A free site visit, a check of what your lot allows and a written, line-item price.
Get your free estimateHow Much Does an ADU Cost to Build?
What’s Happening
ADU construction costs vary widely depending on build method, lot conditions, and local permit requirements, but most homeowners can expect to spend anywhere from $80,000 to $300,000+ for a fully permitted, move-in-ready accessory dwelling unit. Prefab and modular units tend to sit at the lower end; custom stick-built ADUs and those requiring significant site work push toward the higher end.
If you typed this question, you’re probably somewhere between curious and frustrated. You’ve seen a range online that spans $60,000 to $400,000 and it hasn’t told you anything useful. That spread exists because ADU pricing is genuinely job-specific: the size of the unit, the build method, your lot’s topography, your utility connections, and your local permit fees all pull the number in different directions. The only figure that actually matters is the one a builder puts in writing for your specific property. Everything else is an industry average that may have nothing to do with your situation.
Why It Happens: The Factors That Move the Number Most
1. Build Method: Prefab vs. Conventional Construction
This is the single biggest cost lever. A platform-built prefab unit from a manufacturer like Abodu, Boxabl, or Mighty Buildings ships factory-finished, which compresses on-site labor time and typically brings the all-in cost down compared to a stick-built unit. Dvele, Cover, and Plant Prefab sit in a mid-to-upper range because they lead with high-spec finishes and systems integration. Conventional construction using materials like James Hardie siding gives you the most design flexibility but requires the most trade coordination on site. You can usually tell which direction you’re headed by how much you need the unit to match your existing home versus simply function well.
2. Site Conditions and Utilities
A flat, accessible lot with utilities already stubbed near the build zone costs far less to develop than a sloped lot where a crew has to trench 80 feet for sewer lateral and build a retaining wall before framing starts. Check your lot: if the proposed ADU footprint has a slope, existing trees with root systems, or sits far from your home’s main utility connections, budget for site prep costs that may not show up in a rough online estimate. This is one of the most common reasons a homeowner gets a verbal quote over the phone and then sees a higher number once someone actually walks the lot.
3. Local Permit Fees and Code Requirements
Permit fees, impact fees, school fees, and plan-check costs vary dramatically by jurisdiction. Some markets have streamlined ADU permitting with pre-approved plan sets; others require a full architectural submittal and a multi-month review cycle. Neither the manufacturer’s brochure price nor the national average accounts for your specific jurisdiction’s fee schedule. Doing the job by the book – which is the only way we work – means every one of those fees gets factored into your written price upfront, not added as a surprise line item after the slab is poured.
4. Size, Layout, and Finish Level
A 400-square-foot studio ADU and a 1,200-square-foot two-bedroom ADU are fundamentally different projects even when they sit on similar lots. Finish level compounds the difference: builder-grade cabinets and LVP flooring cost considerably less than custom millwork and engineered hardwood. Most homeowners can ballpark their finish expectations pretty quickly – what does your current home look like, and do you want the ADU to match it? That answer narrows the range faster than almost any other variable.
What to Do Right Now
- Find your property survey or plot plan. A site visit goes faster when we can see the lot dimensions, setbacks, and any recorded easements. Check your closing documents or county assessor’s website.
- Photograph your utility connections. Walk around the home and take pictures of where your electrical panel, gas meter, and water/sewer cleanout are located. This gives a builder a head start on estimating utility extension costs before they arrive.
- Write down your non-negotiables. Minimum size, number of bedrooms, whether the unit needs to be accessible, target move-in date. A builder can work with a list like that; vague goals produce vague estimates.
- Check your local zoning basics. Most county or city planning department websites now have an ADU eligibility lookup tool. Run your address and note the maximum allowable size and setback requirements for your zone. Bring that printout to the estimate appointment.
- Be cautious of bids given without a site visit. A phone quote or a quote based only on square footage is a marketing number, not a real price. The lot, the utilities, and the permit picture all have to be in hand before a number means anything.
How We Fix It
When your local Fielding ADU Builders team arrives, we start with the lot – not a clipboard full of packages. We walk the build zone, check utility access, note any grading challenges, and talk through your goals for the unit: size, finish level, timeline, and whether a prefab platform or conventional construction fits better. We work fluently with the major modular and prefab platforms as well as conventional stick-built methods, so we’re not steering you toward one solution because it’s the only one we know.
From that site visit, we build a written price that reflects your actual lot, your permit jurisdiction, and your chosen build method. That price goes in your hands before any work starts – no surprises at the foundation stage, no change-order conversations after you’ve already committed. The work is then covered by our 365-Day Done Right Promise: if anything we built fails within a full year of completion, we come back and correct it at no charge. The estimate is free. Call (361) 315-4177 and your local team will schedule it.
Frequently Asked Questions
Most fully permitted ADUs fall between $80,000 and $300,000+, depending on build method, lot conditions, local permit fees, and finish level. Prefab platforms from manufacturers like Abodu or Boxabl tend to be less expensive than fully custom stick-built units. The only number that matters for your project is the one in your written estimate – call (361) 315-4177 and your local Fielding ADU Builders team will schedule a free site visit to produce exactly that.
Often, yes – but not always by as much as the manufacturer’s base price suggests. Prefab units can reduce on-site labor time, but site prep, utility connections, foundation work, and permit fees apply to every build regardless of where the walls were built. The right build method depends on your lot, your timeline, and your finish expectations. We walk through all of it during the estimate so you’re comparing apples to apples.
Not with us. The price we give you in writing before work starts is the price on your final invoice. That’s not a preference – it’s The Garrison Standard, and it applies to every project we build. The only exception is work you ask us to add after the contract is signed, which gets its own written change order before anyone touches it. No moving numbers at the door.
Timeline varies significantly based on permit jurisdiction and build method. A prefab unit with a pre-approved plan set in a streamlined jurisdiction can move from permit submission to move-in faster than a custom stick-built unit requiring full architectural review. Your local Fielding ADU Builders team will give you a realistic schedule based on your specific jurisdiction’s current processing times – not a marketing timeline built around the best-case scenario. Call (361) 315-4177 to get a straight answer for your area.
Get It Fixed
Your local Fielding ADU Builders team will walk your lot, review your goals, and hand you a written price before any work begins. Call (361) 315-4177 – a live person picks up around the clock – or book your free estimate online. No voicemail, no guessing.
Written by Reed Calloway, Owner at Fielding ADU Builders, a member of the Garrison Partners family serving communities nationwide since 2006.
What Our Customers Say
"Room addition over the back patio. Framing is square, windows are tight, and the roofline matches."
- Danny L."Foundation and slab poured on schedule. Crew kept the yard orderly."
- Marian J."Garage conversion with a new bath and kitchenette. Permit was handled by them. Inspection passed first time."
- Sheila B.
