Secondary suites (also known as accessory dwelling units (ADU), in-law apartments, granny flats, granny annexes or garden suites) are self-contained apartments, cottages, or small residential units, that are located on a property that has a separate main, single-family home, duplex, or other residential unit. In some cases, the ADU or in-law is attached to the principal dwelling or is an entirely separate unit, located above a garage, across a carport, or in the backyard on the same property. Reasons for wanting to add a secondary suite to a property may be to receive additional income, provide social and personal support to a family member, or obtain greater security.

Missing middle housing refers to a lack of medium-density housing in the North American context. The term describes an urban planning phenomenon in Canada, the United States, Australia and more recent developments in industrialised and newly industrialising countries due to zoning regulations favoring social and racial separation and car-dependent suburban sprawl.