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.
|0 to 1,999 square feet of new and/or replaced hard surface||Requires applicants to provide a small parcel SWPPP||Required SWPPP Form|
|2,000 to 4,999 square feet of new and/or replaced hard surface||Targeted Storm Water Plan (MR 1-5) includes report & plan||Targeted Drainage Checklist|
|5,000 plus square feet of new and/or replaced hard surface||Full Drainage Plan Requirements (report & plan)|
Report by WA State licensed engineer
|Full Drainage Checklist (PDF)|
|Some properties are vested to previous drainage code requirements||Visit 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.