Wash. Rev. Code § 35.87A.080
For purposes of the special assessments to be imposed pursuant to this chapter, the legislative authority may make a reasonable classification of businesses and multifamily residential or mixed-use projects, giving consideration to various factors such as business and occupation taxes imposed, square footage of the business, number of employees, gross sales, or any other reasonable factor relating to the benefit received, including the degree of benefit received from parking. Whenever it is proposed that a parking and business improvement area provide more than one of the purposes listed in RCW 35.87A.010, special assessments may be imposed in a manner that measures benefit from each of the separate purposes, or any combination of the separate purposes. Special assessments shall be imposed and collected annually, or on another basis specified in the ordinance establishing the parking and business improvement area.
RCW 35.87A.080