Quick answer: In 2026, most accessory dwelling units cost $150–$300 per square foot, with the national average project landing around $180,000. But that average hides the real story: a garage conversion can come in near $80,000, while a detached new-construction ADU in coastal California routinely passes $375–$600+ per square foot. The type of ADU you choose matters more than any other decision.
This guide breaks down cost by ADU type, the line items that make small units expensive, what permits and fees really add, how people pay for them, and why the cheapest ADU per square foot is usually the biggest one your zoning allows.
ADU Cost by Type (2026)
| ADU type | Typical total | Why |
|---|---|---|
| Garage conversion | $80,000–$150,000 | Walls, roof and slab already exist — you pay for conversion, not construction |
| Basement conversion | $50,000–$140,000 | Similar logic; cost driven by egress, moisture and ceiling height fixes |
| Attached ADU (addition) | $150,000–$300,000 | New construction, but shares one wall and often utilities with the main house |
| Detached new-build | $180,000–$360,000+ | A complete small home: foundation, shell, kitchen, bath, separate utility runs |
| Prefab/modular ADU | 10–20% below custom | Factory-built shell reduces labor; site work still applies |
Why Small Units Cost More Per Square Foot
This is the counterintuitive math that catches most first-time ADU builders:
Kitchens, bathrooms and permits don’t shrink. A 400 sq ft ADU and an 800 sq ft ADU need the same kitchen, the same bathroom, the same utility connections and roughly the same permit stack. Those fixed costs spread over fewer square feet make the small unit more expensive per foot — which is why real-world data shows homeowners overwhelmingly building to the maximum size their zoning allows. If your city caps ADUs at 800 sq ft, the economically rational build is usually 800 sq ft.
The same fixed-cost logic explains why tiny houses carry the highest per-foot prices of any format — see our tiny house cost guide.
The Full Budget: What “ADU Cost” Actually Includes
- Design and plans — stock plans are cheap; custom architectural work and structural engineering add thousands
- Permits and impact fees — the wild card. Some cities have streamlined ADU permitting; others stack school fees, utility connection fees and plan-check costs that add five figures
- Site work and utilities — trenching separate water, sewer and electrical to a backyard unit is a major detached-ADU cost that conversions avoid
- The build itself — labor alone typically consumes ~40% of the project; general contractors charge 10–20% of total project cost
- Interior finish level — same driver as any home: cabinets, flooring and fixtures create most of the range
Permits and Fees: The Line That Varies Most
Two identical backyard units can differ by $20,000 or more on fees alone, depending on the city. What typically appears on the bill:
| Fee | Typical range | Notes |
|---|---|---|
| Plan check and building permit | $2,000–$10,000 | Often scaled to project valuation |
| School impact fees | $0–$10,000+ | Many jurisdictions exempt smaller ADUs |
| Utility connection / capacity fees | $0–$15,000+ | Highest when a separate water or sewer connection is required |
| Separate electrical service | $3,000–$15,000+ | Only if you want or need a separate meter |
| Survey, soils or engineering reports | $1,000–$6,000 | More likely on slopes or new foundations |
Several states have limited what cities can charge for smaller ADUs, so the same unit can be dramatically cheaper to permit under a state ADU law than it was a few years ago. Ask the planning department for a fee estimate before you pay for full plans — it is often free, and it is the fastest way to find out whether your project pencils out.
Garage Conversions: Where the “Cheap” Option Goes Over
Conversions are the lowest-cost route, but they have a predictable list of surprises:
- The slab: garage slabs are often not level, insulated or protected against moisture to living-space standards, and some need a new floor system on top.
- Plumbing runs: getting a sewer line from a new bathroom and kitchen to the existing lateral can mean cutting and trenching through concrete.
- Egress and ceiling height: bedrooms need code-compliant windows or exits, and low headers may need structural changes.
- Replacement parking: some cities still require you to replace the lost parking space, although many state ADU laws now waive this.
A realistic conversion budget holds 15–20% in contingency for these items.
How People Pay for ADUs
| Option | Best for | Watch out for |
|---|---|---|
| Cash | Lowest total cost | Ties up capital for months |
| HELOC or home equity loan | Owners with substantial equity | Variable rates on most HELOCs |
| Cash-out refinance | When today’s rate beats your current mortgage | Resets your whole mortgage |
| Renovation / construction loan | Owners without enough equity today | Some lenders can count projected rental income or the post-build value |
| Builder or prefab financing | Prefab units | Compare the effective rate carefully |
Whatever the source, add a contingency of 10–15% on top of the contract price. Few ADU projects finish exactly on budget, and running out of money with a half-finished unit is the worst outcome.
Regional Reality Check
The national $150–$300/sq ft range stretches badly at the coasts. San Diego turnkey detached ADUs run $375–$600+/sq ft with most complete projects at $300,000–$450,000; New York-area builds run $150–$400/sq ft with premium union labor. Meanwhile, garage conversions in lower-cost Southern and Midwestern markets still land under six figures. Always price locally — state-level cost articles in this section break this down market by market.
Timeline
| Phase | Conversion | Detached new-build |
|---|---|---|
| Design and plans | 3–8 weeks | 1–3 months |
| Permitting | 1–3 months | 2–6 months |
| Construction | 2–4 months | 4–8 months |
| Typical total | 4–9 months | 7–15 months |
Prefab units shorten construction, but not permitting or site work, which is why they rarely cut the total timeline as much as their marketing suggests.
Is an ADU Worth It as a Rental?
The math that matters: monthly rent ÷ total project cost. A $180,000 ADU renting for $1,500/month grosses 10% annually before expenses — strong compared to most real estate. The risks are concentrated up front: permit surprises, utility connection costs, and building in a market where ADU rents don’t support the build cost. Run the numbers with local rents before committing, and subtract realistic expenses — property tax on the added value, insurance, maintenance and vacancy — before comparing returns.
FAQ
What’s the cheapest way to get an ADU?
Converting existing space. A garage or basement conversion skips foundation and shell — often the difference between $100,000 and $250,000.
How long does an ADU take?
Conversions: roughly 4–9 months from design to move-in. Detached new builds: 7–15 months including permitting, and slow-permitting cities can dominate that timeline.
Do I need a separate address and meter?
Depends on jurisdiction. Some cities require separate utility metering for rentals; others allow shared meters. This affects both cost and rental logistics.
Does an ADU raise property taxes?
Generally yes — the new structure is assessed and added. It rarely triggers reassessment of the main home in most states, but verify locally.
Is an ADU a better use of $200,000 than a barndominium or tiny house?
It depends on whether you already own a lot in a place with rental demand. Our comparison of what $200K buys in each format lays out the trade-offs.
Methodology note: figures reflect 2026 published national cost data and builder-reported pricing across multiple markets, cross-checked in September 2026. Fee ranges vary by city and change with state ADU laws — confirm with your local planning department. Last review: September 29, 2026. Written and reviewed by the DwellingMath Editorial Team.