Do I need a permit for accessory dwelling unit (adu) in Sun Prairie?

Most likely yes. Accessory dwelling unit (adu) usually requires a permit in Wisconsin. Confirm the specifics with Sun Prairie.

A detached ADU containing its own dwelling unit is a dwelling, not an accessory building, under the SPS 320.07 definitions, so it needs a Wisconsin uniform building permit before any on-site construction, including excavation (SPS 320.08(1)). Converting a garage, barn or other building to residential use brings it under the UDC at conversion (SPS 320.04(4)); adding a unit inside or onto an existing home is an addition or alteration permitted under the local ordinance. The UDC does not affect local land use, zoning or setback rules (SPS 320.02(2)(b)), so whether an ADU is allowed on your lot is decided by local zoning. An owner may pull the permit without a DSPS dwelling contractor credential only if the owner resides or will reside in the dwelling (Wis. Stat. 101.654(1)(b)); otherwise the applicant needs the credential. A new accessory dwelling unit, detached or added on, is new residential construction. It normally needs a building permit with full plans, plus separate electrical, plumbing and mechanical permits, and zoning approval for a second unit on the lot. Utility connections (water, sewer or septic, and electric service) are usually reviewed as part of the permit, so ask about capacity and connection fees early.