Wash. Rev. Code § 5.60.030
No person offered as a witness shall be excluded from giving evidence by reason of his or her interest in the event of the action, as a party thereto or otherwise, but such interest may be shown to affect his or her credibility: PROVIDED, HOWEVER, That in an action or proceeding where the adverse party sues or defends as executor, administrator or legal representative of any deceased person, or as deriving right or title by, through or from any deceased person, or as the guardian or limited guardian of the estate or person of any incompetent or disabled person, or of any minor under the age of fourteen years, then a party in interest or to the record, shall not be admitted to testify in his or her own behalf as to any transaction had by him or her with, or any statement made to him or her, or in his or her presence, by any such deceased, incompetent or disabled person, or by any such minor under the age of fourteen years: PROVIDED FURTHER, That this exclusion shall not apply to parties of record who sue or defend in a representative or fiduciary capacity, and have no other or further interest in the action.
RCW 5.60.030
Purpose-Intent-Severability-1977 ex.s. c 80: See notes following RCW 4.16.190.