Wash. Rev. Code § 84.40.380
All taxes made payable pursuant to the provisions of RCW 84.40.350 through 84.40.390 shall be due and payable to the county treasurer on or before the thirtieth day of April in the event the date of execution of the instrument of transfer occurs prior to that date unless the time of payment is extended under the provisions of RCW 84.56.020. Such taxes shall be due and payable on or before the thirty-first day of October in the event the date the property lost its exempt status is subsequent to the thirtieth day of April but prior to the thirty-first day of October. In all other cases such taxes shall be due and payable within thirty days after the date the property lost its exempt status. In no case, however, shall the taxes be due and payable less than thirty days from the date the property lost its exempt status. All taxes due and payable after the dates herein shall become delinquent, and interest at the rate specified in RCW 84.56.020 for delinquent property taxes shall be charged upon such unpaid taxes from the date of delinquency until paid.
RCW 84.40.380