The DC Office of Tax and Revenue reminds us that...
When a property owner turns 65 years of age or older, or when he or she is disabled, he or she may file an application immediately for disabled or senior citizen property tax relief. This benefit reduces a qualified property owner's property tax by 50 percent. If the property owner lives in a cooperative housing association, the cooperative will supply and collect the applications. The following guidelines apply:
The Homestead Deduction, and Senior Citizen or Disabled Property Owner Application [PDF] is available on the Real Property Tax Forms page.
If a properly completed and approved application is filed from October 1 to March 31, the property will receive the deduction for the entire tax year (and for all tax years in the future). If a properly completed and approved application is filed from April 1 to September 30, the property will receive one-half of the deduction reflected on the second installment (and full deductions for all tax years in the future).