Why is 30 December an important filing date

If you are obliged to file a self-assessment tax return for 2015-16, and if you have underpaid tax for this year, and if some of your income (including private pension receipts) is taxed under PAYE, then you can apply to have your tax code number adjusted downwards in a future tax year to repay this underpayment by instalments. In effect, your future income tax deductions will be increased.

Many tax payers will find this a palatable option as HMRC would be collecting the underpaid tax sometime in the future. For example, if you had an underpayment for 2015-16 of £2,400 HMRC would adjust your code number for 2017-18. Consequently, instead of paying the underpayment in one amount, on or before 31 January 2017, your income tax deductions from April 2017 would be increased by £200 a month until March 2018.

There are certain conditions that must be met in order to secure this deferred repayment option. HMRC will not allow the coded out payment process if this would unduly reduce your take home pay. There are also graduated limits on the amount of tax that can be recovered in this way.

What is certain, is that you must file your 2015-16 self-assessment tax return, online, before 30 December 2016 otherwise HMRC will not accept a claim to settle income arrears using this method.

Tax payers who have not yet filed their 2015-16 returns have the best part of two months, if they anticipate tax underpaid for the year, and want to spread the repayment cash flow over the tax year 2017-18.