(1) For employees covered by chapter 41.06 RCW who work less than full-time, it shall be presumptively appropriate to include those employees in the same bargaining unit with full-time employees performing similar work.

(2) The presumption set forth in this section is intended to avoid excessive fragmentation and a potential for conflicting work jurisdiction claims which would otherwise exist in separate units of full-time and less than full-time employees.

(3) The presumption set forth in this section shall be subject to modification by adjudication.