Due to the limitation on the deduction for real estate and state and local income taxes (otherwise known as the SALT deduction) to $10,000 in 2018, people are thinking that they can prepay these taxes in 2017 while there is no such limitation. Congress anticipated this and limited the 2017 deduction of prepaid taxes.
The only prepaid taxes that will be deductible in 2017 are those that were assessed in 2017 but do not require a payment until 2018. The payment of these taxes would need to be made before the end of 2017 to be deductible in 2017.
If the tax assessment is related to 2018, the payment of it will not be deductible in 2017.