Mirrormont Environmental Rules: Grading & Drainage Rules (2026)

King County regulates grading and drainage to protect property, water quality, and salmon habitat. KCC 16.82 requires a clearing and grading permit for any grading moving more than 50 cubic yards of earth, clearing over 7,000 square feet, or any activity on or affecting critical areas regardless of size. Permit applications require a site plan showing existing and proposed grades, a drainage analysis, erosion and sediment control plan, and for larger projects a Technical Information Report under the King County Surface Water Design Manual. Steep slope hazard areas (slopes over 40 percent) and landslide hazard areas trigger geotechnical engineering review — a Washington-licensed geotechnical engineer must prepare a report assessing stability and recommending construction methods. Drainage design must meet the following principles: runoff from new impervious surfaces must be controlled to match pre-developed forested conditions (flow control) to prevent downstream erosion; water quality treatment is required for pollution-generating surfaces; discharge points must not adversely affect neighboring properties or natural drainage patterns. Washington's common-enemy doctrine traditionally allowed property owners to discharge natural surface water onto neighbors, but modern case law (Phillips v. King County, 1998) requires reasonable methods and limits liability for concentrated or artificial drainage causing damage. Neighbors affected by grading or drainage can file civil claims for trespass, nuisance, or negligence. Septic system drainfields require separate approval from Seattle-King County Public Health. Cuts and fills on steep slopes face height restrictions and engineering requirements.