Impervious Surface Updates

On September 17, 2024, the Town Council adopted a revised Comprehensive Plan and corresponding revisions to the Steilacoom Municipal Code (SMC). One of these changes is the introduction of a limitation on total impervious surface coverage on lots. Impervious surfaces include roofs on principal and accessory structures, concrete, gravel, oil mat, asphalt or other finished surfaces which do not allow water to absorb into the soil. The limitation was introduced to better align with low-impact development code requirements of the Town’s Phase II Municipal Stormwater Permit from the Department of Ecology. Generally, minimizing impervious surfaces leads to reduced runoff and better stormwater management.

The total impervious surface coverage limit is 50 percent of the lot for most residential neighborhoods in Town (R-7.2 and R-9.6 zones). This new requirement applies in addition to the lot coverage limitation of 30 percent, which applies only to buildings and structures greater than 42 inches in height. Buildings and structures greater than 42 inches in height count towards both the lot coverage limit and the impervious surface coverage limit. Roofs of structures less than 42 inches in height and other impervious surfaces listed above count towards the impervious surface limit, but not the lot coverage limit. Compliance with impervious surface limits will be reviewed with building permit applications where new impervious surfaces are proposed. Review SMC 18.08.378 and 18.20.030 or contact the Community Development Department for additional information.