You do not have to be a Delaware resident to deduct real estate tax (property tax) that you pay in Delaware. On your federal tax return you can deduct real estate tax that you pay on any property you own that is in the United States. It doesn't matter where you live.
Real estate tax is an itemized deduction. It will not make any difference in your tax or refund unless your total itemized deductions are more than your standard deduction. Also, for 2018 through 2025, the total deduction for state and local taxes is limited to a maximum of $10,000 per year ($5,000 if you are married filing separately). The maximum applies to the total of real estate tax, personal property tax, and either state and local income tax or state and local sales tax.
There is no deduction for property tax on the Pennsylvania tax return.
