In-law apartments, also called mother-in-law apartments or secondary suites, are becoming increasingly common in the United States. These kinds of apartments are created when grown children build private space on their own property for their aging parents. Legally, an in-law apartment is called an accessory dwelling unit – an additional separate living space located on a property where only one unit would normally be. In-law apartments are usually not part of the home's original design and are added to existing parts of the home.

пристройка для бабушки

Thank you!

Почему для бабушки, для тещи)

GreatBritain: "бабушкина квартира" (небольшая квартира для пожилого члена семьи; обычно пристраивается к дому)
mother-in-law apartment AmE = granny flat BrE