The lawful use of a building or land existing at the time of the adoption of Ordinance 2816, passed January 5, 1949, may be continued, although such use does not conform to the regulations of the district in which it is maintained. An existing nonconforming use may be extended throughout a building, provided no structural alterations are made. An existing nonconforming use of a building or premises may be changed to another nonconforming use of the same or a higher classification, but no building or premises in which a nonconforming use has been changed to a more restricted use, shall again be devoted to a less restricted use.
   In a Residence A District, Residence B District or a Residence C District, wherever a nonconforming use of a building or portion thereof has been discontinued for a period of at least one year, such nonconforming use shall not thereafter be reestablished and the future use shall be in conformity with the provisions of this Zoning Ordinance.
   Any nonconforming building or structure damaged by fire, explosion, flood or act of God may be reconstructed and used as before such calamity, providing the building is not
substantially destroyed.
(1955 Code. §1151.01)