If any council or board desires to avail itself of the powers conferred by this chapter it shall create a city or county planning commission consisting of from three to twelve members to be appointed by the mayor or chair of the municipality and confirmed by the council or board: PROVIDED, That in cities of the first class having a commission form of government consisting of three or more members, the commissioner of public works shall appoint the planning commission, which appointment shall be confirmed by a majority of the city commissioners. Cities of the first class operating under self-government charters may extend the membership and the duties and powers of its commission beyond those prescribed in this chapter.