Washington · ADU Eligibility Checker

Washington ADU Eligibility Checker

Washington passed HB 1337 in 2023, forcing every GMA-planning city to allow at least two ADUs per residential lot with no impact fees, no off-street parking mandate, and no owner-occupancy requirement. For most Washington residential lots, eligibility is effectively guaranteed by state law — the useful work is confirming your specific lot's constraints and your HOA's (lack of) ability to block.

Statewide ADU mandate
Yes (HB 1337)
ADUs per lot (minimum)
2
Owner-occupancy required
No

ADU Eligibility Checker

Find out if you can build an ADU on your property. Enter your details below for an instant assessment based on state and local regulations.

Property Details

6,000 sq ft
1,000 sq ft50,000 sq ft

Eligibility Assessment

Washington · 6,000 sq ft lot · Single-Family zoning

Likely Eligible

Your property appears to meet state-level ADU requirements

Confidence:
medium — state-level data

Allowed ADU Types

Detached ADU
Attached ADU
Garage Conversion

Maximum ADU size: 1,000 sq ft

Key Rules & Restrictions

Maximum ADU size: 1,000 sq ft (or 50% of lot, whichever is less)

Washington ADU Legislation

Key legislation: HB 1337 (2023)

  • •HB 1337 requires cities planning under GMA to allow at least 2 ADUs per lot
  • •No owner-occupancy, off-street parking, or impact fee requirements for ADUs
  • •HOAs may not prohibit ADUs as of July 2024
  • •Cities over 20,000 population must allow ADUs

Recommended Next Steps

1Verify eligibility with your local planning department — state rules are a floor, not a ceiling
2Get a preliminary site assessment from a licensed contractor
3Use our Cost Estimator to budget your project
4Explore financing options with our Financing Calculator

Important: This is general guidance based on state law. Local ordinances, overlay zones, and specific property conditions may affect eligibility. Always verify with your local planning department before starting any ADU project.

EstimateBuild CostCompareFinancing OptionsEstimateRental Income

How Washington ADU Eligibility Works

HB 1337, signed in 2023 and in effect for most cities by mid-2024, established a statewide floor for ADU rights in every city planning under the Growth Management Act (GMA). Specifically, GMA cities must allow at least two ADUs per residential lot, cannot impose owner-occupancy requirements, cannot require off-street parking for ADUs within one-half mile of a major transit stop, cannot charge impact fees, and cannot require a minimum lot size. For most practical purposes this means a single-family lot in Seattle, Tacoma, Spokane, Bellingham, Olympia, or any other GMA city qualifies to host one detached ADU and one attached ADU (or JADU equivalent) as a matter of right. The calculator marks Washington green for most property configurations.

Where Yellow and Red Lights Appear

Washington still has a small list of situations where eligibility is not automatic. First, HB 1337 applies to cities planning under GMA — about 95% of Washington's urban population but only a subset of rural unincorporated county land. If your lot is in unincorporated King, Pierce, or Snohomish county, the GMA rules may not translate cleanly; check with the county planner. Second, properties in critical areas (wetland buffers, steep-slope landslide hazards, floodways) require environmental review that the ADU permit alone does not short-circuit. Third, shoreline master program jurisdiction (properties within 200 feet of a shoreline of state significance) layers an additional permit path on top. Fourth, Seattle's Environmentally Critical Areas overlay imposes stricter setbacks and tree-retention rules on qualifying lots. None of these block the ADU, but they do require more planning.

HOA Rights Under Washington Law

Washington's position on HOAs is cleaner than most states: HB 1110 (2023) and HB 1337 together establish that HOAs in GMA jurisdictions may not prohibit ADUs in ways that the underlying zoning allows. CC&R restrictions predating these laws are generally unenforceable for the purpose of blocking a compliant ADU. HOAs retain the ability to review exterior appearance (paint, siding, roof material) and require reasonable architectural compatibility, but they cannot impose design requirements so onerous that they effectively prohibit construction. If your HOA is actively resisting, the state's position is strong and the Attorney General's office has supported homeowners challenging CC&R-based ADU denials. That said, practical litigation against a recalcitrant HOA is expensive and slow, so many homeowners end up negotiating cosmetic compromises.

Disclaimer: Estimates on this page are based on state-level data and do not replace consultation with your local planning department, licensed contractor, or tax advisor. Verify rules and costs with local sources before starting any project.

Frequently Asked Questions

I'm in unincorporated Snohomish County. Does HB 1337 apply to me?
HB 1337 applies to cities and urban-unincorporated areas planning under the Growth Management Act. Snohomish County's urban growth areas do fall under GMA, so HB 1337 rules likely apply to your parcel, but it depends on the specific UGA designation and the county's implementation. Call the county planner with your parcel number; they'll tell you in five minutes whether you're in the GMA-covered portion.
Can Seattle still require me to replace a parking space if I convert my garage?
No — HB 1337 prohibits parking-replacement requirements for ADU conversions and for new ADUs within one-half mile of a major transit stop. Most of Seattle is within a half mile of a frequent bus or light rail stop, so this exemption covers the vast majority of lots. The city's older ADU rules that required off-street parking replacement have been superseded.
My lot is only 3,200 square feet. Can I still build a detached ADU?
Probably yes. HB 1337 prohibits minimum lot size requirements for ADUs. The practical constraint then becomes setbacks, lot coverage, and buildable envelope rather than a hard lot-size floor. On a 3,200-square-foot lot, you're typically looking at a smaller detached ADU (say, 400–600 square feet) or an attached ADU built into the primary structure. The calculator's eligibility logic factors this in.
Do I need to live in one of the units?
No. HB 1337 expressly prohibits owner-occupancy requirements for ADUs in GMA-planning cities. You can rent both the primary dwelling and the ADU and live elsewhere. This is a clear improvement over some pre-2023 local ordinances that required owner-occupancy and effectively discouraged ADUs from being built for rental investment.
What about short-term rentals like Airbnb?
HB 1337 does not require cities to allow short-term rentals in ADUs. Many Washington cities — including Seattle, Tacoma, and Spokane — regulate short-term rentals separately from the ADU itself, usually with caps on the number of STR licenses per owner and a primary-residence requirement for non-exempt STRs. If short-term rental income is central to your ROI math, verify the STR rules in your specific city before assuming Airbnb is the default path.

Other Washington ADU Calculators

Cost Estimator — Washington → Financing Calculator — Washington → Rental Income Calculator — Washington → Break-Even Calculator — Washington → Property Value Impact — Washington → ROI Calculator — Washington → Permit Fee Calculator — Washington →

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