Wash. Rev. Code § 39.92.040
The program shall describe the formula or method for calculating the amount of the transportation impact fees to be imposed on new development within the plan area. The program may require developers to pay a transportation impact fee for off-site transportation improvements not yet constructed and for those jointly-funded improvements constructed since the commencement of the program.
The program shall define the event in the development approval process that triggers a determination of the amount of the transportation impact fees and the event that triggers the obligation to make actual payment of the fees. However, the payment obligation shall not commence before the date the developer has obtained a building permit for the new development or, in the case of residential subdivisions or short plats, at the time of final plat approval, at the developer's option. If the developer of a residential subdivision or short plat elects to pay the fee at the date a building permit has been obtained, the option to pay the transportation impact fee by installments as authorized by this section is deemed to have been waived by the developer. The developer shall be given the option to pay the transportation impact fee in a lump sum, without interest, or by installment with reasonable interest over a period of five years or more as specified by the local government.
The local government shall require security for the obligation to pay the transportation impact fee, in the form of a recorded agreement, deed of trust, letter of credit, or other instrument determined satisfactory by the local government. The developer shall also be given credit against its obligations for the transportation impact fee, for the fair market value of off-site land and/or the cost of constructing off-site transportation improvements dedicated to the local government. If the value of the dedication exceeds the amount of transportation impact fee obligation, the developer is entitled to reimbursement from transportation impact fees attributable to the dedicated improvements and paid by subsequent developers within the plan area.
Payment of the transportation impact fee entitles the developer and its successors and assigns to credit against any other fee, local improvement district assessment, or other monetary imposition made specifically for the designated off-site transportation improvements intended to be covered by the transportation impact fee imposed pursuant to this program. The program shall also define the criteria for establishing periodic fee increases attributable to construction and related cost increases for the improvements designated in the program.
RCW 39.92.040