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

Code § 74-174(E) sets impervious surface standards for any riparian lot or a nonriparian lot located entirely within 300 feet of the ordinary high-water mark of a navigable waterway. Percentage of impervious surface is calculated by dividing existing and proposed impervious surface area by the total lot area and multiplying by 100, and the county excludes treated impervious surfaces, such as those draining to storm water ponds, constructed wetlands, infiltration basins, rain gardens, or bioswales, from that count. The general standard under § 74-174(E)(3)(a) caps a lot at 15 percent impervious surface.

A property can exceed that up to a maximum of 30 percent with a county-approved permit and mitigation plan implemented by a deadline the permit specifies. On a DNR-approved highly developed shoreline referenced at § 74-265, § 74-174(E)(4)(a) raises the base allowance to 30 percent impervious surface for residential uses and 40 percent for commercial, industrial, or business uses; those properties can climb further, to a maximum of 40 percent residential or 60 percent commercial/industrial with an approved mitigation plan. Existing impervious surfaces that were legally built before these standards took effect (September 13, 2016, for the highly-developed-shoreline provisions) may be maintained, repaired, or replaced within their existing footprint without triggering mitigation, so long as the lawful percentage isn't increased.

Mitigation itself is scored on a points system tied to specific measures, such as native shoreyard buffers, rain gardens, roof runoff collection, and removal of a second shoreline access, each worth a set number of points toward the increase requested.