While April 15 is the deadline to file federal income tax returns (automatic filing extensions are available until October 15 with Form 4868) the first half of the year's property taxes are due April 30.
Washington State property tax statements are mailed by the county treasurer in February of each year. To avoid interest and penalties, at least half of the amount due must be paid by April 30 (if the tax is less than $50 it must be paid in full by April 30) and the balance by October 31.
Many counties are now accepting electronic payments via the county treasurer website. Check with your county treasurer to see if this option is available. When paying by mail, state officials suggest writing the tax parcel or account number on your check and include the tax statement payment stub.