Washington's HB 1337: Statewide ADU Reform
Washington State passed one of the most direct and comprehensive ADU reform bills in the nation in 2023. House Bill 1337, signed into law on July 23, 2023, is specifically focused on accessory dwelling units — not broad zoning reform like Oregon's HB 2001, but targeted ADU legislation designed to maximize housing production on existing single-family lots statewide. HB 1337's core provisions are significant. It requires all Washington cities and counties planning under the Growth Management Act (GMA) to allow at least 2 ADUs per single-family residential lot by-right. This is more permissive than Oregon and California on this specific point — California allows 1 ADU plus 1 JADU (a California-specific subtype), while Washington explicitly allows 2 full ADUs on the same lot. HB 1337 prohibits local owner-occupancy requirements for ADUs — neither the primary dwelling nor the ADU may be required to be owner-occupied as a condition of approval. It prohibits local ordinances that cap ADU floor area below 1,000 square feet or 60 percent of the primary dwelling size — setting a meaningful minimum size allowance that prevents cities from limiting ADUs to unusably small units. HB 1337 removes off-street parking requirements for ADUs located within 0.25 miles of a major transit stop, a transit access threshold that covers large portions of urbanized Washington cities. It requires ministerial (administrative) ADU approval — no discretionary design review for ADUs meeting objective standards. Local jurisdictions had until July 1, 2024 to update their codes to comply with HB 1337. Most major Washington cities updated their codes on schedule. HB 1337 was paired with HB 1110 (2023), a companion bill that requires cities with 25,000 or more population to allow at least 4 units per lot in areas with frequent transit service and at least 2 units per lot in all other single-family zones — further expanding density allowances.
Seattle ADU Rules: A National Leader in ADU Policy
Seattle was already among the most ADU-permissive major cities in the United States before Washington's HB 1337 required statewide reform. In 2019, Seattle enacted a comprehensive ADU ordinance update that eliminated owner-occupancy requirements, increased maximum ADU size to 1,000 square feet, raised the maximum height for detached ADUs (backyard cottages) to 24 feet, allowed 2 ADUs per single-family lot, and eliminated all off-street parking requirements for ADUs citywide. By the time HB 1337 required these changes statewide in 2024, Seattle had been living with them for five years. Seattle regulates ADUs under Seattle Municipal Code 23.44.041. Detached ADUs — called Detached Accessory Dwelling Units (DADUs) or 'backyard cottages' — and Attached ADUs (AADUs) may each be up to 1,000 square feet. This 1,000-square-foot limit applies to both types, compared to California's 1,200-square-foot limit for detached ADUs. Seattle allows both one DADU and one AADU on the same single-family lot simultaneously — up to 2 ADUs total, consistent with HB 1337. Setbacks for DADUs are 5 feet from side and rear property lines per Seattle Municipal Code 23.44.041. DADUs benefit from a generous lot coverage arrangement — their footprint is not counted against the primary dwelling's lot coverage maximum, which effectively increases total buildable area on the lot. Maximum DADU height is 24 feet, supporting two-story backyard cottages. Seattle does not use the California JADU concept. Maximum AADU height follows the zone height standard. Seattle's transit system — Link Light Rail, bus rapid transit, and extensive frequent bus network — means parking waivers apply throughout most of the city.
What You Can Build in Washington
Washington homeowners in cities subject to HB 1337 have clear options for adding ADUs to their properties. The two-ADU-per-lot allowance is the most distinctive feature of Washington's approach compared to neighboring states. On a standard single-family lot in Seattle or any other Washington city covered by HB 1337, you may build: one Detached ADU (DADU, backyard cottage) up to 1,000 square feet, and simultaneously one Attached ADU (AADU) up to 1,000 square feet. The attached ADU could be a basement apartment with a separate entrance, a main-floor addition, a garage conversion with living space above, or an internal conversion of existing space. Both ADUs can be rented independently — neither the primary dwelling nor either ADU requires owner-occupancy. A property with a primary dwelling, a DADU, and an AADU effectively has three rental units. In high-demand markets like Seattle, Bellevue, Kirkland, and Redmond, this represents significant income potential. Size limits: Washington's HB 1337 prohibits cities from limiting ADUs below 1,000 square feet or 60 percent of the primary dwelling size. In Seattle, the local cap is 1,000 square feet for both DADUs and AADUs. In other Washington cities, the local cap may vary but cannot be below the HB 1337 minimums. Setbacks in Seattle are 5 feet from side and rear property lines for DADUs. Height: Seattle allows DADUs up to 24 feet, enabling two-story backyard cottages. Washington does not use the California JADU (Junior ADU) concept, which is specific to California law. For garage conversions in Seattle, the conversion qualifies as an AADU and is limited to 1,000 square feet. Converting an attached or detached garage to living space is one of the lower-cost ADU options since no foundation or framing work is needed for the exterior shell.
Permit Process and Costs in Seattle
Seattle ADU permits are processed by the Seattle Department of Construction and Inspections (SDCI). The permit process is ministerial — Seattle cannot exercise discretion for ADUs meeting objective standards. SDCI offers both a standard review track and a pre-approved standard plan program for select DADU designs, which can reduce review time significantly. Standard DADU permits take 4-8 weeks in the standard SDCI queue. Pre-approved standard plans — available for select DADU designs — reduce review time to approximately 1-3 weeks. Seattle's pre-approved plans cover common DADU footprints and configurations. If your design matches an approved template, this can save both time and architectural fees. Plan check fees in Seattle typically run $2,000 to $5,000 depending on project value. Building permit fees run $1,500 to $4,000. Total base permit fees typically range from $4,000 to $12,000 for a standard DADU or AADU project. In addition to base permit fees, Seattle charges school impact fees (SEPA mitigation) for new residential units including ADUs. Seattle School District No. 1 charges approximately $3,700 per attached ADU and $7,400 per detached ADU — though rates change annually and should be verified with SDCI before budgeting. This means total fees including school impact fees for a new DADU can reach $11,000 to $19,000. Compared to Portland's ADC total cost (base fees plus SDCs), Seattle's total permit costs are broadly similar for new detached ADUs, though the fee components differ (Seattle uses school impact fees; Portland uses system development charges). Construction costs in Seattle reflect the city's premium construction market. A DADU in Seattle typically runs $150,000 to $350,000 for construction. Garage conversions to AADU are less expensive, often $60,000 to $120,000. Total all-in costs (including design, permits, and construction) for a new DADU in Seattle range from $200,000 to $450,000.
How Washington Compares to Oregon
Washington and Oregon are neighboring states with similar politics and both have landmark ADU reform laws. Comparing them helps homeowners near the border and investors evaluating both markets understand the key differences. ADU quantity per lot: Washington's HB 1337 explicitly allows 2 ADUs per single-family lot (1 attached + 1 detached). Oregon's HB 2001 is primarily a zoning reform bill — Portland's local rules allow 1 ADU per standard single-family lot, though middle housing rules allow up to 4 units total. Washington is more permissive on this specific point. Maximum ADU size: Seattle allows 1,000 sqft for both DADUs and AADUs. Portland limits ADUs to 800 sqft. Washington's larger size allowance means more potential living space and rental value. Both states: neither Washington nor Oregon uses the California JADU concept. Setbacks: both Portland (5 ft side/rear) and Seattle (5 ft side/rear) have identical setback requirements for detached ADUs — both slightly more restrictive than California's 4-foot minimum. Owner-occupancy: both states prohibit owner-occupancy requirements under their respective laws. Parking: both cities (Portland and Seattle) have eliminated all off-street parking requirements for ADUs citywide, predating statewide mandates. Costs: Portland's total permit costs for new detached ADUs are elevated by System Development Charges ($10,000 to $25,000 in SDCs). Seattle charges school impact fees ($7,400 for DADUs). For a new detached ADU, total city fees are broadly comparable but Portland's SDCs can exceed Seattle's school impact fees. Approach: Washington's HB 1337 is more directly ADU-focused (sets size minimums, requires 2 per lot, removes parking mandates). Oregon's HB 2001 is broader housing reform (eliminates single-family zoning, requires middle housing). Both achieve high ADU permissiveness through different legal mechanisms. For homeowners in either state, the result is a regulatory environment where ADU approval is virtually guaranteed if your project meets objective standards.
Recent Changes and the Legislative Timeline
Washington's ADU policy history shows a state that moved progressively toward more permissive ADU rules before state law required it, then codified those advances through legislation. Seattle enacted its landmark 2019 ADU ordinance independently, eliminating owner-occupancy requirements, allowing 2 ADUs per lot, eliminating parking requirements, and increasing size and height limits — all before HB 1337 required these changes statewide. Seattle was demonstrating proof of concept for what eventually became state law. HB 1337 (2023): signed into law July 23, 2023, effective immediately, with local code compliance required by July 1, 2024. This bill required all Washington GMA-planning jurisdictions to allow 2 ADUs per lot by right, prohibit owner-occupancy requirements, allow ADUs up to 1,000 sqft or 60% of primary dwelling, and provide ministerial approval. It was a direct codification of best practices Seattle and other progressive Washington cities had already adopted. HB 1110 (2023): companion bill requiring cities with 25,000 or more population to allow at least 4 units per lot in areas with frequent transit service and at least 2 units per lot in all other single-family zones. This is the 'missing middle housing' expansion that parallels Oregon's HB 2001. HB 1110 goes further than HB 1337 for cities meeting the population threshold, though the focus is on multiplex housing rather than specifically ADUs. Washington does not yet have a statewide residential lot split law comparable to California's SB 9 or Oregon's SB 458. Lot splits in Washington are governed by local subdivision codes. Some Washington cities allow lot splits that could enable separate sale of ADU units, but there is no statewide mandate. The Washington Legislature may revisit this gap in future sessions. PropertyZoned updates state guide data when significant new laws pass.
Getting Started with a Washington ADU
Planning an ADU in Washington is straightforward under HB 1337's permissive framework. Here is the recommended sequence for Washington homeowners. Step 1: Confirm your city is covered by HB 1337. All Washington cities and counties planning under the Growth Management Act must comply. This covers virtually all incorporated cities in Washington. Rural unincorporated areas may have different standards — confirm with your county planning department. Step 2: Look up your city's local ADU rules. While HB 1337 sets a floor, local cities may have specific standards for setbacks, design compatibility, and other objective criteria. In Seattle, consult SDCI and the Seattle Municipal Code 23.44.041. Other Washington cities have their own local codes. Step 3: Use PropertyZoned's ADU Feasibility Checker. The tool incorporates Seattle's specific rules — 1,000 sqft max, 5 ft setbacks, 24 ft maximum height — to give you a preliminary feasibility assessment based on your lot size and zone. Step 4: Decide on DADU or AADU (or both). Under HB 1337, you may build one of each. If your budget allows only one, consider your lot configuration, existing structures available for conversion, and rental income goals. DADUs (backyard cottages) typically command higher rents due to privacy and separate entrance. AADUs (basement apartments, garage conversions) are typically less expensive to construct. Step 5: Budget for school impact fees in Seattle. Add $7,400 for a new DADU or $3,700 for a new AADU to your permit cost estimate when planning in Seattle. Verify current rates with SDCI before finalizing your budget. Step 6: Hire an SDCI-familiar architect and apply for permits. Seattle SDCI offers pre-approved standard plan templates that can reduce design fees and review time. Ask your architect about standard plan eligibility before commissioning custom drawings. Washington's ADU landscape is among the most permissive in the United States. Two ADUs per lot, no owner-occupancy requirement, generous size allowances, and ministerial approval create an excellent environment for homeowners and investors alike.
Frequently Asked Questions
Can I build 2 ADUs on my Washington State property?
Yes. Washington HB 1337 requires all GMA-planning cities to allow at least 2 ADUs per single-family lot by-right — one attached ADU and one detached ADU. In Seattle, this means you can have a backyard cottage (DADU) and a basement apartment (AADU) on the same single-family lot, both rentable without owner-occupancy requirements.
What is the maximum ADU size in Seattle?
Seattle allows both attached ADUs (AADUs) and detached ADUs (DADUs, backyard cottages) up to 1,000 square feet each under Seattle Municipal Code 23.44.041. Washington HB 1337 prohibits cities from limiting ADUs below 1,000 sqft or 60% of the primary dwelling size.
Are there school impact fees for ADUs in Seattle?
Yes. Seattle School District No. 1 charges school impact fees for new residential units including ADUs. Current rates are approximately $7,400 per new detached ADU (DADU) and $3,700 per new attached ADU (AADU). Verify current rates with SDCI before budgeting, as they are updated periodically.
Does Washington have a lot split law like California's SB 9?
No. Washington does not have a statewide residential lot split law equivalent to California SB 9 or Oregon SB 458. Lot splits in Washington are governed by local subdivision codes. HB 1110 (2023) achieves density goals through multiplex housing allowances rather than lot splits. Some Washington cities may allow lot splits under local codes — check with your city planning department.