Homes with ADU · Washington

Buying a home with an ADU in Washington

Washington — and especially Seattle — has one of the most active DADU (detached accessory dwelling unit) markets outside California. Recent state legislation (HB 1337, 2023) preempted local ADU bans across the state, so most jurisdictions now allow ADUs by-right. Seattle's DADU program has been running long enough that mature ADU stock exists, and a typical home with a permitted DADU sells at an 18–28% premium over a comparable single-unit lot.

Washington state-level rules buyers should know

• HB 1337 (2023) preempted local ADU bans statewide — most jurisdictions now allow ADUs by-right.
• Seattle DADU program: up to 2 ADUs per lot allowed since 2019 (DADU + attached).
• Maximum size: typically 1,000 sq ft per ADU in Seattle; varies by city.
• Owner-occupancy: not required at the state level; some cities have legacy rules.
• Parking: state law preempted most parking requirements for ADUs near transit.

What to check when buying

✓ Verify Seattle DADU permit if applicable — Seattle has its own portal separate from the general building-permit system.
✓ Confirm setbacks meet local zoning — many older lots are nonconforming and trigger discretionary review.
✓ Check for sound-mitigation if the unit is over a garage — Seattle's wet climate stresses these connections.
✓ Pull rental-registration records — Seattle requires landlord registration for rental units.
✓ Verify the unit's height — Seattle's height limit is typically 25 ft.

Permit info for Washington cities

The strongest signal of a legitimate ADU is a clean permit history at the city. Use these jurisdictional guides to cross-check what the seller's disclosure tells you.

Permits in Seattle

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Buying a home with an ADU in Washington

Washington state-level rules buyers should know

• HB 1337 (2023) preempted local ADU bans statewide — most jurisdictions now allow ADUs by-right.

• Seattle DADU program: up to 2 ADUs per lot allowed since 2019 (DADU + attached).

• Maximum size: typically 1,000 sq ft per ADU in Seattle; varies by city.

• Owner-occupancy: not required at the state level; some cities have legacy rules.

• Parking: state law preempted most parking requirements for ADUs near transit.

What to check when buying

✓ Verify Seattle DADU permit if applicable — Seattle has its own portal separate from the general building-permit system.

✓ Confirm setbacks meet local zoning — many older lots are nonconforming and trigger discretionary review.

✓ Check for sound-mitigation if the unit is over a garage — Seattle's wet climate stresses these connections.

✓ Pull rental-registration records — Seattle requires landlord registration for rental units.

✓ Verify the unit's height — Seattle's height limit is typically 25 ft.

Buying a home with an ADU in Washington

Washington state-level rules buyers should know

What to check when buying

Permit info for Washington cities

Related

Buying a home with an ADU in Washington

Washington state-level rules buyers should know

What to check when buying

Permit info for Washington cities

Related