prior to the 1920s, commercial rooming houses were often former boardinghouses. after the us civil war, boarding houses became less common, declining from 40% of rental listings in 1875 (in san francisco) to 10% in 1900, and less than 1% by 1910. one reason for this change was that in the decades after the 1880s, urban reformers began working on modernizing cities; their efforts to create "uniformity within areas, less mixture of social classes, maximum privacy for each family, much lower density for many activities, buildings set back from the street, and a permanently built order" all meant that housing for single people had to be cut back or eliminated. by the early 1930s, urban reformers were typically using codes and zoning to enforce "uniform and protected single-use residential district[s] of private houses", the reformers' preferred housing type. in 1936, the fha property standards defined a dwelling as "any structure used principally for residential purposes", noting that "commercial rooming houses and tourist homes, sanitariums, tourist cabins, clubs, or fraternities would not be considered dwellings" as they did not have the "private kitchen and a private bath" that reformers viewed as essential in a "proper home".