ADU Rules & Cost in Washington
A 700 sq ft detached ADU in Washington typically costs $169,500–$277,000 ($242–$396/sq ft). Here is what the rules allow.
Statewide ADU law: RCW 36.70A.681 (HB 1337, 2023)Washington ADU rules at a glance
Washington's HB 1337 (2023) requires cities and counties in urban growth areas to allow two ADUs per lot, of at least 1,000 sq ft each, in single-family zones. Owner-occupancy cannot be required and parking minimums are sharply limited near transit.
- Statewide law
- Yes
- Max detached size
- 1000 sq ft
- Guaranteed minimum
- 1000 sq ft
- Setback
- 5 ft
- Owner-occupancy
- Not required
- Approval time
- Set locally
Parking: No more than 1 space per ADU; no parking required within 1/2 mile of major transit.
Estimate your Washington ADU
Pre-set to Washington. Adjust the type and size for your project.
Your property
ADU typeEstimated cost in Washington
$169,500 – $277,000
≈ $242–$396/sq ft turnkey · midpoint $223,500
- $152,000
- Construction
- $40,200
- Design & permits
- $31,300
- Site work
- ✓Size within the guaranteed minimum. Your 700 sq ft ADU is at or under the 1000 sq ft your city must allow regardless of other rules.
- ✓5 ft setback. State law caps the side and rear setback at 5 ft, so even a tight lot usually works.
- ✓No extra parking likely required. No more than 1 space per ADU; no parking required within 1/2 mile of major transit.
- ✓No owner-occupancy requirement. You can rent out both the main home and the ADU — you do not have to live on the property.
- ✓Lot has room. A 700 sq ft ADU uses about 12% of your 6,000 sq ft lot — comfortably within typical coverage limits.
Rule basis: RCW 36.70A.681 (HB 1337, 2023)
If you rent it out
$1,175–$1,925
est. rent/mo
8.3%
gross yield
12.9 yrs
payback
~$17,298/yr after a 7% vacancy allowance, before financing & maintenance.
Financed over 20 yrs at 7.5%, the build is about $1,801/mo — vs ~$1,550/mo in estimated rent.
Estimates are for planning only and are based on regional construction-cost indices and published statewide ADU statutes. Local ordinances, lot conditions and contractor pricing vary — always confirm with your city planning department and a licensed contractor.
Cost by ADU type in Washington
| ADU type | 600 sq ft | $/sq ft |
|---|---|---|
| Detached new build | $145,000–$237,500 | $242–$396 |
| Attached addition | $132,000–$218,000 | $220–$363 |
| Garage conversion | $79,000–$145,000 | $132–$242 |
| Junior ADU (JADU) | $66,000–$119,000 | $110–$198 |
| Prefab / modular | $119,000–$198,000 | $198–$330 |
Financing your ADU
A handful of loan and grant programmes are built specifically for ADUs. These are informational links we don't currently earn a commission on — not paid placements.
RenoFi
Compare RenoFi ADU loan options →Renovation-specific loans (home equity loan, HELOC or cash-out refinance) that lend against your home's value after the ADU is built, so you can often borrow more than a standard HELOC allows.
Fannie Mae HomeStyle Renovation
See HomeStyle Renovation →A conventional mortgage that rolls ADU construction costs into a single loan, with down payments as low as 3% for qualifying first-time buyers.
CalHFA ADU Grant Program
Check CalHFA grant status →Reimburses up to $40,000 of pre-development costs (design, permits, soil tests) for California homeowners building an ADU.
Current round of funding is fully allocated — the page covers what to check for reopening.
Want financing options matched to your project?
Tell us roughly what you need to borrow and we'll point you to lenders who work with ADU projects.
ADU rules by city in Washington
Estimates are for planning only and are based on regional construction-cost indices and published statewide ADU statutes. Local ordinances, lot conditions and contractor pricing vary — always confirm with your city planning department and a licensed contractor.