How Much Does an ADU Cost to Build?
ADU construction costs vary widely based on unit type, site conditions, and local permit requirements – but most homeowners building a new detached ADU, garage conversion, or junior ADU are looking at a six-figure decision where the design, permitting, and construction costs all land in one place. The number on the page means little without a written scope behind it. At Ellery ADU Studio, every project is quoted with a flat written price before any work starts, under Haven Standard Clause 1 – no exceptions.
What’s Happening
When you search “how much does an ADU cost to build,” you are almost certainly looking at a range of numbers that span $80,000 to $400,000 or more – and none of them tell you what your specific project will cost on your specific lot. That range exists because ADU pricing is genuinely site-dependent: your setbacks, your utility connections, your existing structure (if you are converting), and your local permit fees all move the number.
What the internet cannot give you is a price for your ADU. What it can give you is a clear picture of what drives cost, what questions to ask before you sign anything, and what a legitimate written quote actually looks like. The goal of this page is to get you to that point – so that when you do sit down with a contractor or design-build studio, you know whether the number in front of you is real.
Why It Happens – What Drives ADU Cost
1. Unit Type: New Detached vs. Garage Conversion vs. Junior ADU
A new detached ADU built from scratch on your lot carries the highest base cost because you are building a complete structure with its own foundation, framing, roofing, mechanical systems, and utility connections. A garage conversion reuses the existing shell and foundation, which typically reduces costs significantly – but only if the existing structure is sound and the setbacks comply. A junior ADU carved from existing interior space (a bedroom suite with a separate entrance, for example) is usually the least expensive path, because the structure and utilities are already there.
The clearest way to know which category your project falls into: look at what you are starting with. Do you have a detached garage? An attached garage? A large bedroom with exterior access? Each one leads to a different cost conversation.
2. Site Conditions and Utility Connections
Site prep is the line item that surprises homeowners most often. If your lot is sloped, you may need a retaining wall or engineered foundation before the ADU unit itself even begins. If the nearest sewer lateral is on the far side of your property, the trench run alone adds real money. Electrical panel capacity, gas line routing, and water meter size all get evaluated during site prep – and any one of them can change your budget by tens of thousands of dollars.
A legitimate design-build firm will walk your site before quoting. Any price that arrives before a site visit is a guess, not a quote.
3. Permit Fees and Local Code Requirements
Permit fees vary by jurisdiction and by square footage. School fees, transportation impact fees, and park fees can stack on top of the base permit cost in some markets – and they are not optional. Fire sprinkler requirements, energy code compliance (Title 24 in California, similar codes elsewhere), and accessibility standards add to the design and construction scope in ways that change depending on exactly where your lot is.
This is one of the reasons Ellery bundles design, permitting, and construction under one contract. The permit set is not a separate line item you hand off to a contractor who then discovers the fees. The fees are part of the project from day one.
4. Prefab vs. Site-Built Construction
Prefab and modular ADU systems from brands like Abodu, Cover, Dvele, Mighty Buildings, Boxabl, and Plant Prefab can compress construction timelines and sometimes reduce on-site labor costs. But the unit cost is not the total project cost. You still need a permitted foundation, utility connections, and local inspections. Homeowners who receive a prefab unit quote without a site prep and permitting estimate often face a significant gap when the full scope comes into view.
Ellery specifies and coordinates prefab systems within the same single contract, so the foundation, the unit, and the utility connections are all priced together before the project starts.
What to Do Right Now
- Pull your property survey or lot plat. You need to know your setbacks, your lot size, and the location of your existing structures. This is the first document any legitimate ADU team will ask for, and having it ready speeds up the entire early conversation.
- Check your utility panel. Open the panel cover and note the main breaker amperage (usually printed on the breaker itself). A 100-amp panel serving an existing home often cannot support a new ADU without an upgrade. Write the number down – it comes up early in every site assessment.
- Walk the proposed ADU location and photograph it. Take photos from each corner of the proposed footprint, the path from the street to the site, and any slopes or drainage features. Your local Ellery ADU Studio team will ask for these before the first site visit, and having them ready saves a round trip.
- Look up your local jurisdiction’s ADU rules online. Most building departments post their ADU standards (minimum setbacks, maximum square footage, owner-occupancy requirements) on a public page. Knowing the basic parameters before your first call means the conversation starts at a more useful place.
- Collect any written estimates you already have. If you have received a quote from another firm, Ellery offers a Free Second Opinion on any written estimate in hand. You do not need to start over – you just need to know if the number in front of you reflects the full scope.
How We Fix It – The Ellery Path to a Real Number
When you contact your local Ellery ADU Studio team, the first step is a site assessment, not a sales call. A member of the Ellery team walks the property, reviews your survey, checks utility access, and identifies any conditions – setbacks, grading, panel capacity, sewer distance – that will affect cost. That assessment produces a documented photo record of what was found, not a verbal summary.
From there, Ellery’s design team develops a permit-ready drawing set that reflects your lot’s actual conditions and your jurisdiction’s current code requirements. The drawings and the construction scope are quoted together, in writing, under Haven Standard Clause 1, before any work begins. The services that apply here are ADU Design and Permitting, and whichever construction path fits your project: New Detached ADU, Garage Conversion ADU, Junior ADU and Interior Conversion, or ADU Site Prep and Utilities when utility connections require their own scope.
The written price is fixed. The 365-Day Done Right Promise backs the finished work: if the ADU is not built to the permitted drawings, Ellery makes it right. Timeline from site assessment to a written quote is typically short. Timeline from permit submission to a certificate of occupancy depends on your jurisdiction – Ellery tracks that queue actively, because the permitting timeline is often the actual product.
Frequently Asked Questions
Most ADU projects fall somewhere between $100,000 and $400,000 when design, permitting, and construction are combined – but that range is wide because site conditions, unit type, and local permit fees move the number significantly. A garage conversion on a flat lot with an existing 200-amp panel looks very different from a new detached ADU on a sloped lot that needs a new sewer lateral. The only honest answer to your specific question is a written quote after a site visit. Ellery quotes every project in writing before any work starts – call (971) 497-2402 and your local team will schedule the site assessment.
Junior ADUs and interior conversions are typically the least expensive path because they reuse existing structure, foundation, and often existing utility connections. A garage conversion usually comes in below a new detached ADU for the same reason – the shell is already there. That said, “least expensive” depends on what your existing structure actually is: a garage with a cracked slab, corroded wiring, and undersized framing may cost more to convert than a simple detached build on a clear, flat pad. A site assessment tells you which path is cheaper for your specific property.
Not with Ellery. Every project is quoted under Haven Standard Clause 1: a written price before any work begins, covering the full design, permitting, and construction scope. There is no door upsell and no “while we are in here” revision that resets the number without your written approval. The 365-Day Done Right Promise is the backstop: if the finished ADU is not built to the permitted drawings, Ellery makes it right. If you have a written estimate from another firm that does not include this kind of written scope protection, call (971) 497-2402 for a free second opinion.
Construction timelines vary by unit type and jurisdiction. The build itself – depending on whether you are doing a garage conversion, a prefab install, or a ground-up detached ADU – typically runs from a few months to under a year for the physical construction. The permit review queue is the variable most homeowners underestimate. In some jurisdictions, permit approval takes six to twelve weeks; in others, it runs longer. Ellery tracks the permit queue as part of the project, and the permitting timeline is included in the written project schedule before work starts. Call (971) 497-2402 to get a realistic timeline estimate for your jurisdiction.
Get It Fixed
Your local Ellery ADU Studio team handles design, permitting, and construction under one written price. Call (971) 497-2402 to schedule your free site assessment – a real number, in writing, before anything starts. If you already have a written estimate, we will give you a free second opinion on it.
Written by Nadia Ellery, Principal at Ellery ADU Studio, serving communities nationwide since 2015.
What Our Customers Say
"The permit set and the build were one contract with one number. Our neighbours used two firms and spent a year watching them blame each other."
- Helena V."The rental income has covered the payment since the second month. They were realistic about the numbers rather than optimistic."
- Odette N."They walked us through setbacks and utility capacity before we paid anything, and told us one of our two options was not going to get approved."
- Mireille T.