In all cases where the board shall issue bonds to pay the cost and expense of any county road improvement district and shall provide that the whole or any part of the cost and expense shall be assessed against the lots, tracts, parcels of land, and other property therein, the resolution levying such assessment shall provide that the sum charged thereby against each lot, tract, or parcel of land or any portion of said sum may be paid during the thirty day period provided for in RCW 36.88.270 and that thereafter the sum remaining unpaid may be paid in equal annual installments, the number of which installments shall be less by two than the number of years which the bonds issued to pay for the improvement may run. Interest upon all unpaid installments shall be charged at a rate fixed by said resolution. Each year such installments together with interest due thereon shall be collected in the manner provided in the resolution for the collection of the assessments.