City of Sun Prairie, WI

ADUs are only permitted in certain zoning districts. Most single-family and two-family residential zones qualify, but mixed-use and commercial zones have different rules. Your parcel’s zoning classification is the first thing we verify.

Most Dane County municipalities cap ADU size at 800–900 sq ft or a percentage of the primary home’s square footage, whichever is less. This directly affects what floor plans are achievable on your property.

Setbacks define how close an ADU can be placed to property lines. These vary by zone and municipality — and they’re what most often determine where on a lot an ADU can physically be placed.

ADU height limits typically range from 18 to 25 feet depending on the municipality and zone. Height matters for garage ADUs and any structure with a loft or second level.

ADUs typically require separate utility connections — water, sewer, and electrical. Connection capacity, tap fees, and access to existing lines all affect feasibility and cost. Wisconsin winters also require engineering decisions for water and drain lines.

Some municipalities require one off-street parking space per ADU. Others — including Madison in many districts — have reduced or eliminated this requirement. It depends on your parcel’s location and zone.