ADU Rules in Oshkosh, Wisconsin

Oshkosh is the county seat of Winnebago County, with a population of approximately 66,000. The city sits at the southern end of Lake Winnebago — one of the largest freshwater lakes in the United States — where the Fox River enters and exits the lake. Oshkosh is home to the University of Wisconsin-Oshkosh (UW-Oshkosh) and hosts the world-famous EAA AirVenture annual airshow at Wittman Regional Airport, which draws hundreds of thousands of aviation enthusiasts each year.

No Statewide Wisconsin ADU Law

Wisconsin has no statewide ADU preemption law. Oshkosh's zoning code governs ADU permissions at the local level. Individual zoning districts, lot conditions, and shoreland overlays determine what is allowable on any given property.

ADU Types in Oshkosh

Oshkosh's zoning code may permit the following ADU types in residential districts:

• Attached ADUs (additions to the primary dwelling)
• Detached ADUs (separate backyard structures)
• Basement conversions (interior ADUs)
• Garage conversions (detached garage to living unit)

Oshkosh has a mix of older urban neighborhoods near downtown and the university, and newer suburban development. Older areas often feature larger lots and detached garages well-suited for conversion.

Size, Setback, and Coverage Requirements

City of Oshkosh zoning establishes dimensional standards for accessory structures:

• Rear and side yard setbacks for detached accessory structures
• Maximum ADU floor area (often tied to primary dwelling size or a fixed cap)
• Height limits for accessory buildings
• Total lot coverage maximums
• Parking requirements per dwelling unit

Always confirm current numbers with the Department of Community Development before finalizing ADU plans.

Wisconsin Shoreland Zoning — Lake Winnebago and Fox River

Oshkosh's location on Lake Winnebago and the Fox River means Shoreland Zoning (Chapter NR 115) applies to many properties, administered by Winnebago County:

• Lake Winnebago: Properties within 1,000 feet of the ordinary high-water mark are in the shoreland zone. Minimum 75-foot setback required from the water's edge.
• Fox River: Properties within 300 feet of the Fox River are in the shoreland zone. Minimum 35-foot setback from the ordinary high-water mark.
• Impervious surface limits apply in the shoreland zone
• Vegetative buffer requirements near water's edge

Winnebago County administers these rules separately from City of Oshkosh zoning. Both sets of rules must be satisfied for shoreland properties. Contact Winnebago County before designing a project near water.

Airport Proximity — Wittman Regional Airport

Properties near Wittman Regional Airport may fall within FAA-regulated Runway Protection Zones (RPZs) or airport approach zones. Year-round airport operations — including EAA AirVenture — can create permanent flight path restrictions that affect building height limits and uses. If your property is near the airport, check with the city and the FAA before building.

FEMA Flood Zones

Oshkosh's location at the Fox River/Lake Winnebago nexus creates flood risk for some properties. Check FEMA Flood Insurance Rate Maps (FIRMs) for your parcel. Properties in Special Flood Hazard Areas require flood-resistant construction.

Building Code — Cold Climate

Wisconsin's frost depth in Oshkosh is approximately 48 inches. ADU foundations must be designed for deep footings. Wisconsin's State Building Code governs all construction statewide.

Permit Process

1. Confirm zoning district, shoreland zone status, and airport proximity with Community Development
2. Check Winnebago County shoreland rules if near water
3. Submit building permit application to Oshkosh Building Services
4. Construction inspections and final certificate of occupancy

Contact

• Department of Community Development: ci.oshkosh.wi.us/CommunityDevelopment
• Oshkosh Building Services: ci.oshkosh.wi.us
• Winnebago County Planning: co.winnebago.wi.us/planning

Disclaimer: Zoning regulations change frequently. Always verify current rules with the City of Oshkosh and Winnebago County before beginning any ADU project. This page is for informational purposes only and does not constitute legal or zoning advice.