Waukesha County, WI Accessory Structures: ADU Rules (2026)

Wis. Stat. §59.69 authorizes county zoning, and the Waukesha County Basic Zoning Ordinance (Appendix A, effective December 11, 2024) governs unincorporated land in towns that have adopted the county code. Detached accessory buildings and any secondary dwelling components are evaluated against the underlying residential district's lot-area, floor-area, height, and offset standards. Town-level zoning codes that mirror the county framework permit a "farm dwelling" for additional families or individuals actually engaged in the farm operation as an accessory use, requiring compliance with the floor area, offset, and setback provisions of the R-3 Residential District. Accessory buildings 200 square feet or larger generally require 20-foot side and rear offsets; accessory buildings under 200 square feet may sit 5 feet from a side lot line. All required offsets increase one foot for each foot a building exceeds the district height limit. Because Wis. Stat. §59.69(5) lets towns opt in or out of county zoning, parcel-specific ADU permission depends on which town board has jurisdiction. Incorporated jurisdictions in the county - including the City of Waukesha, City of Brookfield, City of New Berlin, City of Pewaukee, Village of Menomonee Falls, Village of Mukwonago, and Village of Sussex - each adopt their own ADU and accessory dwelling rules under Wis. Stat. §62.23(7) (cities) and §61.35 (villages); the City of Oconomowoc adopted a dedicated ADU ordinance with limits on garage units. Always confirm whether county zoning, town zoning, or city/village zoning controls before submitting plans to the Waukesha County Department of Parks and Land Use, Planning and Zoning Division ((262) 548-7790).