As a result of the Tax Cuts and Jobs Act of 2017, there is now a $10,000 limit on the amount of state and local income taxes, real estate taxes, and general sales taxes that can be taken as itemized deductions. The excess property taxes referenced on Form 8829 would be property taxes in excess of the itemized deductions limit that can still be used (proportionally) in determining the portion of expenses allocable to your home office.
