After the plan of sewerage devised in accordance with section 6119.19 of the Revised Code has been adopted and approved, the board of trustees of the regional water and sewer district shall designate such portions of the work as are required for immediate use. The designation shall be by districts or areas and shall show what districts, areas, or parts thereof are to be improved. The board may order its officers to make an estimate of the cost and expense of constructing the work or such portions thereof as have been designated and report them to the board.