What the law says
Here's what Bothell Municipal Code 18.04.260 says about pressure washing:
"b. Conditional Discharges. The following discharges shall not be considered illegal discharges for the purposes of this chapter if they meet the stated conditions or unless the director determines that the type of discharge, whether singly or in combination with others, is causing or is likely to cause pollution of surface water or ground water:
(4) Street and sidewalk wash water, water used to control dust, and routine external building wash down that does not use detergents are permitted if the amount of street wash and dust control water used is minimized. At active construction sites, street sweeping must be performed prior to washing the street."