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Planning efforts of the mid- to late 19th century were grounded on a belief that the public interest would best be served by the exclusion of undesirable land uses, usually commercial and industrial uses, from residential areas.
By the early 20th century, newly adopted land use zoning controls expanded on the exclusionary nature of community planning by not only physically separating industrial, commercial, and residential zones, but also by distinguishing between single-family and multi-family residential zones. This had the effect of excluding many middle-to lower-income persons -– primarily renters –- from increasingly large swaths of land which (especially by the 1920s) were being designated as single-family districts. With multi-family zones limited to already built up and often deteriorated older neighborhoods, the less well-to-do were locked into the city core.
Many municipalities "refined" their exclusionary zoning practices in the mid-20th century by creating a series of single-family-only zones of increasing minimum property size, ranging from 5,000 square feet or less to 40,000 square feet or more. Since land price is a major contributor to housing affordability, this practice resulted in concentric bands of increasing exclusion, with low and moderate income citizens unable to afford homes in lower-density perimeter residential areas.
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