A nonconforming building or structure shall not be structurally altered or reconstructed, except for such alteration, maintenance and repair work as required to keep said building or structure in safe condition. However, a nonconforming building which is damaged or destroyed by fire, flood, wind, other calamity, act of God or act of the public enemy may be restored, and the nonconforming occupancy thereof may be resumed or continued, provided that the restoration is started within a period of one year, is diligently prosecuted to completion and does not increase either the floor area or the cubical contents of such building.