Renton, WA Environmental Rules: Grading & Drainage (2026)

Renton Municipal Code 4-8 and 4-6-030 govern grading, filling, and drainage activities under the adopted King County Surface Water Design Manual. A grading permit is required for any project that moves more than 50 cubic yards of earth, cuts or fills exceeding 3 feet in depth, or disturbs land within a critical area or its buffer. Permit applications must include topographic surveys, cut-and-fill quantities, erosion control plans, and drainage calculations. Fills in the 100-year floodplain require compensatory storage to maintain flood capacity. Grading on steep slopes (over 15 percent and especially over 40 percent landslide hazard areas) requires geotechnical review. The Surface Water Design Manual requires that post-development peak flows match pre-development conditions for the 2- and 10-year storms, typically achieved via detention ponds, underground vaults, or rain gardens. Drainage must not be directed onto neighboring property or into the public right-of-way without approved tightline to the storm system. Violations including unpermitted filling can trigger stop-work orders, restoration requirements, and fines. Contact CED at (425) 430-7200 before any significant earthwork.