Non-conforming use refers to a structure, which function does not match its zoning district. For example, a factory that exists in a residential district is considered non-conforming. Similarly, a residential property built in a manufacturing zone is non-conforming.
The implementation of NYC’s 1961 zoning resolution created thousands of buildings across the city that no longer conformed to the new zoning regulations. However, the resolution did—and still does in its subsequent updates—allow existing non-conforming buildings to operate, and provides guidelines through which these buildings can eventually conform to the zoning districts where they reside, thus becoming ‘as of right’ properties. (Metropolisny, 2017)
Zoning regulations limit the enlargement, extension or alteration of any existing non-conforming property, but they do provide various guidelines for properties to enlarge and renovate, as long as they abide by the bulk regulations of their particular zoning district. Owners of a non-conforming property can make structural alterations if the purpose is to 1) comply with the law, 2) accommodate a conforming use, 3) conform to district regulations, or 4) enlarge in accordance with the bulk regulations of the specified district. (NYC Zoning Resolution, Chapter II, Article 52-22)
There are many examples, in NYC and other cities, where old factories or hospitals have been converted into lofts or apartment buildings. In some cases this is done in order to conform to residential zoning districts, while in other areas like Williamsburg, industrial areas are being controversially rezoned in order to convert factory space into residential space. (ny.curbed.com, 2019)
Developers may request a zoning designation change if they wish to build a commercial center, say, in a residential zone, or - like the Williamsburg example - convert an old factory into a loft complex within an industrial zone. This will require a Uniform Land Use Review Process (ULURP), where a community board and the city planning commission send a recommendation to the City Council either in favor or against the proposed development. (New York Times, 2018)