No existing building, structure or premises devoted to a use not permitted by existing zoning regulations in the district in which such building or premises is located, except when required to do so by law or ordinance, shall be enlarged, extended, reconstructed or structurally altered, unless such use is changed to one permitted in the district in which such building or premises is located, or an exception has been granted by the Board of Zoning Appeals. (Code 1995 § 48-28.04.)