King County, WA Landscaping Rules: Rainwater Harvesting (2026)

Washington Department of Ecology issued a 2009 interpretive policy confirming that rooftop rainwater harvesting for on-site beneficial use does not require a water right permit and is not restricted under state law. This makes Washington one of the most rainwater-friendly states. King County actively promotes rainwater capture as a stormwater best management practice because Puget Sound receives high annual rainfall and salmon-stream peak flows benefit from slowed runoff. Residents can install rain barrels, cisterns, and underground storage tanks to capture rooftop runoff for landscape irrigation, toilet flushing, or (with appropriate treatment) indoor potable uses under the Washington Plumbing Code and DOH rules. Rain gardens and bioretention are encouraged under the 2019 Stormwater Design Manual for single-family lots. Large cisterns over 5,000 gallons or systems supplying potable indoor fixtures may require plumbing and building permits through the Permitting Division. The RainWise Program (a partnership between King County, Seattle Public Utilities, and participating cities) offers rebates for rain gardens and cisterns in targeted CSO-overflow basins.