Update: The IRS released new guidance on this issue at the end of December after the new tax law was passed.
Prepaid property taxes are only deductible if they are actually assessed -- that is, the town or county has issued a legally binding warrant to collect property taxes. Property taxes are only deductible if they were paid after being assessed. If you made a payment in 2017 for a tax "guesstimate" that was not actually assessed, then it is not deductible. For example, if the assessment date was Friday, December 29, 2017 with a normal payment due date of February 15, 2018, then the taxes would be deductible on your 2017 tax return only if paid on December 29, 30, or 31. If the assessment is dated January 15, 2018 with a payment due date of February 15, 2018, then the property taxes are not deductible if prepaid in 2017 because the taxes were legally assessed and owed at the time the payment was made.
And the payment must actually be made in 2017 -- for example, mailing a check in December that is post-dated to January doesn't count.
