The law signed by Trump last week imposes a $10,000 combined limit on the deduction of state and local income and property taxes. There is no limit on that deduction for 2017.
In a notice on its website, the IRS said that, in general, a full deduction for the prepayment of state or local property taxes depends on whether the taxpayer makes the payment this year and whether the property taxes are assessed prior to 2018.
"A prepayment of anticipated real property taxes that have not been assessed prior to 2018 are not deductible in 2017", the IRS notice said.
"State or local law determines whether and when a property tax is assessed, which is generally when the taxpayer becomes liable for the property tax imposed", it said.