Application deadline date is September 1st of the current year. You can apply online for any of these exemptions through the Cook County Assessor's Office.Application for Property Tax Exemption. To establish eligibility for property tax exemption, the claimant must sign and file an affidavit with the County. I sent in all paperwork that was needed. If you have moved or plan to move in the future, you may be entitled to a prorated Senior Exemption, based on the time of occupancy. Finance CRE News In Your Inbox. Sign up for Connect emails to stay informed with CRE stories that are 150 words or less.