Drainage Plan Requirements

Drainage requirements have become more complicated in recent years due to more restrictive federal and state requirements. National Pollutant Discharge Elimination System (NPDES) standards require applicants to address drainage and storm water runoff from new development projects. The regulations are based upon an established set of thresholds identified in county code which meet the federal and state mandates. These mandates require applicants to provide additional information based on specific project thresholds. Applicants may need to provide a simple small parcel Storm Water Pollution Prevention Plan (SWPPP) or a more complex engineered drainage plan. 


For more detailed information, please see the Land Disturbing Activities page.

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Drainage ThresholdsRequirementsForms/Checklists
0 to 1,999 square feet of new and/or replaced hard surfaceRequires applicants to provide a small project SWPPPRequired SWPPP Form
2,000 to 4,999 square feet of new and/or replaced hard surfaceTargeted Storm Water Plan (MR 1-5) includes report & planTargeted Drainage Checklist
5,000 plus square feet of new and/or replaced hard surfaceFull Drainage Plan Requirements (report & plan)
Report by WA State licensed engineer
Full Drainage Checklist (PDF)
Some properties are vested to previous drainage code requirementsVisit one of our permit assistance guides in our office or submit an Ask Permit Tech question to verify and identify vesting date.Depends on code requirements on vesting date.

Hard surfaces include impervious surface, permeable pavement or vegetated roof. Impervious surface are surfaces that can’t absorb water and increase runoff, roads, driveways, sidewalks, rooftops, parking areas are considered impervious surfaces. Impenetrable materials such as asphalt, concrete, brick and stone including soils compacted by development are also highly impervious.

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