No existing building designed, arranged or intended for or devoted to a use not permitted under the regulations of this title for the district in which such building or premises is located shall be enlarged, extended, reconstructed or structurally altered, unless such is changed to a use permitted under the regulations specified by this title for such district in which the building is located; providing, however, that authorized maintenance shall be permitted not exceeding a total amount, during a period of five years, of fifty percent of the assessed value of the building according to the assessments thereof by the city assessor.
(Prior code § 3124.03)