Extensions of electric distribution lines made after June 5, 1968, which are necessary to furnish permanent electric service to new residential buildings and mobile homes, shall be made underground. The extensions of service shall be made by the utility in accordance with the provisions in these regulations. These regulations apply to: Extensions of electric distribution lines in subdivisions where primary, secondary, and service lines will be placed underground to provide permanent electric service to any number of residential buildings or mobile homes; and An extension to provide permanent electric service to a single residential building or to a mobile home.