The improvement district assessment may be made payable in not to exceed 10 annual installments and the board shall, at the time of the levy of the annual assessments of the district, if any, add to the amount of the annual assessment levied upon the lands within the improvement district, such amount for which they may be liable by reason of the levying of the improvement district assessment, and if the annual district assessment is made payable in two installments, the improvement district assessment or installment thereof shall likewise be made payable in two installments.