How much does an ADU cost?
An ADU often costs $75,000 - $300,000+, depending on whether it is attached, detached, converted from a garage, or built as new construction. Smaller garage conversions can cost less, while detached units with full utilities and high-end finishes can cost more.
ADU pricing is highly local because zoning, impact fees, utility connections, parking, fire separation, design review, and rental rules vary. Compare bids by legal scope, not just size.
ADU cost by build type
Garage conversion ADU
$20,000 - $100,000+How to tellExisting structure becomes a small apartment, studio, guest suite, or rental unit
The fixVerify zoning, parking, fire separation, insulation, plumbing, HVAC, egress, and utility capacity
Attached ADU
$75,000 - $175,000+How to tellNew living space connects to the existing house and may share utilities or walls
The fixPrice foundation, framing, roof tie-in, utilities, kitchen, bath, entrance, and permits
Detached ADU
$150,000 - $300,000+How to tellSeparate backyard cottage or rental with its own structure and utilities
The fixBudget for site prep, foundation, utility trenching, full envelope, kitchen, bath, HVAC, and inspections
Prefab or modular ADU
$100,000 - $250,000+How to tellFactory-built unit still needs site work, utility hookups, permits, and foundation
The fixCompare the delivered unit price against all site and soft costs
Soft costs and approvals
$10,000 - $50,000+How to tellDesign, engineering, survey, utility coordination, permit fees, impact fees, and inspections are separate
The fixAsk for a preconstruction budget before signing a construction contract
Before pricing an ADU
- Check whether your property allows an ADU, what size is allowed, and whether parking, setbacks, owner occupancy, or rental restrictions apply.
- Decide whether the ADU is for family, rental income, aging in place, guest space, or resale value.
- Ask contractors to separate hard construction costs from design, permits, utility connections, surveys, and fees.
- Photograph access paths, utility locations, garage condition, trees, slopes, drainage, and any tight side-yard constraints.
Unlock the hiring checklist
Use this before you call an addition contractor, so you know what to ask and what to avoid.
Call a pro when…
- You need zoning, permit, or rental-rule interpretation
- The ADU needs separate utilities, a kitchen, bathroom, or new foundation
- A garage conversion must become legal living space
- Access, drainage, trees, slopes, or utility trenching may complicate the build
- You are comparing detached, attached, prefab, and garage-conversion options
Repair or replace?
A garage conversion ADU can save money if the existing structure is code-ready. A detached ADU costs more but can offer better privacy, rental flexibility, and long-term value when zoning and site conditions support it.
Need a cleaner scope before you call around?
Use the home additions checklist to compare the same work, questions, cleanup, and warranty before choosing a local company.
Related questions
What costs are often missing from ADU estimates?
Design, engineering, permits, surveys, utility upgrades, impact fees, sewer/water connections, fire separation, site work, landscaping repair, appliances, and contingency are common missing items.
Is a prefab ADU always cheaper?
Not always. The unit may be cheaper, but foundation, delivery, crane access, utility hookups, permits, site prep, and local code requirements can erase part of the savings.
Can an ADU be rented out?
It depends on local rules. Some cities allow long-term rental, restrict short-term rental, require owner occupancy, or set parking and size limits. Verify rules before budgeting around rent.