In-law apartments, also called mother-in-law apartments or secondary suites, are becoming increasingly common in the United States -- up 30 percent from 2000 to 2010. 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.