Mason County, WA Environmental Rules: Erosion Control (2026)

Mason County Code Chapter 14.48, the county's stormwater and drainage ordinance, requires every construction project in unincorporated Mason County that adds at least 2,000 square feet of new impervious surface, or disturbs land, to control erosion during and after work under MCC 14.48.140. Projects under one acre of disturbance qualify as "small parcel" work and must satisfy five numbered requirements. Requirement #1 limits construction vehicle access to a single stabilized route so mud is not tracked onto county roads, and any sediment that reaches pavement must be shoveled or swept clean immediately and again at day's end; hosing it away is allowed only after the sediment itself is removed.

Requirement #2 sets a calendar-based soil exposure limit: bare, worked ground cannot sit uncovered for more than two days between October 1 and April 30, the wet season, or more than seven days from May 1 through September 30, and it must be stabilized with sod, mulch, plastic sheeting or a gravel base in the meantime. Requirement #3 requires vegetative buffers, silt fencing, dikes or filters to keep sediment off neighboring parcels, and Requirement #4 makes the property owner responsible for inspecting and maintaining every BMP for as long as it functions.

A small parcel erosion and sediment control plan documenting these measures must be submitted to Mason County Community Development and, per MCC 14.48.150, is required for any single-family residence or duplex adding 2,000 square feet or more of impermeable surface. Nonresidential work of 5,000 square feet or more, or disturbance of a full acre, escalates the requirement to a large parcel ESC plan and a permanent stormwater quality control (PSQC) plan stamped by a Washington-licensed civil engineer. Mason County Public Works can provide guidance on the accompanying drainage plan or sketch when one is needed.