How Much Does It Cost to Build an ADU?
$80,000 – $250,000
National average: $180,000
Estimate your cost
Adjust the options for a tailored ballpark — figures and the regional adjustment are approximate estimates. Always confirm with local quotes before you budget.
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.
| Component | 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.
Frequently asked 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: ranges reflect typical U.S. pricing for materials and professional installation, compiled and cross-checked against the current (2026) industry sources listed below (see our data & methodology). Your actual cost depends on your location, project size, material grade, and local labor rates — always get multiple written quotes before you commit.
Sources
Cost ranges on this page were checked against current (2026) data from these industry sources: