Wash. Rev. Code § 84.34.380
Farm and agricultural land or timberland on which the right to future development has been acquired by any local government, the state of Washington, or the United States government shall be exempt from special benefit assessments in lieu of assessment for such purposes in the same manner, and under the same liabilities for payment and interest, as land classified under this chapter as farm and agricultural land or timberland, for as long as such classification applies.
Any interest, development right, easement, covenant, or other contractual right which effectively protects, preserves, maintains, improves, restores, prevents the future nonagricultural or nonforest use of, or otherwise conserves farm and agricultural land or timberland shall be exempt from special benefit assessments as long as such development right or other such interest effectively serves to prevent nonagricultural or nonforest development of such land.
RCW 84.34.380