Wash. Rev. Code § 82.45.035
The state department of revenue shall provide by rule for the determination of the selling price in the case of leases with option to purchase, and shall further provide that the tax shall not be payable, where inequity will otherwise result, until and unless the option is exercised and accepted. A conditional sale of mining property in which the buyer has the right to terminate the contract at any time, and a lease and option to buy mining property in which the lessee-buyer has the right to terminate the lease and option at any time, shall be taxable at the time of execution only on the consideration received by the seller or lessor for execution of such contract, but the rule shall further provide that the tax due on any additional consideration paid by the buyer and received by the seller shall be paid to the county treasurer (1) at the time of termination, or (2) at the time that all of the consideration due to the seller has been paid and the transaction is completed except for the delivery of the deed to the buyer, or (3) at the time when the buyer unequivocally exercises an option to purchase the property, whichever of the three events occurs first.
The term "mining property" means property containing or believed to contain metallic minerals and sold or leased under terms which require the purchaser or lessor to conduct exploration or mining work thereon and for no other use. The term "metallic minerals" does not include clays, coal, sand and gravel, peat, gypsite, or stone, including limestone.
The state department of revenue shall further provide by rule for cases where the selling price is not separately stated or is not ascertainable at the time of sale, for the payment of the tax at a time when the selling price is ascertained, in which case suitable security may be required for payment of the tax, and may further provide for the determination of the selling price by an appraisal by the county assessor, based on the full and true market value, which appraisal shall be prima facie evidence of the selling price of the real property.
RCW 82.45.035