Wash. Rev. Code § 8.12.310
On or before the final hearing, the affidavit of one or more of the commissioners shall be filed in said court, stating that they have sent, or caused to be sent, by mail, to the owners whose property has been assessed and whose names and addresses are known to them, the notice hereinbefore required to be sent by mail to the owners of the property assessed. They shall also cause to be filed the affidavit of the person who shall have posted the notice required by this chapter to be posted, setting forth when and in what manner the same was posted. Such affidavits shall be received as prima facie evidence of a compliance with this chapter in regard to giving such notices. They shall also file an affidavit of publication of such notice in like manner as is required in other cases of affidavits of publication of notice of [or] summons.
RCW 8.12.310