Drainage Thresholds | Requirements | Forms/Checklists |
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0 to 1,999 square feet of new and/or replaced hard surface | Requires applicants to provide a small project 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.