Three-family or triplex: three dwelling units that are either attached side by side and share shared walls, or are stacked (in some countries, called a three-decker or triple-decker). Four-family, quadruple, or quadruple: four dwelling units, often two on the first floor and two on the second, or side by side. Five-family: five dwelling units, usually arranged in two rows of two adjoining houses.
Six-family: six dwelling units, usually arranged in three rows of two adjoining houses. Seven-family: seven dwelling units, usually arranged in two rows of three adjoining houses.
Eight-family: eight dwelling units, usually arranged in two rows of four adjoining houses. Nine-family: nine dwelling units, usually arranged in a single row of three adjoining houses with a common wall along the front property line. Ten-family: ten dwelling units, usually arranged in a single row of four adjoining houses with a common wall along the front property line. Eleventh-family unit: an area designated for additional living units. Twelfth-family unit: a building used as a residence that has more than one floor.
Thirteenth-family unit: a structure used as a place of worship. Fourteen-family unit: a large apartment building with fourteen residential units.
Fifteen-family unit: a large office building with fifteen residential units.
A triplex, by extension, is a building with three different dwelling units, whereas a fourplex or quadplex has four independent living units. Thus, a triplex is one type of multi-dwelling unit (MDU) while a quadplex is another type of MDU.
The number of units in an MDU varies, but they usually range from three to eight rooms. Larger or smaller numbers of rooms are possible; for example, a duplex has two units each having five rooms. An MDU can be as small as a single room or as large as a mansion with multiple apartments. There are many different names for these structures including: apartment complex, apartment house, apartment hotel, apartment village, bachelor pad, bed & breakfast, boarding house, college dorm, co-op, efficiency apartment, foster home, group home, hospital apartment, infirmary, industrial building, industrial warehouse, isolator, junkyard cabin, kennel cot, lodging house, nursing home, office building, old people's home, pension house, private residence, rehabilitation center, retirement community, sanitarium, school dorm, shopping mall, theater apartment, trailer park, vacation rental, walk-up apartment.
Dwelling, three-family refers to a structure that is arranged or built to be occupied by three families, with just three dwelling units under one ownership, i.e., a triplex, on one property. In some states, this type of housing is called a "row house". This form of housing is most common in New England where single family homes are less common.
There are several types of triplexes, but they can be divided into two main groups: those containing three independent living units and those containing one large unit with separate apartments for each family member. Sometimes, as in the case of groupings of townhouses or condominiums, only one side of the building is used, while the other three or more stories contain additional apartment units. Other times, such as with quadruples (four-unit dwellings), the entire building is used to provide housing for four families.
Triplexes were very popular between 1960 and 1980 when their use was increasing due to changes in federal income tax laws that allowed homeowners to claim all three units on their taxes as separate properties instead of filing jointly. Since then, their use has declined because of problems associated with maintaining multiple houses under one ownership, increased demand for single family homes, and rising prices of triplexes which make them unaffordable to many first time home buyers.