A duplex is a structure that comprises two separate family units. Duplexes can be attractive because, though they cost more than a single-family dwelling, the net payment for an owner-occupant after rental income can be cheaper than buying a single-family unit.
Single-story duplexes have two one-story units side-by-side, both on the ground floor. This type of duplex, which does not have stairs to climb, may appeal to families with young children and senior citizens.
Two-Story: One Unit Up, One Down
This duplex design separates the units by placing them each on a different floor, which helps it accommodate a more diverse field of tenants. Some people prefer to sleep with a window open, but are uncomfortable doing so on the first floor for security reasons; they would be attracted to the upper level. The lower level would be attractive to renters who prefer single-story duplexes -- those who would prefer not to use stairs.
The two-story, side-by-side duplexes have two, two-level units sharing a common wall. Privacy and footprint size are two advantages of this style of duplex. This style of construction separates the private living spaces -- bedrooms and bathrooms -- from the common areas. It also is useful on smaller lots, offering extra square footage by building up instead of out.