Wash. Admin. Code § 458-12-180
Leasehold interests held by public-exempt bodies are exempt from taxation. The property on which they are located is assessable to the owner and its taxability is in no way affected by the leasehold interest. (AGO 1-30-1937; AGO 8-30-1934)
Improvements made by the public-exempt body in or upon the realty of a private taxpayer shall become part of the realty for taxation purposes unless it clearly appears otherwise. (TCR 5-12-1948)
Whenever it should appear that title to the improvements remain in the public-exempt body, the assessor shall ascertain whether or not the owner of the realty has any taxable interest in the improvements. If he does, he shall be taxed on this interest and not the improvements. If he doesn't, he shall not be taxed on the improvements or anything related thereto.
Wash. Admin. Code § 458-12-180
Order PT 68-6, § 458-12-180, filed 4/29/68.