A housing unit is classified as occupied if it is the usual place of residence of the individual or group of individuals living in it on Census Day, or if the occupants are only temporarily absent, such as away on vacation, in the hospital for a short stay, or on a business trip, and will be returning.
The occupants may be an individual, a single family, two or more families living together, or any other group of related or unrelated individuals who share living arrangements.
Occupied rooms or suites of rooms in hotels, motels, and similar places are classified as housing units only when occupied by permanent residents; that is, occupied by individuals who consider the hotel their usual place of residence or who have no usual place of residence elsewhere. However, when rooms in hotels and motels are used to provide shelter for people experiencing homelessness, they are not housing units. Rooms used in this way are considered group quarters.

A housing unit is classified as vacant if no one is living in it on Census Day, unless its occupant or occupants are only temporarily absent-such as away on vacation, in the hospital for a short stay, or on a business trip-and will be returning.

Housing units temporarily occupied at the time of enumeration entirely by individuals who have a usual residence elsewhere are classified as vacant. When housing units are vacant, the criteria of separateness and direct access are applied to the intended occupants whenever possible. If that information cannot be obtained, the criteria are applied to the previous occupants.

Boats, RVs, tents, caves, and similar shelter that no one is using as a usual residence are not considered living quarters and therefore are not enumerated at all.

Housing specifically for the older population has become more and more prevalent and is being identified by many different names. Living quarters in these facilities, unless they meet the definition of skilled nursing facilities, are housing units, with each residents living quarters considered a separate housing unit if it meets the housing unit definition of direct access. These residential facilities may be referred to as senior apartments, active adult communities, congregate care, continuing care retirement communities, independent living, board and care, or assisted living. People may have to meet certain criteria to be able to live in these facilities, but once accepted as residents they have unrestricted access to and from their units to the outside.

Housing units and group quarters may coexist under the same entity or organization and in some situations, actually share the same structure. An assisted living facility complex may have a skilled nursing floor or wing that meets the definition of a nursing facility and is, therefore, a group quarters, while the rest of the living quarters in the facility are considered to be housing units. Congregate care facilities and continuing care retirement communities often consist of several different types of living quarters, with varying services and levels of care. Some of the living quarters in these facilities and communities are considered to be housing units and some are considered to be group quarters, depending on which definition they meet.

The first Census of Housing in 1940 established the dwelling unit concept. Although the term became housing unit and the definition was modified slightly in succeeding censuses, the housing unit definition remained essentially comparable between 1940 and 1990. Since 1990, two changes were made to the housing unit definition.
The first change eliminated the concept of eating separately. The elimination of the eating criterion is more in keeping with the United Nations definition of a housing unit that stresses the entire concept of separateness rather than the specific eating element. Although the eating separately criterion previously was included in the definition of a housing unit, the data needed to distinguish whether the occupants ate separately from any other people in the building were not collected. (Questions that asked households about their eating arrangements have not been included in the census since 1970.) Therefore, the current definition better reflects the information that is used in the determination of a housing unit.

The second change for Census 2000 and the 2010 Census eliminated the number of nonrelatives criterion; that is, 9 or more people unrelated to the householder which caused a conversion of housing units to group quarters. This change was prompted by the following considerations: 1) there were relatively few such conversions made as a result of this rule in 1990; 2) household relationship and housing data were lost by converting these units to group quarters; and 3) there was no empirical support for establishing a particular number of nonrelatives as a threshold for these conversions.

In 1960, 1970, and 1980, vacant rooms in hotels, motels, and other similar places where 75 percent or more of the accommodations were occupied by permanent residents were counted as part of the housing inventory. However, an evaluation of the data collection procedures prior to the 1990 Census indicated that the concept of permanency was a difficult and confusing procedure for enumerators to apply correctly. Consequently, in the 1990 Census, vacant rooms in hotels, motels, and similar places were not counted as housing units. In Census 2000 and the 2010 Census, we continued the procedure adopted in 1990.