Wash. Rev. Code § 35.92.240
All such assessments shall be levied upon the several parcels of land located within the local improvement district in accordance to the special benefits conferred on such property in proportion to the surface area, one square foot of surface to be the unit of assessment: PROVIDED, That where the water right is acquired or a special improvement is made for a portion of any district, the cost of the water right or the cost of such special improvement shall be levied in the same manner upon such portion of the district as shall be specially benefited thereby: PROVIDED FURTHER, That whenever a special improvement is made for a portion of any district, the land assessed for the cost thereof shall be entitled to an equitable reduction in the annual assessments in proportion to the reduced cost of operation on account of the construction of the improvement.
RCW 35.92.240