Wash. Rev. Code § 35.87A.090
The special assessments need not be imposed on different classes of business and multifamily residential or mixed-use projects, as determined pursuant to RCW 35.87A.080, on the same basis or the same rate. The special assessments imposed for the purpose of the acquisition, construction or maintenance of parking facilities for the benefit of the area shall be imposed on the basis of benefit determined by the legislative authority after giving consideration to the total cost to be recovered from the businesses and multifamily residential or mixed-use projects upon which the special assessment is to be imposed, the total area within the boundaries of the parking and business improvement area, the assessed value of the land and improvements within the area, the total business volume generated within the area and within each business, and such other factors as the legislative authority may find and determine to be a reasonable measure of such benefit.
RCW 35.87A.090