The Board of Appeals may grant a special permit for the alteration and conversion of any dwelling located in a residence district into a dwelling designed and intended for occupancy by not more than two families; provided, however, that such building or structure was in existence prior to March 5, 1951; and provided, further, that such building or structure so altered and converted, in the opinion of the Board of Appeals, will be in harmony with and conform to the character and type of residences generally prevalent in the immediate neighborhood.

A building or portion of a building then or formerly in municipal use may be converted from that use to other use, except manufacturing, if authorized on special permit from the Planning Board, subject to the criteria of § 173-7C and subject to the following: