Accessory Dwelling Unit (ADU) Rules for Mount Vernon, WA

ADU State Law

What can you build in Washington?

• –ADUs are allowed by right, including attached, internal, and detached ADUs.
• –Local governments must allow up to two ADUs per lot.
• –Maximum ADU size is 1,000 sq ft for both attached and detached units.
• –ADU height is limited to 24 ft or the height of the principal dwelling, whichever is less.
• –Setbacks must use the less restrictive of accessory structure or principal dwelling standards.
• –Lot coverage and yard coverage rules may not be more restrictive than those applied to the principal home.
• –Parking requirements are capped, and parking cannot be required within ½ mile of major transit.
• –Owner occupancy may not be required for ADUs.
• –ADUs may be rented long-term or sold separately; local short-term rental rules may still apply.
• –Local governments may not regulate ADUs more restrictively than the principal unit. This includes setbacks, yard coverage, lot coverage, and height.
• –Cities and counties may be more permissive, but not more restrictive, than Washington's state ADU standards.
• –Some zoning rules still come from local code. Standards like FAR, bulk limits, and general lot coverage continue to apply as long as they do not prevent the lot from meeting the state-required ADU entitlements (e.g., allowing two ADUs with the required dimensions).
• –Washington's statewide ADU mandates apply only within Urban Growth Areas.
• –Outside UGAs, local zoning controls whether ADUs are allowed and under what conditions.
• –Additional constraints often apply outside UGAs, including septic capacity, fire access, water availability, and environmental regulations.