In post-war America, changes in the way families used their homes, as well as migration to the suburbs, resulted in the immense popularity of new types of home design.
A conventional two-story home design consists of a floor plan divided into two distinct levels, one directly above the other. The vertical separation between the two levels is full-height. One advantage of a two-story design is that it fits twice as much living space onto the same lot space as a single-story design -- an advantage that's important on small urban lots.
The ranch is a single-story design that was by far the most popular type of home in the United States in the 1950s. All of its living space is on a single level, and it has the advantage of no stairs or inconvenient vertical separation of different parts of the home's floor plan. It has the disadvantage, however, of putting both public and private living spaces on the same level and taking up much more lot space than a two-story home for the same amount of living area.
A split-level home design incorporates aspects of both a two-story design and a ranch home. In a typical split-level design, a conventional two-story section of the home is connected to a single-story section, with the single-story section meeting the two-story section at a level halfway between the two floors. The vertical distance between any two adjacent levels of the home is, then, a half flight of stairs rather than a full flight.
A split-level is sometimes called a tri-level design because it divides the home's floor plan into the three distinct levels. A common way to divide the use of the three levels is to use the upper level as private sleeping space, the middle level as public living space that includes the kitchen and living room, and the lower level as an informal family recreational and utility space. That way, public and private spaces are separated more than they are in a ranch-style home, but at less of a cost in terms of convenience when compared to a two-story design.
The split-level design was at the peak of its popularity in the suburban United States between the 1950s and 1970s.