WAT § 35520.29

The board shall levy an amendatory assessment in the manner provided for original assessments if any of the following occur:

(a) Any land within the drainage service area is omitted from any assessment roll.

(b) Land appears on the assessment roll but is neither assessed or stated not to be assessed.

(c) A final judgment of a court of competent jurisdiction has held that any assessment is not valid as to any part of the land assessed. This amendatory assessment shall be based upon a determination, as nearly as may be, of the amount the original assessment upon the land would have been except for the omission or invalidity.