Seattle, WA Landscaping Rules: Rainwater Harvesting (2026)
Key Facts
- Governing section
- SMC 22.805.070, Stormwater Code
- Category 2 sizing rule
- Must meet on-site performance standard, § .070.C
- Category 4 sizing rule
- Cuts annual runoff volume by 25% or more
- BMP status
- Listed as 'X': allowed but not required
- Applies to
- Single-family and other project types via Tables A-D
Summary
Seattle's Stormwater Code lets rainwater harvesting count as an approved on-site BMP for single-family and other projects under SMC 22.805.070, but only if it's sized correctly: Category 2 systems must meet the chapter's on-site performance standard, and Category 4 systems must cut annual runoff volume by at least 25 percent. It's optional, not mandatory.
Table A for 22.805.070 On-site List for Single-family Residential Projects Category BMPs All Discharge Locations 1 Full Dispersion R, S Infiltration Trenches R, S d Drywells R, S d 2 Rain Gardens a R, S Infiltrating Bioretention R, S Rainwater Harvesting—Category 2 Sizing X b Permeable Pavement Facilities R, S Permeable Pavement Surfaces S Sidewalk/Trail Compost-Amended Strip S 3 Sheet Flow Dispersion R, S Concentrated Flow Dispersion S Splashblock Downspout Dispersion R Trench Downspout Dispersion R 4 Non-infiltrating Bioretention R, S Rainwater Harvesting—Category 4 Sizing X c Vegetated Roofs X 5 Single-family Residential Cisterns R Perforated Stub-out Connections R Trees S ... b Category 2 rainwater harvesting shall be sized to meet the on-site performance standard, subsection 22.805.070.C. c Category 4 rainwater harvesting shall be sized to reduce the runoff volume by 25 percent or more on an annual average basis.
Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: Municipal Code: Supplement 44 Update 1).
Full Breakdown
060, and it lists rainwater harvesting among the approved best management practices (BMPs) a property owner can use to manage roof and hard-surface runoff instead of, or alongside, infiltration or dispersion systems. Under subsection D, applicants work through tiered BMP category tables, Table A for single-family residential projects lists Full Dispersion, Infiltration Trenches, and Drywells in Category 1; Rain Gardens, Infiltrating Bioretention, and 'Rainwater Harvesting, Category 2 Sizing' in Category 2; several dispersion methods in Category 3; Non-infiltrating Bioretention, 'Rainwater Harvesting, Category 4 Sizing,' and Vegetated Roofs in Category 4; and Single-family Residential Cisterns, Perforated Stub-out Connections, and Trees in Category 5.
Applicants must evaluate every feasible BMP in the first category for a given surface before moving to the next; once one BMP is chosen for a surface, no other is required. C, matching pre-developed forested or pasture discharge durations depending on hard-surface coverage and receiving water, while Category 4 rainwater harvesting must reduce runoff volume by 25 percent or more on an annual average basis. 60A).
Violations & Fines
Rainwater harvesting itself isn't separately penalized, but an on-site BMP, including a harvesting system, that's installed undersized or not maintained puts the site out of compliance with the Stormwater Code's on-site performance standard, exposing the responsible party to the same SMC 22.808.050 enforcement track as any other drainage violation: a civil penalty of up to $5,000 per day until the system is brought into compliance.
Frequently Asked Questions
Does Seattle require rainwater harvesting systems on new homes?
How big does a rainwater harvesting system have to be to count?
Which Seattle office administers these BMP sizing rules?
Sources & Official References
Other rules in Seattle
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Rainwater Harvesting in Nearby Cities
How other cities in King County handle rainwater harvesting.