The user surcharge for public storm drainage shall be $2/month/EDU. Effective February 1, 2008 and on February 1 of each calendar year thereafter, the charge shall be adjusted to reflect annual inflation in the cost to construct storm drainage facilities as is shown in the construction cost index for Seattle, Washington, published in the ENR Engineering News-Record in January of each year. Measurement of EDU as to property other than improved single-family dwellings shall be according to the method employed by the Unified Sewerage Agency of Washington County and shall be rounded to the nearest whole number. [BC 4.06.030, added by Ordinance No. 3687, 7/17/89; amended by Ordinances No. 3933, 6/26/95, Ordinance No. 3986, 7/14/97]