This question gets more complex by the day. On Dec. 27, the IRS issued Announcement 2017-210, which can be found on their website. https://www.irs.gov/newsroom/irs-advisory-prepaid-real-property-taxes-may-be-deductible-in-2017-if-assessed-and-paid-in-2017 My first reaction to this was, good for the IRS for doing their job to provide guidance to taxpayers. But there is a caveat. The job of the IRS is not easy because it is subject to political constraints (the acting IRS Commissioner, David Kautter, also serves as Assistant Treasury Secretary for Tax Policy). It is unusual for the Assistant Secretary to also serve as acting IRS Commissioner – indeed this has never happened before, at least in recent memory – and it raises questions precisely about issues like the one about the state and local deduction – does the IRS announcement reflect a nonpolitical view or is it designed to serve the political purposes of the Trump administration?).
Those who are not aficionados of IRS documents may not focus on the fact that the IRS Announcement is not a Revenue Ruling, which would carry some legal status. An Announcement generally does not break new legal ground. The Accouncement states: “ A prepayment of anticipated real property taxes that have not been assessed prior to 2018 are not deductible in 2017.” (this is also ungrammatical, by the way). One would expect an Announcement that states a legal conclusion to provide a citation with legal authority, but the Announcement does not do so.