In 1970, AV Jennings offered to home builders the Chester. The Chester cost $13 740 which they claimed was the same price as building a one storey house. The argument was that by going up, less space at ground level was needed for building, freeing you up for a larger backyard.
Source: AV Jennings. 1970. "Just Built: The First Two Storey Home at a One Storey Price (Advertisement)." The Sunday Telegraph, March 29: 85.