Madison, WI Accessory Structures: ADU Rules (2026)

151's 'Accessory Dwelling Unit' standards, part of the zoning code's Supplemental Regulations, set specific numeric limits rather than leaving the use to case-by-case discretion. A zoning lot may have one attached ADU or up to two detached ADUs, but not both an attached and a detached unit on the same lot, and any detached units must sit in a single structure. The principal building itself can't already contain more than eight dwelling units. Occupancy is capped at one family per ADU, and the ordinance bars selling an ADU separately from the principal building, keeping it tied to the main property.

Detached ADUs, including, per the ordinance's own language, one 'built above a garage or similar space', are capped at twenty-five (25) feet in height, measured the same way as a principal building under Sec. 134(1)(b); the footprint of an accessory building containing an ADU can't exceed 1,000 square feet, matching the 1,000-square-foot cap that applies to ADUs built inside the main house. Setbacks follow whatever the underlying zoning district requires for accessory buildings generally, and any ADU entryway in a rear or side yard must connect to the street by a paved walkway or driveway.

Where an ADU is carved out of the existing principal building, the ordinance requires that the building's exterior appearance not be significantly altered so it still reads as the principal use. The Common Council amended these standards twice in 2025, ORD-25-00068 (10-16-25) on the one-attached/two-detached limit and ORD-25-00057 (9-11-25) on the resale restriction, so the current version reflects recent Council action, not a static older rule.