Madison's Accessory Structures: The Rules That Matter

Every city handles accessory structures a little differently. In Madison, Wisconsin, there are 4 distinct rules that residents and property owners should be aware of. Some are stricter than what neighboring cities enforce, and others are more relaxed. Here is what you need to know.

Shed & Carport Rules

Madison caps the total footprint of sheds and other accessory buildings on residential and mixed-use lots at 1,000 square feet under MGO § 28.131(1)(a), with a conditional use permit needed to go bigger. Height tops out at 15 feet or the principal building's height, whichever is lower, and placement is restricted to specific yard zones with minimum setbacks from property lines.

Key details: Max accessory building area: 1,000 sq ft ground floor per lot. Max height: 15 ft or principal building height, lower. Rear yard setback: 3 feet from property line minimum. Corner lot setback: same as required principal building setback. Over-cap route: conditional use approval required.

Building without a zoning permit, or exceeding the 1,000-square-foot accessory cap without conditional use approval, is a zoning code violation enforced by the Zoning Administrator in the Department of Planning, Community and Economic Development. Noncompliant structures can be ordered removed, relocated, or brought into compliance, and continuing violations trigger Chapter 28's general zoning enforcement remedies, including citations and forfeitures through the City's municipal court process.

ADU Impact Fees

Madison doesn't have a separate ADU impact fee: accessory dwelling units are billed under the Multi-Family Park Impact Fee category set by Madison General Ordinances § 20.08(2). Developers owe a Park-Land Impact Fee (734 square feet per unit, or cash in lieu) plus a Park-Infrastructure Impact Fee of $1,032 per unit, indexed annually from a 2016 base.

Key details: ADU fee category: Multi-Family Park Impact Fee (with 3-family+ units). Land dedication rate: 734 sq ft per unit, or fee in lieu. Infrastructure fee: $1,032 per unit (2016 base, indexed annually). When fee is due: Before building permit issuance. Low-cost housing exemption: Available under Wis. Stat. § 66.0617(7).

Under § 20.06(1)-(3), no rezoning permit, conditional use permit, or building permit may be issued for a development, including one adding an ADU, until the Park Impact Fee is calculated and noticed for collection, and for unplatted land the fee must be noted on the face of the recorded plat or certified survey map.

Tiny Homes

Tiny homes on foundations are allowed in Madison if they meet WI Uniform Dwelling Code and zoning minimum dwelling size. Tiny houses on wheels are treated as RVs and not allowed for permanent residence outside RV parks.

Key details: Foundation Tiny: Allowed as ADU or primary. THOWs: Not for permanent residence. Code: WI UDC applies. ADU Option: MGO 28.151. Min Size: Per UDC standards.

ADU Rules

Madison lets a zoning lot carry one attached accessory dwelling unit or up to two detached ADUs under Madison General Ordinances § 28.151, so long as the principal building has no more than eight dwelling units total. Detached ADUs, including ones built above a garage, are capped at 25 feet in height, and units inside the main house top out at 1,000 square feet.

Key details: Attached ADUs per lot: 1 maximum. Detached ADUs per lot: 2 maximum, single structure. Detached ADU height limit: 25 feet. Size cap (in-building ADU): 1,000 sq ft. Footprint cap (detached ADU): 1,000 sq ft.

An ADU built or occupied outside these standards, too many units on the lot, a detached unit over 25 feet, a footprint or floor area over 1,000 square feet, or a unit sold off separately from the principal building, is a Chapter 28 zoning violation. Sec. 28.207(1) sets the penalty: a forfeiture of not less than one dollar ($1) and not more than one thousand dollars ($1,000) for any violation of the zoning chapter's requirements.

The Bottom Line

Madison's accessory structures rules are a mixed bag. Some areas are strict, others are relaxed, and the details matter. The best approach is to check the specific rule that applies to your situation rather than assuming Madison is broadly strict or permissive.

These rules come from Madison's publicly available municipal code. For complete penalty schedules, exemption details, and answers to common questions, see the individual ordinance pages throughout this guide.