HMRC can correct income tax returns for obvious errors or omissions within nine months of the submission date. You should be informed of any changes to the tax calculated on your return and given 30 days to object, otherwise the adjustment stands. If HMRC miss this nine-month window they can only change the amount of tax due if they open an enquiry or discover an error made by you or your adviser.
HMRC have been using their power to correct tax returns for 2016-17 and 2017-18 more than nine months after they were submitted. The corrections relate to incorrect tax calculations made by the HMRC computer for those years and not to errors on the part of the taxpayer.
If you receive a tax demand out of the blue relating to a tax return for 2016-17 or 2017-18, ask us to check it immediately. If HMRC have waited too long to demand the extra tax it may not be legally payable.