A dwelling unit is defined as the living quarters occupied by, or intended for occupancy by, one household. A dwelling unit may be a detached house; a tenement, flat, or apartment in a larger building (apartment house, apartment hotel, or section of a hotel devoted entirely to apartment rather than transient use); or a room in a structure used primarily for business or other nonresidential purposes. It may be a tourist cabin, trailer, railroad car, boat, etc., if occupied by person having no other place of residence.

Dwelling units are classified into 11 groups, according to the type of structure in which they are located. A 1-family detached structure has open space on all four sides and contains only one dwelling unit. A 1-family attached structure has one or both sides built directly against an adjoining structure or structures. Each dwelling unit in a group of three or more row houses is a 1-family attached structure, but two attached dwelling units by themselves constitute a 2-family side-by-side structure.