits in under 30 days. That changes the math for homeowners, and it changes the timeline. But the real conversation in Union City isn’t about how fast the permit lands. It’s about what the salt air from San Francisco Bay does to the building once it’s up. We see it on nearly every older ADU we’re called to repair or replace. If you’re weighing a detached ADU or a JADU on your lot, call (669) 266-4319 and we’ll give you a written price before any work starts.
Why Lodgewick ADU San Jose Is Union City’s Preferred ADU Builders Company
We’ve built and repaired enough ADUs in Union City to know that this city punishes inland-spec materials. The combination of bay salt air, wet winters, and clay-heavy soil near the flatlands means a fastener or frame that works fine in Fremont’s warmer interior can corrode or rot within a few seasons here. Our crews show up knowing that. We specify hot-dip galvanized hardware, marine-grade aluminum frames, and sealed paver joints as standard on Union City projects, not as an upsell. Russ Delmar, who leads our team, has walked enough Decoto Road and Alvarado-area job sites to know exactly what fails first and what holds.
Every project operates under The Kingswell Guarantee. That means a written price before work begins, a defined arrival window, a clean job site, and a written guarantee on every job. If the work fails within the 365-day guarantee period, we come back and make it right. No phone tag, no debate. Our ADU Builders team puts that commitment in writing before a single nail is driven.
Union City homeowners call us because they’ve been burned before. A price that changed at the door. A crew that ghosted. A cheap exterior that started showing rust blooms and efflorescence within two winters. We answer those calls with a live person, 24/7. You won’t reach a voicemail loop. You’ll reach someone who can look up your project and tell you where things stand.
Why Salt Air Changes Everything: Building a Coastal-Spec ADU in Union City
Union City sits close enough to San Francisco Bay that airborne salt and coastal fog reach inland, especially in neighborhoods west of the hills and along Alvarado Boulevard, Decoto Road, and the older tract housing near the BART station. The salt isn’t dramatic day to day, but it works constantly. Standard galvanized joist hangers, post bases, and roof clips that would last decades in a dry inland city rust through in five to seven years here. Window frames pock and corrode. Untreated lumber in ground-contact applications wicks moisture and invites dry rot and termite damage faster than most homeowners expect.
On a 1960s rancher off Decoto Road, we replaced rusted galvanized roof clips and corroded window frames on a detached ADU with hot-dip galvanized fasteners and marine-grade aluminum frames. The original hardware was standard interior-grade spec. It rusted through after just a few seasons of bay exposure. The homeowner hadn’t done anything wrong. The material choice was wrong for the site. That’s the lesson we carry into every new build here: Union City is a coastal-spec market, even when it doesn’t look like one.
The price gap is real. A coastal-spec detached ADU in Union City typically runs $320 to $420 per square foot, where an inland-spec version might come in at $280 to $360 per square foot elsewhere in the East Bay. The difference is in hardware, framing treatment, window and flashing materials, and foundation drainage details. The cheaper spec costs more within a decade, once you’re paying to replace corroded clips, repair spalled concrete, or reframe rotted sill plates. That’s the math we walk every homeowner through before we write the quote.
Detached ADU in Union City
A Detached ADU is a separate living unit built in the backyard of your Union City property, with its own kitchen, bath, and entrance. Union City’s ADU ordinance makes these unusually practical here: no impact fees, up to 1,200 square feet on any residential lot, and permits through the ADU Express counter in under 30 days. Most Union City homeowners build a detached ADU to create rental income near the BART station, to house aging parents, or to give adult children a place of their own on a lot that already has enough backyard to spare. The 1950s-to-1970s ranchers and split-levels that dominate Union City’s housing stock tend to sit on 5,000 to 7,000 square foot lots with usable rear yards, which makes a detached footprint more viable here than in denser cities.
Coastal-Grade Foundations and Drainage
Union City’s flat, clay-heavy soil near the bay expands and contracts with moisture, and winter groundwater can rise and sit against slab and crawlspace edges. For a detached ADU, we specify foundation drainage that handles that movement: perimeter drains, proper slab elevation, and sealed joints that keep groundwater from wicking up into the structure. Skip this step and you see efflorescence and spalling within a few winters, especially on the west side of town.
Salt-Resistant Exteriors and Hardware
We frame Union City detached ADUs with treated lumber on any ground-contact application, and we use hot-dip galvanized or stainless fasteners throughout, including joist hangers, post bases, and roof clips. Exterior window frames and flashing get marine-grade aluminum or equivalent corrosion-resistant materials. It’s a small line-item difference at the start. It prevents a five-figure repair later.
JADU in Union City
A JADU, or Junior Accessory Dwelling Unit, is a compact living unit created within the existing footprint of a single-family home, typically by converting a garage, a large utility room, or an underused wing. In Union City, JADUs are especially common in the 1950s and 1960s ranchers where a two-car garage is doing nothing but storing boxes. Because a JADU reuses existing walls, roof, and foundation, it costs significantly less than a detached build, and under California law, Union City must approve qualifying JADUs ministerially, without discretionary review. A typical JADU conversion in Union City runs $120,000 to $190,000, depending on whether you’re working within a garage shell or rebuilding a portion of the interior.
Working Within a 1950s Rancher Shell
The single-story slab foundation ranchers common in Union City are actually well suited to JADU conversion. Most have an attached garage with direct access to the main electrical panel and a flat slab that can accept new plumbing lines with proper trenching. The shallow sewer laterals typical of these older tracts sometimes require creative routing or a pump, and we inspect the existing line before quoting, because replacing a failed lateral mid-project is an expensive change order. We write that into the scope up front.
Separate Utility Connections and Compliance
One of the most common questions we get from Union City homeowners is whether a JADU can be added without separate utility connections. The short answer is yes for sewer and water, provided the existing service can handle the additional fixture load, and no for electrical, which typically requires a small subpanel. Union City’s building department accepts shared utility services for JADUs under state law, which keeps the conversion cost lower than a detached unit. We verify the panel capacity and sewer lateral condition during the estimate, not after the walls are open.
Trusted Brands We Service in Union City
We work hands-on with the leading prefab and panelized ADU brands that fit Union City’s lot sizes, budget ranges, and coastal conditions. For homeowners who want speed and factory-built precision, we install units from Abodu, Cover, and Dvele, handling the site prep, foundation, utility connections, and finish work those systems require. For panel products and exterior finishes that hold up to bay salt air, we specify James Hardie siding and trim regularly on Union City builds. Because we work with these brands and materials consistently, our crews know their tolerances, lead times, and installation requirements, which means fewer surprises on site. We keep the fasteners, flashing, and hardware that pair correctly with each system in stock for Union City projects.
Common ADU Builders Problems We See in Union City Homes
- Rusted-out galvanzied hardware on roofs and decks. Standard galvanized joist hangers and post bases corrode within five to seven years in Union City’s salt air, especially on the west side and near Alvarado Boulevard. We replace them with hot-dip galvanized or stainless hardware and specify the same on new builds.
- Spalling and efflorescence at slab edges. Winter groundwater wicks up through unsealed paver joints and attacks ADU slabs in the flat, clay-heavy soil near the bay. The fix is proper perimeter drainage and sealed exterior joints, installed before the slab goes down.
- Dry rot and termite damage at ground contact. Untreated lumber placed against soil or concrete in a damp coastal climate invites rot and termites. Union City’s older homes often have this problem at sill plates and deck posts, and it carries into older ADUs built without treated materials.
- Corroded window frames and flashings. Aluminum windows without marine-grade finish pit and fail early near the bay. We see this often on ADUs built within the last decade using standard inland-spec frames. Marine-grade frames and stainless flashing prevent the repeat failure.
Pricing for ADU Builders in Union City, CA
Here are honest ranges for Union City based on what we’ve quoted and built here:
- Detached ADU (400 – 800 sq ft): $200,000 – $340,000, depending on size, site prep, and coastal-grade materials.
- Detached ADU (800 – 1,200 sq ft): $340,000 – $500,000, including foundation drainage and corrosion-resistant exterior packages.
- Prefab or modular ADU installation (full site work): $180,000 – $350,000, brand and finish dependent.
- JADU conversion (garage or interior): $120,000 – $190,000, assuming an existing slab and panel capacity.
- Coastal-spec upgrade on a quoted build: typically 8 – 12% over inland-spec pricing, covering hardware, marine-grade windows, treated framing, and sealed joint details.
What moves the number most: foundation work on Union City’s clay soil, sewer lateral condition on older ranchers, and the level of coastal-grade exterior you choose. We put every line item in writing before work starts. No price changes at the door. Call (669) 266-4319 to schedule a free estimate and get a written price for your specific lot.
We Also Serve Cities Near Union City
While this page focuses on Union City, our crews also build ADUs in nearby Union City area communities: Fremont, Palo Alto, Milpitas, and Los Altos. If you’re in one of those cities or anywhere else in the East Bay or Peninsula, we can assess your lot and give you the same written price and guarantee. Call (669) 266-4319 and we’ll talk through what’s possible on your property.
Serving Union City, CA - Our Local Coverage Area
We’re based in the Union City 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 Builders in Union City
Union City’s process is faster and cheaper than either neighboring city. Union City waives all ADU impact fees and allows up to 1,200 square feet on any residential lot, and the planning department’s ADU Express counter issues permits in under 30 days. Fremont and Hayward both have more fee structures and longer review timelines. That’s one reason Union City has become such an active ADU market. If you’ve been putting off a build because of permit horror stories, Union City is the wrong place to worry. Call (669) 266-4319 for an exact quote and we’ll handle the permit work.
At minimum, require hot-dip galvanized or stainless steel roof clips, fasteners, and flashing on any detached ADU in Union City. Standard electro-galvanized hardware rusts through within a few seasons of bay salt air, especially on west-facing roofs and anything within a mile or two of the shoreline. Marine-grade aluminum for window frames and fascia also prevents the pitting and corrosion we see on older ADUs here. A builder who doesn’t know to specify these materials hasn’t built enough in this climate. We write the hardware spec into every quote, so you can see exactly what you’re paying for.
Yes. Union City’s flat, clay-heavy soil expands when wet and shrinks when dry, and winter groundwater can rise within a few feet of the surface near the bay. Without perimeter drains and proper slab elevation, groundwater wicks into the slab and causes efflorescence, spalling, and eventually structural cracking. We specify drainage on every detached ADU we build here, not as an option. It’s part of the written price. Call (669) 266-4319 to get a quote that includes it.
In Union City, a coastal-spec detached ADU typically runs 8 to 12 percent more than an inland-spec version of the same footprint. On a 600 square foot unit, that’s roughly $20,000 to $35,000 extra for hot-dip galvanized hardware, marine-grade windows, treated ground-contact lumber, and proper drainage details. The inland-spec version appears cheaper on paper, but within a decade you’re paying to replace rusted fasteners, repair spalled concrete, and reframe rotted sills. The coastal spec is the cheaper build over a ten-year window. We show both numbers side by side in our quotes so you can make the call with your eyes open.
Yes, for water and sewer. A JADU inside an existing single-family home in Union City can share the home’s water and sewer service, provided the existing lines can handle the added fixtures. Electrical is the exception: you’ll typically need a small subpanel dedicated to the JADU. We inspect the panel capacity and sewer lateral condition during the estimate, so there are no surprises once walls are open. Most 1950s ranchers here have the bones for a straightforward conversion, though the shallow sewer laterals on these older tracts occasionally need creative routing. Call (669) 266-4319 for a free estimate, and we’ll check your specific house.
Contact Lodgewick ADU San Jose for Your Union City ADU Project
If you’re ready to build a detached ADU or add a JADU to your Union City home, start with a conversation. We’ll walk your lot, listen to what you’re trying to solve, and give you a written price that covers the coastal-grade materials this city actually demands. No price changes at the door. No fuzzy scope. No hardware that rusts out in five years. Call (669) 266-4319 to schedule your free estimate. We answer with a live person, 24/7.
Written by Russ Delmar, Owner at Lodgewick ADU San Jose, serving Union City and the surrounding Bay Area.
