Fitchburg, WI Accessory Structures: ADU Rules (2026)

Fitchburg City Code § 22-230 permits a maximum of one accessory dwelling unit (ADU) per lot of record across all residential districts. The entrance to an ADU may only be added to the side or rear of the primary dwelling, and the entryway must connect to the street frontage by a paved walkway or driveway. ADUs must meet the setback standards of the underlying zoning district, though the plan commission can reduce rear and side setbacks by conditional use approval. A travel trailer or recreational vehicle cannot serve as an ADU: every unit must comply with the State of Wisconsin Uniform Dwelling and Energy Code, sit on a permanent foundation compliant with that code, and connect to permanent water and sewer utilities, though it may share existing utility connections with the primary residence if the building inspector confirms adequate capacity.

For ADUs on the same lot as a single-family home or duplex, § 22-230(10) adds design limits: the unit cannot exceed 75 percent of the principal dwelling's floor area (excluding garage, screened porch, patio and deck space), and a detached ADU building, including one built above a garage, is capped at 25 feet in height. The exterior finish, roof pitch, trim, eaves and window proportions of the ADU must match the principal dwelling, and the ADU cannot be sold separately from the principal home.