More Adding Living Space: Home Addition ·Sunroom ·Garage Conversion ·Basement Finishing
Accessory dwelling units — backyard cottages, in-law suites, converted garages — have become one of the most popular ways to add space, house family, or earn rental income. Building one costs $80,000 to $250,000 in 2026, averaging around $180,000, or about $100–$300 per square foot.
What you’re paying for
An ADU is a complete, independent living unit, so you’re funding everything a small home needs: foundation and shell, a kitchen and bath, finishes, mechanical systems, site work, utility connections, and permits. The kitchen, bath, and utility connections are why ADUs cost more per square foot than a plain room addition.
Cost by type
How you build is the biggest lever. The table below shows typical pricing from a budget garage conversion to a detached new build.
The hidden cost: site and utilities
People price the structure and forget the site work. A detached ADU needs water, sewer, and electrical run from the main house or street, plus grading and access — easily $5,000–$30,000 before the building itself. A garage conversion sidesteps much of this by reusing an existing, already-served structure, which is why it’s the budget route.
Is it worth it?
ADUs are among the higher-return home investments because they can both add resale value and generate rent. The math depends heavily on local rents and regulations — in high-rent metros an ADU can pay for itself over time, while in low-rent areas the value is more about flexible space for family.
How to save on an ADU
- Convert a garage instead of building detached.
- Consider prefab/modular for lower cost and faster timelines.
- Keep finishes rental-grade unless you’ll live in it yourself.
- Check your local ADU ordinance early — rules drive both cost and feasibility.
| Item | Typical range | Notes |
|---|---|---|
| Foundation & shell | $25,000 – $80,000 | – |
| Interior finishes & cabinetry | $20,000 – $70,000 | – |
| Plumbing, electrical & HVAC | $3,400 – $10,400 | – |
| Site work & utility connections | $5,000 – $30,000 | – |
| Design & permits | $3,000 – $20,000 | – |
| Option | Typical range | Notes |
|---|---|---|
| Garage conversion | $20,000 – $140,000 | Cheapest; uses existing structure |
| Attached ADU | $125 – $225 / sq ft | – |
| Detached ADU | $150 – $250 / sq ft | Most common new-build |
| Prefab / modular | $80 – $160 / sq ft | Lower cost, faster |
What affects the price
- ADU type Converting a garage is cheapest; a detached new build is priciest. Prefab can cut both cost and time.
- Size Most ADUs run 400–800 sq ft; larger units cost more in total.
- Site & utilities Running sewer, water, and electrical to a detached unit — plus grading and access — is a big variable.
- Finishes A rental-grade ADU costs far less than one with high-end kitchen and bath finishes.
- Local rules & region Permit complexity and labor vary widely; California and high-cost metros run well above the national average.
ADU cost by state
Where you live can change the price as much as the options you pick. Labor is a big part of the bill, and construction workers are paid more in some states than others. A typical ADU job costs about $93,600 to $292,500 per sq ft in Hawaii (17% above the U.S. average). In Arkansas, it costs about $68,000 to $212,500 per sq ft (15% below the U.S. average).
See the typical cost in all 50 states and D.C.
| State | Typical range |
|---|---|
| Alabama | $69,600 – $217,500/sq ft |
| Alaska | $92,000 – $287,500/sq ft |
| Arizona | $76,000 – $237,500/sq ft |
| Arkansas | $68,000 – $212,500/sq ft |
| California | $89,600 – $280,000/sq ft |
| Colorado | $80,000 – $250,000/sq ft |
| Connecticut | $85,600 – $267,500/sq ft |
| Delaware | $78,400 – $245,000/sq ft |
| District of Columbia | $86,400 – $270,000/sq ft |
| Florida | $72,800 – $227,500/sq ft |
| Georgia | $73,600 – $230,000/sq ft |
| Hawaii | $93,600 – $292,500/sq ft |
| Idaho | $75,200 – $235,000/sq ft |
| Illinois | $92,000 – $287,500/sq ft |
| Indiana | $80,800 – $252,500/sq ft |
| Iowa | $77,600 – $242,500/sq ft |
| Kansas | $76,000 – $237,500/sq ft |
| Kentucky | $74,400 – $232,500/sq ft |
| Louisiana | $72,000 – $225,000/sq ft |
| Maine | $78,400 – $245,000/sq ft |
| Maryland | $80,000 – $250,000/sq ft |
| Massachusetts | $92,000 – $287,500/sq ft |
| Michigan | $80,000 – $250,000/sq ft |
| Minnesota | $87,200 – $272,500/sq ft |
| Mississippi | $69,600 – $217,500/sq ft |
| Missouri | $81,600 – $255,000/sq ft |
| Montana | $78,400 – $245,000/sq ft |
| Nebraska | $75,200 – $235,000/sq ft |
| Nevada | $82,400 – $257,500/sq ft |
| New Hampshire | $79,200 – $247,500/sq ft |
| New Jersey | $91,200 – $285,000/sq ft |
| New Mexico | $73,600 – $230,000/sq ft |
| New York | $88,000 – $275,000/sq ft |
| North Carolina | $72,000 – $225,000/sq ft |
| North Dakota | $80,800 – $252,500/sq ft |
| Ohio | $80,000 – $250,000/sq ft |
| Oklahoma | $72,800 – $227,500/sq ft |
| Oregon | $88,000 – $275,000/sq ft |
| Pennsylvania | $80,800 – $252,500/sq ft |
| Rhode Island | $83,200 – $260,000/sq ft |
| South Carolina | $72,800 – $227,500/sq ft |
| South Dakota | $72,000 – $225,000/sq ft |
| Tennessee | $73,600 – $230,000/sq ft |
| Texas | $72,800 – $227,500/sq ft |
| Utah | $75,200 – $235,000/sq ft |
| Vermont | $77,600 – $242,500/sq ft |
| Virginia | $76,000 – $237,500/sq ft |
| Washington | $92,800 – $290,000/sq ft |
| West Virginia | $76,000 – $237,500/sq ft |
| Wisconsin | $83,200 – $260,000/sq ft |
| Wyoming | $79,200 – $247,500/sq ft |
These are numbers to plan with, not quotes. Each one is this guide's U.S. range, adjusted for what construction workers earn in that state. See every state cost guide or how we estimate.
Common questions
- How much does an ADU cost on average?
- Nationally, ADUs average around $180,000, with most between $80,000 and $250,000. Cost per square foot runs $100–$300 depending on type, finishes, and location.
- What's the cheapest type of ADU?
- A garage conversion is usually cheapest because it reuses an existing structure — often $20,000–$140,000. Prefab/modular units are the most affordable new-build route.
- Does an ADU add value or income?
- ADUs can add substantial resale value and generate rental income, often making them one of the higher-return additions — though returns vary by local rents and regulations.
- Do I need permits to build an ADU?
- Yes. ADUs require permits, plans, and inspections, and many areas have specific ADU ordinances covering size, setbacks, and parking. Permit and design fees run $3,000–$20,000.
- How long does it take to build an ADU?
- A garage conversion can take 2–4 months; a detached new build typically runs 6–12 months including design and permitting. Prefab units install faster once the site is prepped.
- Prefab vs. site-built ADU — which is better?
- Prefab/modular is faster and often cheaper per square foot, with predictable quality. Site-built offers more customization. Both still require a foundation, utilities, and permits.
- Why are California ADUs so expensive?
- High labor and material costs, strict codes, and site/utility requirements push California ADUs to $150,000–$350,000+, with detached LA units often $350–$550 per square foot.
How we estimate: These are typical U.S. prices for materials and for hiring a pro to install them. We checked them against the 2026 sources listed below (see how we get our numbers). Your price depends on where you live, the size of the job and the materials you pick. Get two or three written bids before you choose a contractor.
Sources
We checked the prices on this page against 2026 numbers from these sources: