All executive and administrative offices, agencies, and instrumentalities of the city and their respective powers and duties, except as otherwise provided by law or this Charter, shall be allocated among and within the following six principal departments. The Council, based on recommendations of the City Manager, to the extent permitted by law, may create or abolish other departments, make changes in the assignment of functions among the six principal departments, or add to, or subtract from the functions allocated to any one department when it considers such change necessary for the efficient administration of city government.