Why the numbers vary so much
When you see ADU cost estimates ranging from $40,000 to $400,000, it's not that someone is lying — it's that "ADU" covers an enormous range of projects. A garage conversion in a low-cost rural market is a fundamentally different proposition from a purpose-built detached unit in California with full permitting and a licensed contractor.
The main variables that drive cost are: whether you're converting existing space or building new, your location and local labour costs, whether the unit needs its own utilities connections, the level of finish, and whether you're owner-managing the project or hiring a general contractor to handle everything.
The four types and what they cost
Garage or basement conversion: $40,000–$120,000
The most cost-effective option in most markets, because the structure already exists. You're adding insulation, flooring, a bathroom, a kitchenette, and potentially HVAC. The wide range reflects the starting condition of the space — a finished basement with existing plumbing runs far cheaper than a raw concrete garage that needs everything.
What people consistently underestimate: egress windows (often required by code), electrical panel upgrades, and the cost of a separate entrance that feels genuinely private rather than like a side door into someone's home.
Internal conversion (carving out a self-contained unit): $60,000–$150,000
This is what most multigenerational families actually do — take an existing large home and reconfigure part of it into a separate unit with its own entrance, kitchen, and bathroom. Costs depend heavily on how much structural work is needed and whether plumbing needs to be rerouted.
The big hidden cost here is soundproofing. A conversion that's quiet enough to feel like two genuinely separate homes costs significantly more than one that isn't. It's worth budgeting for properly upfront rather than living with the compromise.
Attached ADU (new addition to the home): $100,000–$250,000
A purpose-built addition — typically extending the footprint of the house at ground level or adding a new storey. Full construction costs apply. Permitting is more involved than a conversion. On the upside, you get exactly what you design rather than working around existing constraints.
Detached ADU (separate structure in the garden): $150,000–$400,000
The most expensive option and the most complex to permit. Requires its own foundation, roof, and typically its own utility connections. In high-cost markets like California, New York, or the Pacific Northwest, $300,000+ for a modest detached unit is not unusual once you factor in land preparation, permitting, and contractor margin.
Prefabricated ADU units have brought costs down somewhat — a prefab shell can be installed in days — but site preparation, foundation work, and utility connections still add $50,000–$100,000 on top of the unit cost in most markets.
What builders don't always tell you
Permitting takes longer than you expect. In many jurisdictions, ADU permits take 3–6 months to approve. Some markets have streamlined the process; others haven't. Factor this into your timeline before you commit to a contractor start date.
Utility connections are a wildcard. Connecting a new unit to water, sewer, and electricity can cost anywhere from $5,000 to $50,000+ depending on distance from existing connections and local utility requirements. Always get a utilities assessment before finalising your budget.
The quote and the final cost are different numbers. Construction overruns of 15–25% are common even on well-managed projects. Budget for this explicitly rather than hoping it won't happen.
Owner-builder savings are real but have limits. Managing subcontractors yourself rather than hiring a GC can save 15–20% on a project. But it requires time, knowledge, and the ability to manage multiple trades simultaneously. For most families doing this once, the GC markup is worth it.
Does the construction cost make financial sense?
This is the question that matters most, and it has two parts.
First, the break-even: how long before the monthly savings from the shared arrangement cover the construction cost? A $100,000 conversion for a family saving $1,800 a month breaks even in under five years. A $280,000 detached ADU for a family saving $1,500 a month takes over 15 years — by which point the arrangement may have run its natural course.
Second, the property value impact: a well-executed ADU typically adds 60–80% of its construction cost to the property's market value. It's not a dollar-for-dollar return, but it's not money lost either. A $120,000 conversion that adds $85,000 to your home's value has a real net cost of $35,000 — which changes the break-even calculation considerably.
Before you call a contractor
The order matters: run the financial model first, then get planning advice, then get quotes. Families who reverse this order often fall in love with a design before they've confirmed it makes economic sense — which puts them in a weak negotiating position and sometimes leads to projects that don't pay back.
Get at least three quotes from licensed contractors. Ask each one to itemise the estimate so you can compare like for like. And ask specifically what's not included — the gap between the quoted scope and the finished project is where overruns live.