Wash. Rev. Code § 31.12.860
Neither a credit union nor its members may be taxed upon its shares and deposits as property. A credit union shall be taxable upon its real property and tangible personal property, and every credit union shall be termed a mutual institution for savings and neither it nor its property may be taxable under any law which exempts savings banks or institutions for savings from taxation. For all purposes of taxation, the assets represented by the regular reserve and other reserves, other than reserves for expenses and losses of a credit union, shall be deemed its only permanent capital, and in computing any tax, whether it be property, income, or excise, appropriate adjustment shall be made to give effect to the mutual nature of such credit union.
RCW 31.12.860