Section 29.45.250 - Rates of penalty and interest(a) A penalty not to exceed 20 percent of the tax due may be added to all delinquent taxes, and interest not to exceed 15 percent a year shall accrue upon all unpaid taxes, not including penalty, from the due date until paid in full. A municipality may impose a penalty not to exceed 20 percent of the tax due upon the late return of personal property assessment forms. A penalty under this section may be imposed according to a formula that increases the amount of the penalty as the length of time increases during which payment is delinquent or assessment forms are not returned.(b) If a taxpayer is given the right to pay the tax in two installments, penalty and interest on an unpaid installment accrues from the date the installment becomes due.