The IRS
issued an advisory that states prepaying 2018 property taxes in 2017 will only
be deductible if the 2018 property taxes were assessed prior to 2018. Thus a
prepayment of anticipated property taxes that have not been assessed prior to
2018 are not deductible in 2017. State and local law determines whether and
when a property tax is assessed, which is generally when the taxpayer becomes
liable for the property tax imposed.
For more
information and examples see the IRS news article IRS Advisory: Prepaid Real Property
Taxes May Be Deductible in 2017 if Assessed and Paid in 2017 on the IRS website.