When you submit a tax return, HMRC typically has 12 months from the date a tax return was filed to open an enquiry. This is called an ‘open year’. In certain circumstances where HMRC discover an issue that wasn’t fully disclosed, they can look back further than this and raise an assessment for additional tax. Broadly speaking, these assessments can be raised:
Four years from the end of the relevant tax period
Six years from the end of the relevant tax period when the error has arisen from careless behaviour
12 years from the end of the relevant tax period when the error relates to an offshore asset
20 years from the end of the relevant period when the error has been caused by deliberate behaviour
If you have not filed a tax return, HMRC can raise a determination (an estimate of the amount of tax that should have been paid), for a 20-year period, unless you have what HMRC deems to be a reasonable excuse, which restricts the window to four years.