RCW 82A.04.430: Allocation and apportionment—Nonresident compensation.

(1) Unless provided otherwise in this chapter, a nonresident individual is subject to tax on the portion of federal adjusted gross income derived from employment within the state of Washington, regardless of the location of the commercial domicile of the employer.

(2) Compensation for services performed by a nonresident as part of their employment must be allocated to this state to the extent such services are rendered within the state. If services are performed both within and outside the state, the compensation must be apportioned based on the ratio of days worked in the state to total days worked, or by another reasonable method approved by the department.

(3) For the purpose of this section, the following definitions apply:

(a) "Compensation" means wages, salaries, commissions, and any other form of remuneration paid to employees for personal services.

(b) "Employment" means personal service, of whatever nature, as known to the common law or any other legal relationship performed for an employer by an individual for compensation or under any contract calling for the performance of personal services, written or oral, express or implied, where the employer is subject to tax under RCW 50.24.010 on any portion of compensation paid by the employer to the individual for the performance of the personal services.