Without knowing the specifics, no one can give a certain answer. However, if it predates the zoning ordinance, it is likely an existing non-conformity. Or if built after the zoning ordinance was adopted, a variance might have been granted.
Look at the zoning ordinance, which is likely available online. It will explain about such situations, and limitations on expanding such non-conforming structures, provisions so that such buildings can be maintained, etc.