In plain words

Wisconsin does not have a statewide ADU law. An accessory dwelling unit (ADU) is a small second home on the same lot as a house. Examples are a basement apartment, a unit over a garage, or a backyard cottage. Each city, village, town and county decides whether to allow ADUs through its zoning. Zoning means the local rules about what can be built where. Milwaukee and Madison now allow ADUs on many lots. Other places allow them only with a conditional use permit, which needs a public hearing, or not at all. Lawmakers tried to change this in 2025. A bipartisan bill, AB 449, and its Senate twin, SB 473, would have made every community with zoning allow at least one ADU on each lot with a house. Committees later narrowed it to ADUs made by converting an existing building, like a garage. A broader bill, AB 365 and SB 247, went further. Both bills died when the session ended on March 23, 2026. Two existing state rules still help. When a complete permit application for housing meets every local rule, the community must grant it. That rule does not cover rezonings or conditional use permits. State law also limits how far towns can restrict renting a home for stays of seven days or more. You may be able to build an ADU if your local rules and your property allow it. Start by asking your local zoning office.

The rules at a glance

State ADU lawNo. Each city, town or county decides.
Owner living on siteTowns may require the owner to live on the property.
ParkingState law doesn’t set this. Each community sets ADU parking rules.
SizeState law doesn’t set this. Local caps vary; Milwaukee allows up to 1,000 sq ft for internal or attached units and 1,300 sq ft or 16% of the lot for detached units (counting any garage space), capped at 1,600 sq ft, and Madison allows a detached ADU building footprint of up to 1,000 sq ft.
Lot size and setbacksState law doesn’t set this. Lot size, coverage and setbacks come from each local zoning code.
FeesState law doesn’t set this.
Short-term rentalsWis. Stat. § 66.1014 bars local bans on rentals of 7 or more consecutive days. For rentals longer than 6 but shorter than 30 consecutive days, a community may cap total rental days at no fewer than 180 in any 365-day period and may require a local license, and anyone operating a short-term rental more than 10 nights a year needs a state tourist rooming house license. Communities may otherwise regulate rentals in ways consistent with that statute.

What to do next

  1. Find out who zones your property (city, village, town or county) and ask the zoning office whether ADUs, in-law suites or accessory apartments are allowed in your district.
  2. Ask whether an ADU is a permitted use (staff approval) or a conditional use (public hearing), and get the list of conditions in writing.
  3. Write down local rules on owner occupancy, size, height, setbacks (how far a building must sit from property lines), parking and utility connections before you design.
  4. Check your deed and any homeowners association covenants; current state law does not override private restrictions on ADUs.
  5. If you plan to rent the ADU, ask about local rental licenses and short-term rental limits, and check the state tourist rooming house license rules.
  6. Watch the 2027-2028 Legislature, which starts in January 2027, for a reintroduced ADU bill.

What your town still decides

Counties (s. 59.69), towns (s. 60.61) and cities and villages (s. 62.23) each write their own zoning, so ADU rules vary by community. Where ADUs are listed as a conditional use, Wis. Stat. § 62.23(7)(de) (and matching county and town rules) requires a public hearing, but also says conditions must be reasonable, measurable where practicable and backed by substantial evidence, and the permit must be granted if the applicant meets them. Where ADUs are a permitted use, § 66.10016 says a complete application that meets all existing rules must be approved. Both 2025-2026 ADU bills would have created new subsections in ss. 59.69, 60.61 and 62.23 requiring at least one ADU per qualifying lot; neither passed.

Examples of local rules

  • Milwaukee: An ordinance effective August 2, 2025 allows internal, attached and detached ADUs as a limited use in single-family, two-family and multifamily residential districts and several business districts. One ADU per lot; internal and attached units only with a single-family home, detached units with a single- or two-family home. The owner must live in the main home when applying for the permit. Minimum 300 sq ft; internal and attached up to 1,000 sq ft; detached up to 1,300 sq ft of living and parking area or 16% of the lot, whichever is greater, but never more than 1,600 sq ft or the main home's floor area; no taller than the main house, 3-foot side setback. Detached units need their own sewer and water connection. Source
  • Madison: ADUs are a permitted use on a lot with a building of up to eight dwelling units, with site plan approval required before construction. Attached ADUs can have up to 1,000 sq ft of living area. A detached ADU building can have a footprint of up to 1,000 sq ft and a height of 25 feet. The city adopted changes in early 2025 to simplify ADU construction; lots in Planned Development zoning may have extra limits. Source

What we’re still confirming

  • Madison's detached ADU page doesn't mention an owner-occupancy rule; confirm with the city whether one applies.

Laws, bills and citations

The laws behind this (2)
  • Permits for residential housing developments (Wis. Stat. § 66.10016, created by 2023 Wisconsin Act 16 (2023 AB 266), signed June 22, 2023, in effect January 1, 2025). Not ADU-specific. Since January 1, 2025, when someone files a complete permit application for single-family or multifamily housing that meets every rule in effect on the filing date, the city, village, town or county must grant it, and the applicant can go to court to force approval. It does not cover rezonings or conditional use permits, and it does not require any community to allow ADUs. Read it
  • Limits on local rental restrictions (Wis. Stat. § 66.1014 (created by 2017 Wisconsin Act 59), signed January 1, 2017, in effect January 1, 2017). Not ADU-specific. Local governments may not ban renting a residential dwelling for 7 or more days in a row. For rentals longer than 6 but shorter than 30 days in a row, they may cap the total at no fewer than 180 days in any 365-day period and may require a local license. The statute defines a residential dwelling as any building or part of a building used as a home, so it would generally reach an ADU used as a rental. Read it
Proposed and failed bills (2). None of these is law.
  • AB 449 / SB 473 (2025-2026): died as of October 3, 2026. Bipartisan bill led by Republicans Goeben and Bradley. As introduced, every county, town, city and village with zoning would have had to allow at least one ADU (inside, attached or detached) as a permitted use on each residential or mixed-use lot with a single-family home. Communities could cap size at the main home's square footage, cap height at the district limit, apply normal setbacks and lot coverage, and ban new ADUs as short-term rentals. Committee amendments (Assembly Amendment 1, adopted in committee 8-6; Senate Amendment 1, recommended 5-0) narrowed it to ADUs created by converting an existing structure. The Assembly housing committee recommended passage 13-1 on October 3, 2025 and the Senate committee 5-0 on February 12, 2026, but neither chamber voted on it; it failed to pass on March 23, 2026 under Senate Joint Resolution 1. Bill page
  • AB 365 / SB 247 (2025-2026): died as of October 3, 2026. Democratic bill (Hong, Roys). Would have required at least one ADU allowed by right on every lot where single-family or multifamily homes are allowed by right, and barred extra fees beyond standard building permit fees, stricter size or design rules than for single-family homes, extra parking, and separate utility connections. It also would have voided deed covenants and HOA rules that block or unreasonably restrict ADUs. Never got a hearing; failed to pass on March 23, 2026 under Senate Joint Resolution 1. Bill page

Sources

  1. Wis. Stat. § 66.10016, created by 2023 Wisconsin Act 16 (2023 AB 266) · official source
  2. Wis. Stat. § 66.1014 (created by 2017 Wisconsin Act 59) · official source
  3. 2025 Assembly Bill 449 history, Wisconsin Legislature · official source · checked 2026-10-03
  4. 2025 Senate Bill 473 history, Wisconsin Legislature · official source · checked 2026-10-03
  5. 2025 Senate Bill 473 text and LRB analysis · official source · checked 2026-10-03
  6. Assembly Amendment 1 to 2025 AB 449 (conversion-only limit) · official source · checked 2026-10-03
  7. Senate Amendment 1 to 2025 SB 473 · official source · checked 2026-10-03
  8. Assembly Substitute Amendment 1 to 2025 AB 449 (Allen) · official source · checked 2026-10-03
  9. 2025 Assembly Bill 365 history · official source · checked 2026-10-03
  10. 2025 Assembly Bill 365 text and LRB analysis · official source · checked 2026-10-03
  11. 2025 Senate Bill 247 history · official source · checked 2026-10-03
  12. Wisconsin Assembly Journal, October 7, 2025 · official source · checked 2026-10-03
  13. Wis. Stat. § 62.23(7)(de), conditional use permits · official source · checked 2026-10-03
  14. City of Milwaukee, Accessory Dwelling Units · official source · checked 2026-10-03
  15. City of Madison, New Detached Accessory Dwelling Unit · official source · checked 2026-10-03
  16. City of Madison, Attached Accessory Dwelling Unit · official source · checked 2026-10-03
  17. WPR, Assembly approves GOP housing package, including building code bill (Oct 7, 2025) · checked 2026-10-03

This is general information, not legal advice. Your town, village, city or county decides what can be built on your property. If something here looks out of date, email the team.