In general, you must file an affidavit when ownership or title to real property transfers by conveyance, deed, grant, assignment, quitclaim, or any other document that will result in a transfer.
Washington State Department of Revenue REET Links
DOR REET Forms
Taxability of Transfer Exemptions
REET Exemption Rules - Chapter 458-61A WAC: Washington Administrative Code
Whatcom County Treasurer's Office
Property Transfers FAQ
Completed Real Estate Excise Tax Affidavits
Search for completed Real Estate Excise Tax (REET) affidavits by Excise Number