King County, WA Permit Requirements: Deck & Patio Permits (2026)

Under the Washington State Building Code as adopted by King County, decks are exempt from building permits if they meet all of the following: floor area does not exceed 200 square feet, the deck is not more than 30 inches above grade at any point, and the deck does not serve a required exit door. Decks exceeding any of these thresholds require a building permit from King County's Department of Local Services, with plans showing structural details, footings, guard rails (required for decks over 30 inches above grade — minimum 36 inches tall per Washington code), and setback compliance. Attached decks that connect structurally to the primary dwelling typically require permits regardless of size due to the structural connection. Concrete slab patios at grade level generally do not require building permits. Covered patios and pergolas may require permits depending on size and attachment method. In flood hazard areas (mapped by FEMA), decks may need to be constructed to flood-resistant standards and elevated above the base flood elevation. King County's critical areas regulations (KCC 21A.24) may affect deck construction near wetlands, streams, and steep slopes — any development in critical area buffers requires additional review and potentially a critical areas alteration permit. The Pacific Northwest's rain demands proper flashing, drainage, and rot-resistant materials (cedar or pressure-treated lumber).