King County Post-Construction Soil Standard
King County's clearing and grading regulations include a post-construction soil standard for site development activities in unincorporated King County (KC Code, Chapter 16.82). In effect since January 1, 2005, the soil standard is meant to preserve and restore healthy soils to better manage stormwater and reduce stormwater runoff and its negative effects. These regulations help prevent costly environmental and landscape problems by requiring permit holders to preserve topsoil, restore soils by adding compost after construction, or implement other measures to maintain the soil's moisture holding capacity.
Except for areas that will be covered by impervious surface or have been incorporated into a stormwater facility, areas that have been cleared and graded must have the soil moisture holding capacity restored to that of the original undisturbed soil native to the site to the maximum extent practicable.
Regulations require that soil amendments take place between May 1 and October 1, when soils are typically driest. Replaced topsoil must be a minimum of eight inches thick and have an organic matter content of 5 and 10 percent dry weight for turf areas and planting beds, respectively and a pH suitable for the proposed landscape plants.
It is recommended that compacted subsoils be tilled or plowed before placing amendments or topsoil and that planting beds be mulched with two inches of forest duff, ground bark, wood chips or other organic material after planting.
Surface mine operations are exempt from these requirements.
Soil management options
There are four soil management options permit holders can use to meet the soil standard. Option 1 applies to areas where the soil will not be disturbed or compacted during construction activities. Choose Option 2, 3 and/or 4 to restore soil quality in areas where grading and soil disturbance are unavoidable:
A pre-approved amendment rate of 1.75 inches of compost or a custom amendment rate for turf areas and 3 inches for planting beds can be used for the options calling for soil amendment.
Learn more about the King County soil standard in the booklet, Achieving the Post-construction Soil Standard (PDF, 830 K). The booklet includes links to additional resources including the Online Compost and Topsoil calculator, which should be used in conjunction with the booklet and a Soil Management Plan form (PDF, 950 K) required by DDES. Other resources include:
For questions about the soil standard or how to implement it, contact:
Steve Bottheim, Environmental Scientist
Karen May, Project Manager
Updated: Sep. 29, 2011