Accessory Dwelling Units

The City of South Milwaukee adopted a new zoning code in 2025, allowing Accessory Dwelling Units (ADUs) in residential districts that have a single-family home. An ADU is defined as a smaller, independent residential dwelling unit located on the same lot as a stand-alone (i.e., detached) single-family home.  ADUs go by many different names throughout the U.S., including accessory apartments, secondary suites, and granny flats. ADUs can be converted portions of existing homes (i.e., internal ADUs), additions to new or existing homes (i.e., attached ADUs), or new stand-alone accessory structures or converted portions of existing stand-alone accessory structures (i.e., detached ADUs).

ADU Informational Guidelines for constructing an ADU in the city. 

Zoning

  • A second dwelling unit is allowed in Z-2 and Z-3 zoning districts that have a single-family dwelling. Z-2 and Z-3 districts are the majority of the city’s residential neighborhoods.  
  • Must function as an independent living unit and the ADU may not be sold independently from the principal dwelling. The property owner does not need to reside at the property.
  • In Z-2 zones, no more than 50% and in Z-3 zones, no more than 60% of your lot may be impervious, that includes all hard surfaces, such as patios, paths, driveways and sheds.
  • 1 parking space is required per dwelling unit. On-street parking credits apply for abutting street parking.
  • Refer to the Zoning Code and information flyer for details about ADU setbacks, size, and heights.

Zoning Information Page is found in the Economic Development Department

Types of ADUs

ADU type graphic

Design & Material Standards 

  • Whether detached or attached, the ADU must be compatible with the style and materials of the primary dwelling. Use the same architectural details, such as siding materials, roof styles, and trim, as the main house, including matching windows and doors. Check with the zoning administrator for alternative materials.
  • ADUs require a 3-foot clear path to the property line. Show path on site plan.
  • Exterior new lighting shall meet color temperature and shielding/non-glare requirements.

Permits and Process Guide

ADU process permit graphic

Permits

Community Development Permit - For detached and addition ADUs, submit a Community Development Permit application for zoning review, confirming height, area, setback, flood plain designation, and other site standards are met. Draw the proposed ADU building envelope on a property plat of survey or separate site plan to scale. All hard surfaces and structures must be shown on the survey, including patios and paths.

Building Permit - Building permits for an ADU must be taken out by either the homeowner or a contractor licensed by the State of Wisconsin. Plans must meet Wis. Uniform Dwelling Code, Fire Safety Regulations, and accessibility requirements. Submit scaled constructions drawings and elevations of the proposed building and site plans based on a survey plat, showing its location on the lot, with dimensions. Property surveys may not be greater than 10 years old.

Electrical, Plumbing and HVAC Permits - A licensed electrician, licensed plumber and licensed HVAC contractor submit affiliated permits. Exterior lighting must meet lighting standards, including maximum color temperature and light cutoff.

Utility Connections: Water, Sewer, Electrical/Gas - You’ll need proper connections for water, sewer, and electrical/gas power. Detached ADUs may access power through the main dwelling or separate.  Detached ADUs must have separate water and sewer connections from the main dwelling unless a property-based hardship is identified. The property owner is responsible for all expenses that require additional sewer or water line extensions in the public way to the private property if existing lines are not present. Check with the city engineering department to identify where existing utilities are located.  

ADU contractor