RCW 19.150.150: Late fees.

Any late fee charged by the owner shall be provided for in the rental agreement. No late fee shall be collected unless it is written in the rental agreement or as an addendum to such agreement. An owner may impose a reasonable late fee for each month an occupant does not pay rent when due. A late fee of twenty dollars or twenty percent of the monthly rental amount, whichever is greater, for each late rental payment shall be deemed reasonable, and shall not constitute a penalty.