Zoning is a legal tool employed by local governments to regulate land development. It determines the use, intensity, and form of development in localities through enforcement of the zoning ordinance, which consists of a text and an accompanying map that divides the locality into zones. Zoning is an exercise of the police powers by local governments, typically authorized through state statutes. Components of what became part of the zoning process emerged piecemeal in U.S. cities during the 19th century in response to development activities deemed injurious to the health, safety, and welfare of the community. American zoning was influenced by and drew upon models already in place in German cities early in the 20th century. Following the First National Conference on Planning and Congestion, held in Washington, DC in 1909, the zoning movement spread throughout the United States. The first attempt to apply a version of the German zoning model to a U.S. city was in New York City in 1916. In the landmark U.S. Supreme Court case, Ambler Realty v. Village of Euclid (1926), zoning was ruled as a constitutional exercise of the police power, a precedent-setting case that defined the perimeters of land use regulation the remainder of the 20th century.
Zoning was explicitly intended to sanction regulation of real property use to serve the public interest, but frequently, it was used to facilitate social and economic segregation. This was most often accomplished by controlling the size and type of housing, where high density housing (for lower income residents) could be placed in relation to commercial and industrial uses, and in some cases through explicit use of racial zoning categories for zones. The U.S. Supreme Court ruled, in Buchanan v. Warley (1917), that a racial zoning plan of the city of Louisville, Kentucky violated the due process clause of the14th Amendment. The decision, however, did not directly address the discriminatory aspects of the law. As a result, efforts to fashion legally acceptable racial zoning schemes persisted late into the 1920s. These were succeeded by the use of restrictive covenants to prohibit black (and other minority) occupancy in certain white neighborhoods (until declared unconstitutional in the late 1940s). More widespread was the use of highly differentiated residential zoning schemes and real estate steering that imbedded racial and ethnic segregation into the residential fabric of American communities.
The Standard State Zoning Enabling Act (SSZEA) of 1924 facilitated zoning. Disseminated by the U.S. Department of Commerce, the SSZEA created a relatively uniform zoning process in U.S. cities, although depending upon their size and functions, there were definite differences in the complexity and scope of zoning schemes. The reason why localities followed the basic form prescribed by the SSZEA was to minimize the chance of the zoning ordinance being struck down by the courts. Nonetheless, from the 1920s through the 1970s, thousands of court cases tested aspects of zoning, but only a few reached the federal courts, and typically, zoning advocates prevailed.
In the 1950s and 1960s, critics of zoning charged that the fragmented city was an unintended consequence. This critique was a response to concerns that zoning created artificial separations among the various types of development in cities, and that this undermined their vitality. Zoning nevertheless remained a cornerstone of U.S. urban and suburban land regulation, and new techniques such as planned unit developments, overlay zones, and form-based codes introduced needed flexibility to reintegrate urban functions previously separated by conventional zoning approaches.