WHAT IS AN ADU?

The City of Milwaukee defines an Accessory Dwelling Unit (ADU) as a room or set of rooms with its own cooking, sleeping, and sanitary facilities that is located on the same lot as a single-family home or 2-family home. An ADU also has a smaller living area than the main dwelling to which it is accessory.

ADUs are a housing option that can work for just about everyone! Upsizing, downsizing, just starting out, single, divorced, or widowed, families, adult children, aging parents, and extended family. ADUs can also provide additional income from homeowners. 


ADUs come in many different shapes and sizes. Your ADU will fall into one of the following categories:

 

Internal ADU:

An additional residential unit located within the walls of an existing or newly-constructed residence. This could be either a second-story, attic, or basement conversion.

  

        

Attached ADU:

An additional residential unit located in a separate addition to an existing residence.

Detached ADU:

A freestanding residential unit located on the same property containing a separate principal residence. Detached ADUs are commonly known as in-law suites or garage apartments.

 

CAN I BUILD AN ADU?

 

Zoning Districts:

An accessory dwelling unit (ADU) is allowed in the following zoning districts as a Limited Use:

• Residential Single-Family: RS1, RS2, RS3, RS4, RS5, RS6
• Residential Two-Family: RT1, RT2, RT3, RT4
• Residential Multi-Family: RM1, RM2, RM3, RM4, RM5, RM6, RM7
• Residential and Office Districts: RO1 and RO2
• Local Business: LB1, LB2, LB3
• Regional Business: RB1, RB2
• Neighborhood Shopping: NS1, NS2
• Commercial Service: CS
• Industrial: IM

Visit milwaukee.gov/mymilwaukeehome to identify the zoning district for your property. Go to the "Special Interest" section.

 

 

In general, ADUs are allowed when:

• There is no more than one ADU on each property.
• The ADU is smaller in gross livable area than the principal residence.
• Internal ADUs and Attached ADUs are only allowed if the ADU is located in or attached to a single-family residence.
• Detached ADUs are only allowed if the property has a single-family or 2-family residence.
• The principal residence on a property is occupied by the owner at the time of submitting a permit.
• The ADU is not taller than the principal residence.

 

 

Water & Sewer

The current City plumbing code (Milwaukee Code of Ordinances 225-17) and Water Works Service Rules and Regulations (3.1.1) require that each residential structure on a property have a direct connection to a sewer and water main. Detached ADUs permitted by zoning would need a connection to a public or private main sewer separate from sewer main of the principal residence on the property.

 

Lot Coverage & Lot Size

• In RS1 to RS5 and RT1 to RT3 districts, properties with an ADU may increase their maximum lot coverage by up to 15%.
• In RT4 and RS6 districts, the same 15% increase in lot coverage is allowed for properties with an ADU, if the lot coverage of accessory structures is reduced by an equal amount.
• Additionally, ADUs do not count toward the required lot area per dwelling unit for the principal residence.

 

DESIGN STANDARDS

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	Internal ADU	Attached ADU	Detached ADU
Floor area, minimum	300 sq. ft.	300 sq. ft.	300 sq. ft.
Floor area, maximum	1,000 sq. ft., but not larger than the largest dwelling unit; may exceed 1,000 sq. ft. if structure existed as of the effective date of this ordinance [August 2, 2025]; shall be located entirely on one level and shall not exceed the area of the first floor.	1,000 sq. ft., but not larger than the largest dwelling unit.	1,300 sq. ft. of habitable and parking areas on all levels, or 16% of the lot area, whichever is greater, but not to exceed 1,600 sq. ft. or the floor area of the largest dwelling unit.
Height, minimum	none	none	none
Height, maximum	Same as principal building.	Same as principal building.	Not greater than the height of the principal building on the parcel.
Front setback, minimum	Same as principal building.	Same as principal building.	Not less than the front setback of the principal building on the parcel.
Side setback, minimum	Same as principal building.	Same as principal building.	3 ft.
Rear setback, minimum	Same as principal building.	Same as principal building.	4 ft. from the alley lot line if the accessory dwelling unit building includes a garage with an overhead door facing the alley; 3 ft. if the accessory dwelling unit does not include a garage or includes a garage but the overhead door does not face the rear lot line.
Distance from main dwelling, minimum	Not applicable.	Not applicable.	The front-to-back minimum distance between the 2 buildings shall be 10 ft. The side-to-side minimum distance between the 2 residential buildings shall be 5 ft.
Pedestrian connections to street frontage	An accessory dwelling unit entry within a rear or side yard shall be connected to a street frontage by a paved walkway or access drive.	An accessory dwelling unit entry within a rear or side yard shall be connected to a street frontage by a paved walkway or access drive.	An accessory dwelling unit entry within a rear or side yard shall be connected to a street frontage by a paved walkway or access drive.
Balconies	Same as principal building.	Same as principal building.	Same as principal building.

FINANCING AN ADU

While there are no loans specific to ADU construction, some homeowners utilize financing tools for home rehabilitation. Others borrow against their home’s equity. Commonly homeowners will use a combination of tools to finance their ADU. Here are some examples of potential financing options. 

 

Cash-Out Refinance:

If your home has gone up in value, you can refinance your mortgage and take out some of that extra value as cash. This gives you money to build your ADU, and you’ll have just one mortgage that covers both your home and the new unit.
 

Home Equity Loan:

Homeowners may choose to take out a second mortgage on their home called a Home Equity Loan. You get a lump sum of money to use for your ADU, and you pay it back with fixed monthly payments. This loan will remain separate from your main mortgage.
 

Home Equity Line of Credit (HELOC):

HELOC opens a line of credit for homeowners for home repairs and additions. Loan holders pay interest on the money borrowed during the construction of their ADU. After the “draw period” ends, the homeowner will begin paying off the total loan amount. It’s flexible for projects with changing costs.
 

Construction-to-Permanent Loan:

Otherwise known as a “one-time close" loan, this financing tool combines the costs of building your ADU with your long-term mortgage. While the ADU is being built, the bank pays your contractor and you pay interest. Once it’s done, the loan turns into a regular mortgage you pay off over 15 or 30 years.
 

203(k) Rehabilitation Mortgage:

This FHA loan helps homeowners pay for repairs or upgrades for single-family homes. You must spend at least $5,000, and your home’s value must be less than the FHA mortgage limit for the area. New detached ADUS are not eligible for this mortgage. 

 

List created September 2025.