(1) Except as provided in RCW 39.76.020, every state agency, county, city, town, school district, board, commission, or any other public body shall pay interest at a rate of one percent per month, but at least one dollar per month, on amounts due on written contracts for public works, personal services, goods and services, equipment, and travel, whenever the public body fails to make timely payment.
(2) For purposes of this section, payment shall be timely if:
(a) Except as provided otherwise in this subsection, a check or warrant is mailed or is available on the date specified for the amount specified in the applicable contract documents but not later than thirty days of receipt of a properly completed invoice or receipt of goods or services, whichever is later. If a contract is funded by grant or federal money, the public body shall pay the prime contractor for satisfactory performance within thirty calendar days of the date the public body receives a payment request that complies with the contract or within thirty calendar days of the date the public body actually receives the grant or federal money, whichever is later.
(b) On written contracts for public works, when part or all of a payment is going to be withheld for unsatisfactory performance or if the payment request made does not comply with the requirements of the contract, the public body shall notify the prime contractor in writing within eight working days after receipt of the payment request stating specifically why part or all of the payment is being withheld and what remedial actions must be taken by the prime contractor to receive the withheld amount.
(c) If the notification by the public body required by (b) of this subsection does not comply with the notice contents required under (b) of this subsection, the public body shall pay the interest under subsection (1) of this section from the ninth working day after receipt of the initial payment request until the contractor receives notice that does comply with the notice contents required under (b) of this subsection.
(d) If part or all of a payment is withheld under (b) of this subsection, the public body shall pay the withheld amount within thirty calendar days after the prime contractor satisfactorily completes the remedial actions identified in the notice. If the withheld amount is not paid within the thirty calendar days, the public body shall pay interest under subsection (1) of this section from the thirty-first calendar day until the date paid.
(e)(i) If the prime contractor on a public works contract, after making a request for payment to the public body but before paying a subcontractor for the subcontractor’s performance covered by the payment request, discovers that part or all of the payment otherwise due to the subcontractor is subject to withholding from the subcontractor under the subcontract for unsatisfactory performance, the prime contractor may withhold the amount as allowed under the subcontract. If the prime contractor withholds an amount under this subsection, the prime contractor shall:
(A) Give the subcontractor notice of the remedial actions that must be taken as soon as practicable after determining the cause for the withholding but before the due date for the subcontractor payment;
(B) Give the contracting officer of the public body a copy of the notice furnished to the subcontractor under (e)(i)(A) of this subsection; and
(C) Pay the subcontractor within eight working days after the subcontractor satisfactorily completes the remedial action identified in the notice.
(ii) If the prime contractor does not comply with the notice and payment requirements of (e)(i) of this subsection, the contractor shall pay the subcontractor interest on the withheld amount from the eighth working day at an interest rate that is equal to the amount set forth in subsection (1) of this section.
(3) For the purposes of this section:
(a) A payment is considered to be made when mailed or personally delivered to the party being paid.
(b) An invoice is considered to be received when it is date-stamped or otherwise marked as delivered. If the invoice is not date-stamped or otherwise marked as delivered, the date of the invoice is considered to be the date when the invoice is received.