The City Council may provide for the payment of special assessments in annual installments, not to exceed 30 in number, the first installment being due upon confirmation of the assessment roll and the deferred installments being due annually thereafter, or in the discretion of the Council, may be spread upon and made a part of each annual city tax roll thereafter until all are paid. Interest may be charged upon deferred installments at a rate not to exceed 6% per year payable annually. Under any installment plan adopted, the whole or any deferred installments with accrued interest to date of payment may be paid in advance of the due dates established.