Reno, Nevada. Nov. 29, 2018. The Washoe County Treasurer would like to remind taxpayers that supplemental tax bills are mailed throughout the fiscal tax year ending on June 30th. Supplemental tax bills are not provided directly to your mortgage company. If you have recently purchased any newly constructed property or if you receive a notice and you aren’t sure what it is for, please call our office at 775-328-2510 or visit us at 1001 E 9th St Bldg, D140 for an explanation.