Most ADU conversations start with zoning — how big, how tall, how close to the lot line. That's only half the approval. Your ADU also has to meet Wisconsin's Uniform Dwelling Code (UDC), the state building code for one- and two-family homes. Zoning decides where your ADU can go. The UDC decides how it's built.
Here's what the code means for your project in plain terms, and where it tends to shape your design and budget.
Once Someone Lives in It, It's a Home
Under Wisconsin law, once a space is used as someone's home, it's a "dwelling" — not an accessory building like a shed or detached garage. So your ADU is held to the same building code as any house.
The code is also the same statewide. Zoning varies from town to town across Dane County, but local governments can't write their own versions of the building code.
Attached or Detached Changes the Rules
- Detached ADU: built as its own single-family home.
- Attached to a single-family house: the building becomes a two-family home, and the two units need fire separation between them.
- Attached to a duplex or larger building: Madison zoning allows an attached ADU on a building with up to eight units, but once a building has three or more attached units, it falls under the commercial building code — a different set of rules entirely.
Attached ADUs: Separation Between Units
In an attached ADU, the two homes have to be separated from each other and from shared spaces like attics, basements, and garages. That means:
- Fire-rated 5/8-inch Type X drywall (or an equivalent) on both sides of the shared wall, and on the ceiling below if one unit sits above the other
- Doors between the units rated for at least 20 minutes
- Fire protection where ducts, pipes, and wires pass through the separation
That separation has to wrap the whole unit, which is why basement and attic conversions often take more work than people expect. It's far easier to plan for during design than to add later.
Detached ADUs: Distance Sets the Wall Rating
A detached ADU doesn't need unit separation, but how close it sits to your house or the lot line can call for fire-rated walls, doors, and windows (measured wall to wall, not from the overhang):
- Less than 5 feet from the house: fire-rated construction on both facing walls
- 5 to 10 feet: on one of the two facing walls
- More than 10 feet: no requirement
- Less than 3 feet from a property line: a fire-rated wall; at 3 feet or more, none
Madison lets a detached ADU sit as close as 3 feet from the house, so where it goes and how its walls are built are really one decision.
Exits, Ceilings, and Safety Alarms
- Exits. The standard is two exit doors. Small ADUs get a break: one with a first floor of 400 square feet or less and no second story (a loft up to half that size is fine) needs just one door plus an egress window.
- Egress windows. A window used as an exit must open without tools, have at least 20 by 24 inches of clear opening, and sit no higher than 60 inches off the floor, with a platform required above 46 inches.
- Ceiling height. Living and sleeping rooms, kitchens, hallways, and bathrooms need 7-foot ceilings. A sloped room still counts if at least half its floor area is 7 feet or higher — often the deciding factor for an attic conversion.
- Smoke alarms. In every bedroom, outside sleeping areas, and on every level — hardwired, interconnected, and with battery backup.
- Carbon monoxide alarms. Required on each level, unless the home has no attached garage, no fireplace, and no fuel-burning appliance. An all-electric detached ADU may not need them, but confirm with your inspector.
Converting a Garage? Same Standards
The code is clear: a garage, barn, or other building converted into a home must meet the UDC. A conversion is held to the same standards as new construction, so whether it makes sense depends on the existing structure.
Inspections and Your Certificate of Occupancy
State-certified inspectors check the work at each major stage, from the foundation to a final inspection, and generally no one can move in until the final passes. In Madison, the City issues your Certificate of Occupancy once all permits have passed final inspection.
Energy code is part of plan review, too. Madison's ADU plan checklist requires REScheck energy calculations — our post on insulation and envelope standards covers that side.
What This Means for You
Most of these rules are simple to meet during design and expensive to fix later. Unit separation, wall ratings, exits, and ceiling heights all shape your layout, so they should be settled before plans are drawn.
That's where we come in. We prepare and submit all permit applications, and every Boundless project is delivered to CO. If you're weighing attached vs. detached, or wondering whether your garage can be converted, start with a free feasibility check.
