How Much Does an ADU Cost to Build?
ADU construction costs typically range from $100,000 to $400,000 or more, depending on size, site conditions, the type of unit (detached, attached, or garage conversion), and the materials and prefab systems you choose. A garage conversion almost always lands at the lower end of that range. A fully custom detached ADU with a foundation, utility connections, and finished interior sits at the higher end. The only number that actually matters is the written estimate for your specific lot, your specific unit, and your specific goals – and that’s exactly what your local Seedling ADU Builders team produces before any work begins.
What’s Happening
When families start researching ADU costs, they almost always find a range so wide it feels useless. That’s not a coincidence – it reflects how genuinely variable this work is. Every lot is different. Every city has different permit fees, setback rules, utility hook-up requirements, and soil conditions. A number pulled from a national average tells you almost nothing about what your project will actually cost.
What the range does tell you is this: ADU construction is real construction. It involves foundations, framing, electrical panels, plumbing rough-ins, insulation, drywall, roofing, and finishes. Whether you’re converting your garage, adding onto your house, or building a freestanding unit from scratch, the scope of work is substantial. The best way to anchor to a real number is to have someone walk your lot, look at your utility access, understand your goals, and put a written price in your hands.
Why Costs Vary So Much
Type of ADU
A garage conversion costs less because the shell is already there. Your local Seedling team is converting a structure that already has a roof, walls, and a concrete slab – the work is about insulation, interior finishing, plumbing, electrical upgrades, and egress compliance. A detached ADU built from the ground up requires a full foundation, framing, and all new utility runs. An attached home addition falls somewhere in between, depending on how much of the existing structure ties in. The type you choose has the single biggest impact on where your number lands.
Site Conditions and Utility Access
A flat lot with a nearby electrical panel and an accessible sewer lateral costs less to build on than a sloped lot that needs retaining work, or a property where the utility connections require trenching across a long distance. Soil conditions matter too – some sites need more substantial foundations than others. You can’t know any of this without a site visit, which is why your local Seedling team walks the lot before putting anything in writing.
Prefab vs. Site-Built
Prefab and modular ADU systems – brands like Abodu, Cover, Dvele, Mighty Buildings, Boxabl, and Plant Prefab – are engineered off-site and delivered to your property in a more finished state. This can compress the construction timeline and sometimes the cost, depending on the unit. Tuff Shed structures and James Hardie materials come into play for specific finishes and applications. None of these is automatically cheaper or better – they’re different tools for different lots and different families, and knowing which one fits your situation is part of what your local Seedling team helps you sort out.
Local Permit and Code Requirements
Permit fees, impact fees, and inspection requirements vary significantly by jurisdiction. Some municipalities charge a few hundred dollars to permit an ADU. Others charge several thousand. Some require owner-occupancy conditions. Some have maximum unit sizes that affect your design choices before a single drawing is made. These aren’t costs Seedling controls, but they are costs that get included in your written estimate – no surprises after work starts.
What to Do Right Now
- Walk your lot and note what’s already there. Is there an existing garage or outbuilding that could be converted? Where does your sewer cleanout sit? Where is your electrical panel? This information helps your local Seedling team give you a more accurate picture during the estimate conversation.
- Write down your goals for the unit. Full kitchen or kitchenette? One bedroom or two? Will a family member live there, or are you planning to rent? Your intended use shapes the design, and the design shapes the cost.
- Check your local jurisdiction’s ADU rules online. Most cities and counties post their ADU ordinances publicly. Look for maximum square footage, setback requirements, and whether owner-occupancy is required. Note what you find – your local Seedling team will know the specifics, but having the basics helps the conversation move faster.
- Take photos of your lot from each corner, your driveway approach, and your utility panel. These photos give the team a head start before they arrive for the site visit.
- Do not commit to a design or a contract with anyone until you have a written price in hand. Verbal estimates and “ballpark figures” have a way of changing once crews arrive. Every Seedling project starts with a written price – the actual number, before any work begins, no exceptions.
How We Fix It
When you call your local Seedling ADU Builders team, the first step is a site visit – not a sales presentation, a genuine walkthrough. We look at your lot, your existing structures, your utility access, and your setbacks. We ask about your goals: who is living there, how long you plan to stay in the house, what level of finish feels right for your family.
From there, we produce a written estimate that accounts for the full scope: design, permitting, foundation or conversion work, framing, electrical, plumbing, insulation, finishes, and any prefab or modular systems that fit your situation. If a brand like Abodu, Cover, or Plant Prefab makes sense for your lot, we’ll explain why. If site-built makes more sense, we’ll explain that too.
Once you approve the written price, the build begins – permitted, inspected, and covered by the 365-Day Done Right Promise. Every Seedling project is covered by it. If something isn’t right after we finish, we make it right. That promise is in writing, just like the price.
Timeline depends on the type of unit and your local permitting office, but your team will walk you through a realistic schedule during the estimate conversation – no guessing.
Frequently Asked Questions
ADU costs generally run from $100,000 to $400,000 or more, with garage conversions typically at the lower end and fully detached new-construction units at the higher end. That range is wide because the variables – site conditions, type of unit, local permit fees, finish level, and prefab versus site-built – swing the number significantly. The only reliable figure is a written estimate based on your actual lot. Call (707) 383-5338 to schedule yours – it’s free, and it’s in writing before anything starts.
Rough numbers exist, but they’re risky to plan around. A garage conversion on a straightforward lot can look very different in cost from a garage conversion that needs major electrical upgrades, a new sub-panel, and significant weatherproofing work. The only way to give you a number you can actually build a budget around is to see the site. Your local Seedling team offers free written estimates – a site visit costs you nothing, and you leave with a real number in hand.
The written price Seedling gives you before work begins covers the scope we agreed on – no surprise line items appear after crews arrive. If you decide mid-project to change the design or add scope, that conversation happens in writing before any new work begins. The principle is simple: explain the price before the work. That applies to the original estimate and to any changes along the way. Call (707) 383-5338 if you have questions about how project pricing works.
Timeline depends heavily on the type of unit and how quickly your local permitting office moves. Garage conversions typically move faster than new detached construction because you’re not waiting on a foundation pour or a full framing sequence. Prefab and modular systems can compress the on-site build phase significantly. Your local Seedling team will give you a realistic timeline during the estimate conversation – one that accounts for permit review periods in your specific jurisdiction, not a generic national average. Call (707) 383-5338 to get the conversation started.
Get It Fixed
Your local Seedling ADU Builders team is ready to walk your lot, answer your questions, and put a written price in your hands – no obligation, no pressure, no guessing. Call (707) 383-5338 or book online. A real person picks up, every time.
Written by Ben Ortega, Owner at Seedling ADU Builders, serving families across the country with ADUs built with care.
What Our Customers Say
"Garage conversion into a studio. They sealed off the work area so dust didn't get into the house, and cleaned up every day before leaving. Calm, careful team to have around."
- Darnell Hobbs"Added a backyard cottage. The team protected our existing landscaping and left the yard clean every evening. Gentle, respectful, and they walked us through each stage calmly."
- Tamara Welsh"Garage to studio conversion with us living right there. They sealed off the dust, cleaned up daily, and respected our quiet hours. No surprise costs. Beautiful."
- Terrence Boylan