An ADU is the largest check most homeowners will ever write to a contractor — bigger than a kitchen, bigger than a roof, sometimes bigger than the car in the driveway several times over. And unlike a kitchen remodel, the pricing is opaque enough that two legitimate builders can bid the same backyard unit tens of thousands of dollars apart. Here's the real math for 2026.
The 2026 ranges
| ADU type | Typical all-in cost | Notes |
|---|---|---|
| Detached new-build ADU | $180,000 – $360,000 (often $200,000–$400,000+ in high-demand states) | The backyard home; most expensive, most value added |
| Garage conversion | $60,000 – $180,000 (up to $225,000 for larger detached-garage conversions) | Structure and slab exist; new utility runs drive cost up |
| Attached ADU / addition | $100,000 – $250,000 | Shares a wall and often utilities with the main house |
| Basement or interior conversion | $80,000 – $180,000 | Egress, ceiling height, and moisture work set the price |
Per square foot, expect roughly $150–$300 all-in nationally — design, permits, site work, construction, and utility connections included. High-cost coastal metros and larger custom builds push toward the top of that range or beyond; rural markets and permissive states run lower.
Where the money actually goes
The construction itself is only part of the number. The line items that surprise homeowners:
- Site work and utilities — for a garage conversion with short runs, expect $10,000–$20,000; for a detached backyard unit needing new water, sewer, and electrical lines, $25,000–$55,000 or more. It's the single most common source of bid divergence.
- Design and permits — plans, structural engineering, and city fees typically total $10,000–$35,000, with the widest swings driven by local permit fees, which can run from a few thousand dollars in smaller towns to well over $20,000 in expensive metros. Some cities now offer pre-approved plans that cut this meaningfully.
- The panel problem — many older homes need an electrical service upgrade before an ADU can connect. A straightforward panel swap runs $1,300–$4,000; if the utility company has to upgrade the service line and meter too, that climbs to $7,300–$12,800.
- Finishes — the spread between builder-grade and custom finishes on a 600 sq ft unit is $20,000+ by itself.
Why bids for the same ADU spread 20–30%
When three licensed builders bid the same plans, a five-figure spread is normal — and informative. The usual causes: different assumptions about site work (did they actually walk the utility path?), different allowances for finishes (a $5,000 flooring allowance vs $15,000 is a hidden $10,000 gap), and different overhead models. A bid that's dramatically lower than the others usually has thin allowances that come back as change orders — on a six-figure project, comparing bids line by line isn't optional. It's five figures of your money.
Does the investment pencil out?
Two ways an ADU pays: rent and value. Long-term rents for a well-built one-bedroom ADU in high-demand metros commonly run $1,500–$2,500+ per month, and appraisals increasingly credit permitted ADUs at resale. The math is strongest where rents are high and lots are large — run your own numbers with real local rents before committing, and get real bids before running the numbers. Estimating with internet averages on a project this size is how budgets break.
How to choose the builder
On an ADU, generic contractor vetting isn't enough. You want: ADU-specific experience (ask how many they've completed in your city — permitting fluency is worth months), a permit track record they'll show you, license and insurance verified with your state board, and references from ADU clients you actually call. Then get at least three bids on the same plans. On this project, the comparison isn't about saving a few hundred dollars — it's routinely the difference of $30,000 or more for identical work.
What's new for 2026
California's Department of Housing and Community Development released an updated ADU Handbook for 2026, reflecting several state law changes that took effect January 1, 2026. If you're building in California, don't rely on last year's rules or a builder's memory of them — pull the current handbook or check with your city's planning department, since local ordinances still layer on top of state law. Other states have moved on ADU-friendly legislation too; check your state or city planning site before you budget, not after.