Louisville will allow new accessory dwelling units — sometimes known as “granny flats” or “mother-in-law apartments” — with the City Council aiming to make sure the size of the housing units are more equitable in size for larger and smaller homes.

An ADU is an attached or detached housing unit located on the same lot as the primary home, and it can be rented out. ADUs provide a place for living, sleeping, eating, cooking and sanitation. Louisville code previously did not address ADUs, meaning they were not in compliance with city policy. The City Council unanimously voted to amend the code in order to comply with a state law requiring certain municipalities, like Louisville, to allow ADUs in all zoning districts that allow single-family housing. The deadline to comply with the law is June 30, meaning Louisville needed to have an ordinance allowing ADUs by then.

“Zoning” means a city’s rules for what types of property developments can be built in different parts of town.

While the state law outlines what regulations municipalities can and cannot include regarding ADUs, the Louisville City Council expanded on what an ADU in Louisville can look like during its Tuesday night meeting.

Councilmember Judi Kern said the intent of the city’s ADU code is to make it easier for residents to age in place.

City staff previously acknowledged some number of ADUs may already exist in Louisville and said any existing units would be grandfathered into the new rules.

Attached ADUs in Louisville can be built in front of the primary home, with the addition that detached ADUs in the Old Town neighborhood can also be in front of the primary residence, city Planning Manager Jeff Hirt said.

During an April 15 meeting, the council looked at ways to ensure that homes in small and large lots have more equitable access to creating an ADU of a decent size. The ADU is able to be built up to 75% of the size of the primary home, capped at 800 square feet for a detached ADU and 1,000 square feet for an attached ADU.

While the city made changes to ensure that ADUs at small and large homes can be built more equally in size, ADUs built in basements are the exception. Multiple councilmembers argued putting certain limits on the size of a basement ADU that does not change the facade of the home is an unnecessary rule.

Instead, basement ADUs, whether fully or partially below grade, do not count toward size limitations. An ADU cannot be larger than the primary home.

“On a big lot, you could have quite a large ADU, which I think is probably a good thing,” Councilmember Dietrich Hoefner said.