American Museum of Science and Energy in Oak Ridge, Tennessee, has a display of a so-called "flat-top" house. The population of Oak Ridge grew rapidly during the Manhattan Project, outstripping all of the planning that was done and producing a persistent housing shortage. The Tennessee Valley Authority, which was working closely with the Army on the project, offered plans for small houses they had used during the construction of Fontana Dam in North Carolina. These small flat-top houses could be built quickly in an off-site factory and trucked in sections to the home site, often with furniture already inside.
alphabet houses, most flat-tops (they're still called that) are rental units today. Many rental units are in poor repair, but those in private ownership often have been remodeled and enlarged to the point they are no longer recognizable as flat-tops.