After the receipt of the report and recommendations of the planning agency on the matters referred to in RCW 36.70.430, or after the lapse of the prescribed time for the rendering of such report and recommendation by the commission, the board may approve by motion and certify such plan, change or addition without further reference to the commission: PROVIDED, That the plan, change or addition conforms either to the proposal as initiated by the county or the recommendation thereon by the commission: PROVIDED FURTHER, That if the planning agency has failed to report within a ninety day period, the board shall hold at least one public hearing on the proposed plan, change or addition. Notice of the time, place and purpose of the hearing shall be given by one publication in a newspaper of general circulation in the county and in the official gazette, if any, of the county, at least ten days before the hearing. Thereafter, the board may proceed to approve by motion and certify the proposed comprehensive plan or any part, amendment or addition thereto.