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

Portage County's zoning tables at Code §§ 7.1.2-7.1.5 do not list accessory dwelling units among permitted or conditional uses; the only county-level rule addressing ADUs sits inside the Private Sewage System (POWTS) Ordinance, Chapter 7.9, which every septic-served lot in the towns must satisfy before Planning and Zoning issues a sanitary permit. Code § 7.9.9(5) declares that 'Holding tanks are not permitted except as allowed by this section or by variance,' and subsection (c) lets the Portage County Board of Adjustment approve a sanitary permit for a holding tank as a special exception.

The first item on that special-exception list allows 'An accessory dwelling unit (ADU) or accessory use if a code compliant POWTS already exist on the property,' which means the county will not permit a brand-new stand-alone holding tank to serve a new ADU; the lot must already carry a functioning conventional private sewage system before the Board will even consider the request. A holding tank approved this way 'can only accept the waste generated from the home occupation' where the accessory use is a home occupation, so it cannot be shared with unrelated wastewater sources.

Before granting the exception, the Board weighs site suitability under Wis. Admin. Code ch. SPS 383, along with proximity to surface water, floodplains, wetlands, and groundwater depth, plus the setback and use standards of Code § 7.1.6(5) of the Zoning Ordinance. Code § 7.9.40(3) separately defines an ADU as 'a, generally smaller, independent residential dwelling unit located on the same lot as a stand-alone (i.e., detached) single-family home,' confirming the county's rule targets detached units rather than basement or attached-addition conversions.