Can I Build an ADU in Madison, WI?

Direct Answer: Yes, Madison allows accessory dwelling units, but you cannot skip review. The City says you need permits to build any ADU, and you must first obtain site plan approval. Detached and attached ADUs have their own Development Services Center pages, and both point applicants to zoning review, Building Inspection permits, licensed contractors, address assignment, and Parks impact-fee review.

Verified against official municipal and state sources: July 13, 2026. Requirements change — confirm with the city before applying.

Key Takeaways

• Madison's detached ADU page says you need a permit to build any ADU and need site plan approval first (New Detached Accessory Dwelling Unit).
• Madison's attached ADU page says attached ADUs are a permitted use on the lot and can be up to 1,000 square feet of living area (Attached Accessory Dwelling Unit).
• The principal building may have up to eight units under the City's published ADU guidance.
• Engineering assigns a new address for the ADU.
• You must pay a Parks impact fee for a new ADU; Madison's page says to check with Parks for the current rate.
• Licensed contractors are needed to obtain permits and complete the work.

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Attached vs. Detached ADU

An attached ADU is built inside or attached to an existing principal building. Madison's guidance says attached ADUs may occupy any area inside the building, including a full basement, and may be up to 1,000 square feet of living area. A detached ADU is a separate structure, such as a backyard cottage or converted accessory building, and triggers site placement, utility, and address questions.

The practical sequence is the same: confirm zoning, prepare a site plan, obtain site plan approval, get the right permits, use licensed contractors, and schedule inspections. If you are converting a garage or basement, the building-code questions — egress, fire separation, ceiling height, heating, plumbing, and electrical — can be more important than the zoning question.

What to Prepare

• A site plan showing the existing house, proposed ADU, parking, lot lines, and setbacks.
• Existing floor plans if converting interior space.
• Proposed floor plans, elevations, and construction details.
• Utility information and any new address requirements.
• Contractor information for building, plumbing, electrical, and HVAC work.

For broader conversion issues, see Can I Turn My Garage Into an Apartment?.

FAQ

Are ADUs a permitted use in Madison?

Madison's attached ADU page says ADUs are a permitted use on the lot, but you still need site plan approval and permits.

Can a Madison ADU be in a basement?

Yes, Madison's attached ADU guidance says ADUs may occupy any area inside the building, including a full basement.

Can I pull every permit myself?

Not generally. Madison says licensed contractors are needed to obtain permits and complete the work.

Verify Your Address

Use GovCodex's Madison permit catalog or run a permit check before design work so the ADU answer is tied to your parcel and project type.