Seattle, WA Landscaping Rules: Rainwater Harvesting (2026)

Seattle regulates harvested rainwater through its Stormwater Code, not through a standalone cistern permit. Section 22.805.070 sets the minimum requirements for on-site stormwater management, and Table A for 22.805.070 is the on-site list for parcel-based projects, meaning building lots as opposed to trail, sidewalk, and roadway projects, which are sent to Tables B and C. Rainwater harvesting appears in Table A twice, as Category 1 sizing and Category 4 sizing, and a Residential Cistern line sits in Category 4.

The list only applies once a project reaches a trigger in Section 22.805.050.B. On a lot created or altered by a plat recorded with the King County Recorder on or after January 1, 2016, the trigger is 750 square feet or more of new plus replaced hard surface, or 7,000 square feet or more of land disturbing activity. For any other lot, the trigger is 1,500 square feet of hard surface or 7,000 square feet of land disturbance. The code defines hard surface as an impervious surface, a permeable pavement, or a vegetated roof.

Table A works category by category. For each roof or other hard surface, the designer considers every applicable BMP in the first category and uses any that is feasible. If none is feasible, the designer moves to the next category, and once one BMP is used for a surface, no other BMP is necessary for it. Category 1 rainwater harvesting therefore competes with full dispersion, infiltration trenches, and drywells, while Category 4 harvesting is only reached after Categories 1 through 3 fail. Feasibility is judged against the design criteria and infeasibility criteria in the Director's rules, and the Director can supersede or reduce the list for competing needs such as historic preservation, the Americans with Disabilities Act, FAA airport requirements, Superfund or the Model Toxics Control Act, special zoning districts, and public health and safety.

The two columns separate projects by where the water goes. The first column covers discharges to a receiving water not designated by Section 22.801.050, a public combined sewer, or a capacity-constrained system. The second covers discharges to a designated receiving water or its basin, a list that names the Duwamish River, Puget Sound, Lake Washington, Lake Union, Elliott Bay, Portage Bay, Union Bay, and the Lake Washington Ship Canal. Category 1 rainwater harvesting and the Residential Cistern are marked X in both columns: evaluation is not required but is allowed. Category 4 harvesting is marked R in the first column, so all roof runoff must be evaluated unless the project has less than 20,000 square feet of new plus replaced rooftop surface, and X in the second column. Category 1 systems are sized to meet the on-site performance standard in subsection 22.805.070.C, and Category 4 systems are sized to cut runoff volume by 25 percent or more on an annual average basis.

The 2026 amendments by Ordinance 127444 govern project applications submitted on or after July 1, 2026.