King County Post-Construction Soil Standard

Background

King County's post-construction soil standard is part of clearing and grading regulations under KC Code, Chapter 16.82. Requirements apply to site development activities in unincorporated King County.

These regulations help avoid costly environmental and landscape problems by restoring healthy soils. Permit holders must take measures to preserve the soil’s ability to hold moisture. This helps better manage stormwater and reduces the harmful effects of stormwater runoff.

How to meet code requirements

Download the Achieving the Post-construction Soil Standard brochure for detailed instructions.

These additional resources are required for your permit application:
Download theSoil Management Plan form
Use the online Compost and topsoil calculator

Code requirements

Permit holders must restore the soil moisture holding capacity to areas they cleared and graded. The capacity needs to be the same as the original undisturbed soil native to the site. Areas covered by impervious surface or incorporated into a stormwater facility are exempt.

Soil amendments must take place when soils are typically driest between May and October 1st. Replaced topsoil must meet these requirements:

• Minimum of 8 inches thick
• Contains organic matter content of 5% dry weight for turf areas and 10% dry weight for planting beds
• Have a pH suitable for the proposed landscape plants

We recommend tilling or plowing compacted subsoils before placing amendments or topsoil. After planting, mulch your planting beds with two inches of forest duff, ground bark, wood chips or other organic material.

The requirements don’t apply to surface mine operations.

Additional compost resources

• Compost made with Loop®
• Northwest Natural Yard and Garden
• Soils for Salmon