The Internal Revenue Service has a message for the homeowners rushing to prepay their property taxes before new rules take effect on New Year’s Day: Not so fast.

The tax bill that President Trump signed into law last week sharply limited the itemized deductions for state and local taxes while raising the standard deduction for individuals and couples. Those rules do not take effect until 2018, however. That has led some homeowners, particularly in high-tax, affluent areas, to try to prepay their 2018 property taxes before the deduction disappears.

In an advisory notice posted to its website on Wednesday, the I.R.S. said that maneuver could work, but only under limited circumstances. To qualify for the deduction, property taxes not only need to be paid in 2017, they must also be assessed in 2017 — meaning that homeowners who prepaid their taxes based on estimated assessments, or who tried to pay several years’ worth of taxes at once, will probably be out of luck.

“Those individuals now are not getting the benefits of those prepayments,” Nicole Kaeding, an economist at the Tax Foundation, said of people who paid taxes that had not yet been assessed. “All that you’ve done is provided an interest-free loan to your municipal government.”