The declaration made in 2018 relative 2017’s income, will be the one used by the tax office to calculate the applicable withholding tax rate.
In order to avoid a double taxation in 2019 a special tax credit will erase your tax on 2018’s rental income.
HOWEVER only NON EXCEPTIONAL REVENUES will give you rights to a tax credit.
No need to wonder whether or not you should postpone paying your charges to 2019!
You are better off paying a lot in 2018 and very little if any in 2019.