It shall be the function and duty of the planning commission, after holding public hearings, to make and adopt and certify to the city council a general plan for the physical development of the city. The general plan, with the accompanying maps, charts, and descriptive and explanatory matter, shall show the commission recommendation for the said physical development and may include, among other things, the general location and extent of street. The planning commission, after holding a public hearing thereon, may from time to time amend, extend, or add to the plan or carry any part of the subject matter into greater detail.