A building or structure, the use of which does not conform to the use regulations for the district in which it is situated, shall not be enlarged or extended unless such building or structure, including such enlargement or extension, is made to conform to all regulations, including use, for the district in which it is situated; provided, however, that where the public welfare, health or safety warrants a reasonable and necessary extension, such extensions may be temporary or permanent, and permission therefor shall be obtained by application to and approval by the Board of Appeals.