From the 1910s through 1930s, the usual way people built houses was by going to the local lumberyard or contractor and looking through booklets of designs called plan books. They’d pick out a design, purchase the blueprints, and let their contractor take care of the rest. “What we’ve found is that the styles of these houses are often almost exact matches to models Sears offered,” says Judith Chabot, a member of the research group behind the National Database of Sears Houses who runs the popular Sears House Seeker blog.

To make matters more confusing, kit house manufacturers “purloined each other’s designs with impunity,” says Jeffrey Gabel, an architectural historian and kit home hunter. For example, the 1913 cover of the Chicago Millwork Supplies catalog displayed a drawing of a home that was a direct copy of the kit home manufacturer Sterling’s “Vernon” design. The “Vernon” was then copied again in 1923 by the National Homebuilder’s Society in Chicago, which offered catalog plans for its version of the design—renamed “The Oakland”—even brazenly using Sterling’s own photograph for its catalog illustration.

By 1928, a decline in the volume of lumber sold by local lumberyards caused the Northeastern Retail Lumber Association to describe kit home companies as “insidious competition,” and “a ready cut racket.”  The widespread stealing of designs by both lumberyards and kit home manufacturers means that even if a house looks nearly identical to one in a kit home catalog, it might not be. Original mortgage records or blueprints and shipping labels on millwork are essentially the only inarguable proof you’ve got a kit house on your hands. Short of that, researchers often rely on a combination of highly specific stencil markings (down to the serif or the angle on the curve of a “2” printed on a piece of wood) and a physical catalog match to make a strong, though rarely absolute, case.