Adding an accessory dwelling unit (ADU) typically costs $80,000-$400,000, with many projects landing near $200,000. The final price depends on whether the unit is detached, attached, or converted from a garage, as well as its size, utility connections, local labor rates, and permit requirements.
| Item | Low | Average | High | Notes |
|---|---|---|---|---|
| Garage conversion | $80,000 | $125,000 | $200,000 | Existing structure is usable |
| Attached ADU | $100,000 | $190,000 | $300,000 | Roughly 400-800 sq ft |
| Detached ADU | $150,000 | $250,000 | $400,000 | Roughly 500-1,000 sq ft |
| Cost per square foot | $150 | $275 | $400 | Construction cost; land excluded |
Typical ADU prices by size and build type
A basic 400-square-foot garage conversion may cost $80,000-$160,000 when the foundation, framing, and roof need limited work. A new 600-square-foot detached unit often costs $180,000-$300,000, while a larger or high-spec 900-square-foot build can reach $300,000-$400,000 or more.
For a typical detached ADU, budgeting about $250,000 is a reasonable starting point, not a guaranteed quote. This assumes standard finishes, normal site access, and a U.S. market with average construction rates. Design fees, land purchase, major grading, and unusual utility upgrades may be additional.
Assumptions: 500-800 sq ft, standard kitchen and bath, ordinary access, and no major structural or utility work.
How West Coast and Midwest ADU quotes compare
Location can shift a similar project’s quote substantially. High-cost coastal cities may run 20%-40% above a national midpoint because of labor rates, permitting, seismic standards, and limited contractor availability. Smaller Midwestern and Southern markets may come in 10%-20% below that midpoint, although remote sites and local code rules can narrow the gap.
Compare local bids for the same floor area and included work, rather than relying on a national per-square-foot average. A low bid may exclude utility trenching, appliances, design, or permit fees. Some cities also require impact fees or code upgrades that add thousands of dollars.
Where the $250,000 detached ADU budget goes
Construction estimates typically combine materials, skilled labor, equipment, and project overhead. The ranges below show common shares for a standard detached unit; site conditions and local pricing can change each line item.
| Quote item | Typical range | Unit or scope | What it covers |
|---|---|---|---|
| Materials | $75,000-$120,000 | $125-$200 per sq ft | Framing, finishes, fixtures |
| Labor | $60,000-$100,000 | $50-$100 per hour blended | Trade and installation work |
| Equipment and site work | $15,000-$45,000 | $25-$75 per sq ft | Excavation, access, machinery |
| Permits and design | $10,000-$35,000 | Per project | Plans, engineering, reviews |
| Delivery and disposal | $3,000-$12,000 | Per project | Material delivery and debris |
Labor and materials usually make up the largest portions of the construction quote. A planning estimate for labor can use ; for example, 900-1,200 hours at a blended $65-$85 per hour equals roughly $58,500-$102,000.
Three ADU quotes show how scope changes the total
A 450-square-foot garage conversion with existing utilities, 450 labor hours, and modest finishes might total $110,000-$145,000, or about $245-$320 per square foot. The relatively small size does not eliminate kitchen, bathroom, design, and permit expenses.
A 600-square-foot detached unit on a level lot could cost $210,000-$275,000. At 900-1,100 labor hours and $65-$85 per hour, labor alone may be $58,500-$93,500, with the remaining budget covering materials, site work, design, and fees.
A 900-square-foot detached ADU with upgraded finishes and a long utility run may reach $310,000-$400,000. Its size, electrical and sewer work, and finish choices push the per-square-foot price toward the upper end.
These examples are planning ranges; site-specific bids can differ even for matching floor plans.
Square footage and utility runs drive ADU price changes
Each additional 100 square feet may add roughly $15,000-$40,000, depending on whether the added area requires more bathrooms, kitchen cabinetry, or upgraded finishes. Compact plans can cost more per square foot because kitchens, baths, and mechanical systems take up a larger share of the build.
Utility distance is another major variable. A sewer, water, or electrical connection within 25 feet may need limited trenching, while runs over 50 feet can add $5,000-$20,000 or more, especially where pavement must be cut or capacity must be upgraded. Slopes above about 10%, poor soil, or retaining walls can also add substantial site-preparation costs.
Confirm utility routes and capacity before treating a design’s base price as the project total. A garage with a sound slab and accessible plumbing can be far less expensive to convert than one requiring foundation repair or a new sewer connection.
Construction time and site extras to budget for
A straightforward conversion may take 4-8 months from design through completion, while a detached unit often takes 8-14 months, including permitting. Construction itself may take 4-8 months for a new build; plan review, inspections, weather, and contractor scheduling extend the full timeline.
Demolition, tree removal, asbestos testing, or a difficult equipment-access route can add $2,000-$20,000 or more. A contingency of 10%-15% is prudent for unknown conditions, particularly when opening walls or excavating around older utilities.
Ask each contractor to list exclusions, allowances, and site-work assumptions in writing. This makes it easier to distinguish a genuinely lower price from a quote that leaves likely expenses out.
Keep an ADU budget down without cutting essential scope
Use a compact rectangular floor plan, keep plumbing close to the main house’s existing connections when feasible, and choose standard-size windows, doors, and cabinets. Simplifying rooflines and limiting custom finishes can reduce labor and material waste without compromising structural requirements.
Compare at least three itemized bids based on the same plans and finish schedule. Ask whether a garage conversion is structurally viable before pricing a new detached building, and verify that the proposed design meets local zoning and building rules before paying for extensive revisions.
Preserve budget for permits, utility work, and a contingency before adding premium finishes or optional upgrades. Savings from cheaper fixtures can be outweighed by a missed sewer upgrade or a redesign after permit review.