Fast, Reliable New Detached ADU Across Antioch
A new detached ADU in Antioch typically runs $280,000 to $390,000 all-in, drawings through final inspection, depending on square footage, site conditions, and whether you go stick-built or prefab. Most Antioch projects take 10 to 14 months from first survey to certificate of occupancy. If you own a home in the Lone Tree Road or Deer Valley corridors, you’re sitting on one of the few East Bay lot sizes that can actually fit a detached unit without a variance fight. Call (971) 497-2402 and we’ll walk your property, check the soils, and hand you a written price before any work starts.
Why Ellery ADU Studio Is Antioch’s Preferred New Detached ADU Company
We’ve designed, permitted, and built 900+ ADUs since 2015, and the Antioch ones have taught us things no regional brochure ever would. We know which HOA boards in the Lone Tree corridor still send pre-2020 rejection letters. We know the Delta clay under Deer Valley turns to gumbo at four feet. And we know that a 1998-era 200-amp panel feeding a garage subpanel will not carry a new ADU plus the primary home through a 100°F July afternoon. That’s the kind of local knowledge that keeps a project from stalling at permit review or falling apart two winters in.
Nadia Ellery founded this studio because the designer-versus-builder gap was ruining good projects. Our answer is one contract, one written price, one accountable team. Permit set, structural drawings, construction, final inspection. If the finished ADU doesn’t match the permitted drawings, the 365-Day Done Right Promise is in writing before we break ground. Independent owner, real accountability, no franchise script.
Our New Detached ADU Services in Antioch
Custom Built ADU
For Antioch lots in the eastern half of the city, a custom built detached ADU is usually the highest-value path. We frame it to your footprint, on an engineered slab sized for Delta clay, with a mini-split HVAC system rated for 100°F-plus summers. Most custom builds we do in Antioch land between 600 and 1,000 square feet. The permit set is the core product: every drawing, every load calc, every HOA preemption letter filed before excavation.
Studio ADU
A studio ADU in Antioch runs $210,000 to $270,000 and is the most common starting point for owners who want rental income without overbuilding. On a 6,000-square-foot Lone Tree lot, a 480-to-600-square-foot studio tucks cleanly into the rear yard while preserving usable outdoor space. We spec a dedicated mini-split and a compact kitchenette, and we engineer the slab for the clay expansion cycles that are standard in lower-lying Antioch parcels.
1-Bed & 2-Bed ADU
One-bedroom and two-bedroom detached ADUs in Antioch run $300,000 to $390,000 depending on bedroom count, bathroom count, and whether you add a separate laundry. A two-bedroom unit near the 2018 eBART station genuinely rents to Bay Area commuters, not just local tenants. That changes the math on your project. We factor that demand into the floor plan conversation from day one, because a well-laid-out 2-bed ADU in Antioch is a cash-flow asset, not a guest room with extra steps.
Prefab ADU Installation
Our New Detached ADU team specifies and coordinates prefab systems from Cover, Dvele, and Boxabl within the same single contract as a custom build. Prefab works well on Antioch lots where the backyard access is clear and the HOA’s architectural review wants a faster, cleaner construction window. The foundation requirements don’t change because the unit is prefab: engineered slab, proper drainage, correct setback from the property line. We handle the site prep, the utility laterals, and the final inspection. You get one written price for the whole thing.
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 Antioch
We specify and install prefab and modular systems from Cover, Dvele, and Boxabl, among others. The brand matters less than the spec, and the spec matters less than the foundation under it. We’ve seen a Dvele unit set perfectly on an Antioch slab and we’ve seen a cheaper panelized system fail at the seam because the site prep was rushed. Our job is to make sure the unit you choose sits on a foundation engineered for Antioch’s clay-heavy Delta soils, with HVAC sized for inland summer heat, and with every utility connection documented in the permit set. No brand-blame games. One contract.
Common New Detached ADU Problems We See in Antioch Homes
- HOA rejection letters citing pre-2020 CC&Rs. This is the most Antioch-specific problem we deal with. A 2005-era HOA in the Lone Tree corridor will often send a letter claiming detached ADUs are banned. California Civil Code §4751 (AB 670) voids HOA provisions that effectively prohibit ADUs, but you have to invoke it formally, with documentation, before excavation. We file that notice as part of the permit process.
- Undersized or maxed-out electrical panels. Homes built between 1995 and 2005 in Antioch’s eastern tracts frequently have 200-amp panels that are already full. Adding a detached ADU means a new subpanel or a full service upgrade, and that line item needs to be in the written price before you sign. Not discovered mid-project.
- Clay expansion cracking standard slabs. The Delta soils in lower-lying Deer Valley and the parcels near the San Joaquin River hold water and expand. A standard 4-inch slab poured without a soils report can crack within two wet winters, shifting the structure and the condenser pad. We spec engineered slabs on every Antioch foundation.
- Failed waterproofing on existing patios and walkways. When a detached ADU ties into an existing patio or exterior wall, we regularly find failed membranes or missing flashing behind original stucco. It’s not dramatic. It’s just what 20 to 30 years of Antioch sun and Delta moisture does. We price those repairs before we touch them and get written sign-off.
Pricing for New Detached ADU in Antioch, CA
A detached ADU in Antioch runs $280,000 to $390,000 all-in. Here’s the line-item reality: site prep and engineered foundation, $38,000 to $55,000 depending on soil conditions. Electrical service upgrade, $6,000 to $12,000 if the panel is maxed out. Mini-split HVAC sized for inland heat, $8,000 to $14,000. Framing and shell, $90,000 to $140,000 depending on square footage. Interior finishes, $60,000 to $100,000. Permit fees and HOA compliance documentation, $8,000 to $15,000. Every project is quoted with a written price before any work begins, under Haven Standard Clause 1. If you have a written estimate from another contractor, bring it in. We give a Free Second Opinion on any written estimate already in hand. Call (971) 497-2402 and we’ll walk the lot and hand you documented numbers.
We Also Serve Cities Near Antioch
We serve Antioch and the surrounding East Bay, including Oakley, Pittsburg, Brentwood, and Concord. The lot sizes differ, the HOA landscape differs, and the soil conditions change as you move west. What doesn’t change is the written price before work starts. If you own property in any of these cities, the same inspection-first process applies.
Serving Antioch, CA - Our Local Coverage Area
We’re based in the Antioch 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 Antioch
Yes, in most cases. Under Civil Code §4751 (AB 670), HOA governing documents adopted before 2020 cannot effectively prohibit ADU construction on a single-family lot. We prepare a formal preemption notice citing the statute, file it with the association before excavation, and proceed with the city permit. We’ve done this repeatedly in the Lone Tree and Deer Valley corridors. The key is doing it before you dig, not after the HOA files a complaint. Call (971) 497-2402 and we’ll review your CC&Rs and tell you exactly where you stand. The review is part of the written price, not a separate consulting fee.
You need an engineered slab, and saving money here is false economy. Antioch’s clay-heavy Delta soils expand and contract with moisture. A standard 4-inch slab poured without a soils report will crack within two wet winters in lower-lying parts of the city. The repair costs more than the engineered slab would have. Our written price includes the soils report and the engineered foundation as a single line item. If you want to verify the soil on your specific lot before quoting, we do that during the walkthrough. Call (971) 497-2402 for a free estimate on your property.
Realistically, $6,000 to $12,000, depending on whether you need a subpanel feed or a full service upgrade. In the 1990s tract homes around Lone Tree Way, we often find 14-gauge wiring feeding the garage subpanel and a main panel that’s already at capacity. Adding a detached ADU means a dedicated circuit for the mini-split, the kitchenette, and the laundry. We write the electrical allowance into the contract before we quote, not after we open the wall. Call (971) 497-2402 and we’ll tell you exactly what your panel will and won’t carry, documented in writing.
Before we quote, we do an inspection-first walkthrough. We look behind the stucco where the ADU will tie in, check the existing foundation, read the panel, and test the soil. What we find goes into the written price as a numbered allowance. When we opened a 2003 ranch off Lone Tree Way, we found a maxed-out panel, 14-gauge wiring, and gumbo clay at four feet. We wrote an $8,500 electrical allowance and a $12,000 engineered-slab allowance into the contract. When we hit a failed waterproofing membrane later, we priced the repair and got written sign-off before we dug a single pier. That’s Clause 5: the scope is locked at signing, and any change is priced and approved in writing before we touch it. Call (971) 497-2402 to schedule a walkthrough of your lot.
Yes, and that’s one of the main reasons detached ADUs work in Antioch when they don’t pencil out in Brentwood or Oakley. The 2018 eBART station connects to BART at the transfer platform, making a well-built two-bedroom ADU genuinely attractive to Bay Area commuters who can’t afford Walnut Creek or Pleasant Hill rents. A 2-bed detached unit near the Lone Tree corridor rents at a premium to exactly this tenant pool. That demand shapes the floor plan conversation. We design for rentability, not just for the permit. Call (971) 497-2402 and we’ll show you what actually rents in Antioch right now.
Get Your Written Price
Every project starts with a walkthrough. We look at your lot, your panel, your soil, your HOA documents, and your existing structures. Then you get a written price, a documented photo record of everything we found, and a scope that doesn’t move unless you approve it in writing. This is the only New Detached ADU page that tells you exactly what opening up an older Antioch home will turn up: undersized framing, failed waterproofing, and the HOA preemption letter you didn’t know you needed. We price those before they become budget explosions. Call (971) 497-2402 to schedule your free estimate. A live person answers, 24/7. No voicemail queue.
Written by Nadia Ellery, Owner at Ellery ADU Studio, serving Antioch since 2015.
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