Washington Revised Code Title 35. Cities and Towns § 35.44.390. Supplemental assessments--When authorized

If by reason of any mistake, inadvertence, or other cause, the amount assessed was not equal to the cost and expense of a local improvement or that portion thereof to be paid by assessment of the property benefited the city or town council shall make supplemental assessments on all the property in the district. The property found to be specially benefited shall not be limited to the property included in the original assessment district.

These assessments shall be made in accordance with the provisions of law, charter, and ordinances existing at the time of the levy.