Any lot lawfully in existence on March 16, 1966, located in a Residence C District and conforming to the requirements for said districts then in effect, may be built upon for a one-family house and the uses and buildings accessory thereto, or the one-family use of such lot may be added to, expanded or structurally altered, provided that all said uses, buildings, expansion or alterations conform to the requirements of the bylaw for Residence C Districts in effect on March 16, 1966.