A basement apartment is an apartment located below street level, underneath another structure—usually an apartment building, but possibly a house or a business. Cities in North America are beginning to recognize these units as a vital source of housing in urban areas and legally define them as an accessory dwelling unit or "ADU". Rent in basement apartments is usually much lower than it is in above-ground units, due to a number of deficiencies common to basement apartments. The apartments are usually cramped, and tend to be noisy, both from uninsulated building noises and from traffic on the adjacent street. They are also particularly vulnerable to burglary, especially those with windows at sidewalk level. In some instances, residential use of below-ground space is illegal, but is done anyway in order for the building owner to generate extra income.

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.