Duplexes can be configured in two ways. They might be one or two-story individual flats next to one other. This is a two-by-two arrangement. A duplex is likewise two levels, but each unit occupies one floor, with one apartment above the other. This is a stacked duplex, sometimes known as a one-up-one-down duplex. Which method you choose depends on what you're trying to accomplish. A one-or-two-story flat next to another similar flat usually requires four units per lot, assuming a standard width between buildings. If there's room for more parking, then five or six units can be built. Each unit needs its own front door, which increases the cost of construction. A duplex uses only two units per lot, so it takes fewer lots to cover the same ground. It's also cheaper because you don't need as many doors. The most efficient use of space is important when building small projects. For example, if the total combined area of your duplexes is less than 200 square feet (19 sq m), then they should be one-story structures with no more than two units per lot. Otherwise, you'll need too much land and the project will become too expensive. Large projects often use stacked duplexes. These can be two different sizes, like 100 square feet (9.3 sq m) and 150 square feet (13.6 sq m). Or they may be one large apartment divided into two separate rooms.