Duplexes, or duplex homes, are structures in which two housing units share a common central wall. In most cases, a duplex home is configured to allow the two units to share a common vertical wall. However, there are some designs where the units are placed on separate floors, with the barrier servings as the floor for one unit and the ceiling for the ground unit. For the most part, a true duplex home will occupy no more ground space than an average sized house, even though the structure provides living space for two households.

In actual practice, many people use the term duplex home to refer to any type of structure where up to four living units are connected by central wall. This is not completely accurate, since a two-story structure containing four living units would be more accurately referred to as a quadplex or quadraplex home. However, the two latter terms are rarely used, and the building is much more likely to be referred to as a duplex apartment.

There are two basic building plans that can rightly be referred to as duplex homes. The first and more common plan involves a structure that is two stories in height. The design is usually characterized by entrances for both units on the front and back of the building on the ground floor. Inside, the bottom floor of each unit will contain common rooms such as a living room, den, kitchen, bath, and dining room. The second floor is reserved for full baths and bedrooms. This design is normally referred to as a full duplex.

The half duplex is another common design for a duplex home. With this design, there is no second floor for either of the two units. All living space is confined to the main floor. As with the full duplex, each unit in a half duplex home will have two entrances, although the design may call for side entrances instead of the more traditional back entrances. Half-duplex living spaces are usually more compact and better suited to singles or couples, while a full duplex can more easily accommodate a family.

A duplex home may be purchased as a single building, even if it carries two different mailing addresses. It is not unusual for owners of duplexes to live in one unit and rent out the other as a source of income. However, the laws in some metropolitan areas make it possible to treat both units as a separate house, making it possible to sell each unit in the duplex house to different owners.