Washington County, WI Building Setbacks & Zoning: Lot Coverage Limits (2026)

Washington County's impervious surface standard applies only inside the shoreland zone, not to lots generally. Under § 275-11A, it governs any impervious surface built, expanded, replaced or relocated on a riparian lot, or on a nonriparian lot located entirely within 300 feet of the ordinary high-water mark of a navigable lake or stream, in the unincorporated towns of Washington County. Section 275-11C sets the general cap at 15% impervious surface on the portion of the lot within 300 feet of the ordinary high-water mark, calculated by dividing existing and proposed impervious area by the total lot area within that 300-foot band and multiplying by 100.

Higher caps apply on mapped 'highly developed shorelines,' land identified as an urbanized area or urban cluster in the 2010 census, or land the County has separately mapped after a DNR hearing: § 275-11D allows up to 30% impervious surface for residential use and up to 40% for commercial, industrial or business use. A property that exceeds even those caps can still get a permit with a mitigation plan meeting § 275-8, up to hard maximums of 30% under the general standard or 40%/60% under the highly-developed-shoreline standard, per § 275-11E. Certain treated surfaces, such as those draining to stormwater ponds, rain gardens or bioswales, are excluded from the calculation entirely under § 275-11F, and lawfully placed existing surfaces that already exceed the cap may be maintained, repaired or relocated within the existing building envelope under § 275-11G.