Prior to adoption of the current stormwater codes, residential development, agricultural development (regardless of size), and projects which added less than 5,000 sq. ft. of impervious surface were exempt from any stormwater review in unincorporated Whatcom County. To capture stormwater impacts in the rural areas, all projects within unincorporated Whatcom County which are outside of a NPDES Phase II Area, and Birch Bay, Drayton Harbor, Lake Whatcom, Lake Samish, and Lake Padden watersheds are separated in to three categories; Low, Medium, and High Land Use Intensity (These are different than the land use intensity designations within the Critical Areas Ordinance). Whether a project is considered Low, Medium or High Intensity will determine the level of stormwater management that is required.

The revised stormwater codes are designed to capture stormwater impacts from large projects in the rural areas.  It should be noted that Projects that result in new plus replaced hard surface greater than or equal to 10% of the gross parcel size or 20,000 sq. ft., whichever is greater, or converts 1.5 acres of vegetation to lawn or five acres of vegetation to pasture, are considered a High Intensity Land Use and are subject to all thresholds and requirements of the 2012 Washington State Department of Ecology Stormwater Management Manual For Western Washington.

When determining whether your project is considered Low, Medium, or High Intensity, please refer to the following lists and subsequent stormwater requirements:

The following land uses are considered Low Intensity development for the purposes of stormwater review:

• Single family residences and accessory buildings on parcels larger than 25,000 sq. ft.,
• Construction of agricultural buildings on agricultural land,
• Seasonal roadside stands,
• Roads (other than those exempt as pavement maintenance)

For all Low Intensity development (as listed above), a stormwater site plan will be required for projects which propose:

•  7,000 sq. ft. or greater of new or replaced hard surface or,
• 14,000 sq. ft. or more of land disturbing activity
• The stormwater site plan will show where stormwater from proposed hard surfaces is being directed.

The stormwater site plan shall show how stormwater from the proposed development is to be directed along with the locations of any proposed stormwater infrastructure.

The following land uses are considered Medium Intensity development for the purposes of stormwater review :

• Single family residences and accessory buildings on parcels smaller than 25,000 sq. ft.,
• Short subdivisions,
• Minor utility developments,
• Trails and trailheads:

For all Medium Intensity development (as listed above), a stormwater design which is consistent with all applicable minimum requirements of the 2012 Washington State Department of Ecology Stormwater Management Manual For Western Washington will be required if the project proposes:

• 4,000 sq. ft. or more of new or replaced hard surface or,
• 14,000 sq. ft. or more of land disturbing activity,

This may require consultation from a civil engineer

The following land uses are considered High Intensity development for the purposes of stormwater review:

• Long subdivisions
• Uses on parcels bisected by the NPDES Phase II Area Boundary
• Projects that result in new plus replaced hard surface greater than or equal to 10% of the gross parcel size or 20,000 sq. ft., whichever is greater, or converts 1.5 acres of vegetation to lawn or five acres of vegetation to pasture 
• Other uses, including commercial, industrial, institutional, and urban or multifamily residential uses,

All High Intensity development (as listed above) must provide a stormwater design which is consistent with the 2012 Washington State Department of Ecology Stormwater Management Manual For Western Washington.

A Stormwater Pollution Prevention Plan (SWPPP) which indicates how soils exposed during construction will be prevented from eroding into critical areas, surface drainages, or onto neighboring properties will be required for all projects.

These regulations can be found in their entirety in WCC 20.80.630(1)(d).