• Use the drainage worksheet to look up information about your property and answer the questions. You'll also need information from the Impervious Surface Worksheet. Links to records lookup and instructions are on the sheet.
• Allow yourself a little time.
• If you can't find the information or need help, contact us at DevRev_Tech@co.thurston.wa.us

Option 1 No Plan Needed

Your project may be exempt from drainage plan requirements if all the following criteria are met:

1. Rural Area property located outside the county's Municipal Stormwater Permit (NPDES) Boundary area.
2. Less than 5% total impervious surface on the property including new and existing.
3. Grading less than 5,000 cubic yards.
4. Less than 10% of the total property is disturbed by new and existing development.
5. No increase in runoff to neighboring property. (Your site if very flat, your disturbed area is sufficiently away from neighboring property lines, or you have very gravelly soils.)

Option 2 Professionally Designed Plan Needed 

If your completed drainage worksheet is marked YES for even one of the conditions below, your project will need a drainage plan developed by a licensed professional stormwater drainage designer or engineer

• • If any part of your project is within 200 feet of a Critical Areas.
  • If any part of your project is within 200 feet of a Marine Bluff Hazard Area.
  • Lots less than 1-acre and no A/B soils, or
  • Lots sloped greater than 10%, or
  • Lots with greater than 15% slope with 10 feet or greater of height difference, or
  • Historic drainage problems in the area.

Option 3 Abbreviated Plan Needed (Abbreviated Plan example )

If one of these cases and all conditions within that case apply, your project can be submitted with a simpler drainage plan, not drawn by a licensed engineer.

• Case 1: Single Lot
  • Less than 2,000 square feet (sf) of new impervious surface.
  • Less than 7,000 sf of disturbed area created by the total project, including clearing, buildings, well, septic, utilities, yard, garden, driveways, and construction materials staging areas.
    
     
• Case 2
  • 85% of site is A/B soils.
  • Less than 5,000 sf of new impervious surface.
  • Less than three-quarters of an acre converted to lawn or landscape.
  • Less than 2.5 acres converted to pasture.
    
     
• Case 3: Subdivision with Stormwater System
  • The project property is in a development that already has an accepted and functioning stormwater system. 
    
     
• Case 4: Outside Thurston County regulated stormwater area
  • The property is located outside the county's Municipal Stormwater Permit (NPDES) Boundary area.
  • Parcel greater than 1 acre.
  • 85% of site is A/B soils.
  • Total impervious surface of less than 10% on the entire property including proposed new and existing development.
  • Non-native pervious surfaces like yards or garden, PLUS impervious surfaces like roofs and concrete make up less than 35% of the entire property.
    
    ​​​​​​​
• Case 5
  • Rural Area property located outside the county's Municipal Stormwater Permit (NPDES) Boundary area.
  • Parcel greater than 2.5 acres.
  • Total impervious surface of less than 10% on the entire property including proposed new and existing development.
  • Non-native pervious surfaces like yards or garden, PLUS impervious surfaces like roofs and concrete make up less than 35% of the entire property.
  • No increase in runoff to neighboring property. 

• Submit your completed drainage worksheet with your application packet even if a drainage plan is not required for your property.
• County staff will review your worksheet and may ask for more information or find other information that may affect the required plan determination. Staff will explain and discuss any alternate findings with you if that should occur.