Zoning bylaws are meant to reflect the desired physical composition of an urban area, specifying the permitted uses and standards of individual properties. They outline a site's authorized functions, built form, height, and density, but as the city evolves, mass updates to the zoning bylaws can alter these permissions. The revisions often result in legal nonconforming uses or structures, where the use or built form of a property that complied with previous zoning regulations no longer adheres to the new regulations. This is also known as "grandfathering."

Toronto's zoning map outlines the permitted property uses across the city, image via City of Toronto

While the use or built form of a particular site may be contrary to prevailing attitudes and the public interest, a nonconforming use or structure does not need to be altered to reflect the new zoning rules. The property can continue to operate legally despite not meeting the most recent zoning specifications. It is generally believed that requiring a property to immediately cease its use would be unfair, and in some countries, likely unconstitutional. However, if a property is abandoned or its use is discontinued — usually for six months or one year — it loses its legal nonconforming status. Local restrictions will often prohibit or place strict limitations on the addition and expansion of nonconforming uses and structures.

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