Wash. Rev. Code § 39.04.040
Whenever plans and specifications shall have been filed the work to be done shall be executed in accordance with such plans and specifications unless supplemental plans and specifications of the alterations to be made therein shall be made and filed in the office where the original plans and specifications are filed.
In the event that the probable cost of executing such work in accordance with the supplemental plans and specifications shall be increased or decreased from the estimated cost as shown by the original estimate to an amount in excess of ten percent of such estimate, then a supplemental estimate shall be made of the increased or decreased cost of executing the work in accordance with the supplemental plans and specifications and filed in the office where the original estimate is filed.
RCW 39.04.040