HMRC can normally enquire into your tax return up to 4 years after the end of the tax year. For example, for the 2025/26 tax year (ending 5 April 2026), HMRC has until 5 April 2030 to open an enquiry.
How the 4-year rule works
The default enquiry window runs to the first anniversary of the filing date, but for most Self Assessment taxpayers HMRC effectively has four years from the end of the tax year to open an enquiry. If HMRC believes a return is wrong because of careless behaviour, the window extends to six years. If it thinks the error was deliberate, it stretches to up to 20 years.
A practical way to think of it: keep your records for at least four years after the tax year they relate to. For the 2025/26 tax year that means holding on to records until April 2030.
When the enquiry window opens and closes
- Standard window — 4 years from the end of the tax year (12 months after the 31 January filing deadline)
- Careless behaviour — 6 years
- Deliberate behaviour — up to 20 years
Exceptions: fraud and negligence
The longer windows exist so HMRC can chase under-declared tax where taxpayers were careless or deliberate. If an enquiry finds you simply got something wrong with no fault, you normally owe the tax plus interest. Where behaviour was careless or deliberate, penalties apply on top — which is why accurate record-keeping and complete quarterly MTD updates matter.
How MTD affects the 4-year rule
MTD does not shorten the 4-year enquiry window. It changes what HMRC can see: your quarterly updates give HMRC a rolling view of your income and expenses, which can make discrepancies more visible earlier. Keep the same records you would for any tax year, and remember MTD’s own digital record-keeping rules require you to retain records for the relevant period.
FAQ
How long does HMRC have to investigate my tax return?
Normally 4 years from the end of the tax year. The window extends to 6 years for careless behaviour and up to 20 years for deliberate behaviour.
What is the 4 year rule?
HMRC can open an enquiry into your tax return within 4 years of the end of the tax year concerned. You should keep records for at least that long.
Does MTD change how long HMRC can investigate me?
No. MTD does not change the 4-year enquiry window, though quarterly updates give HMRC a more up-to-date picture of your income and expenses.
How long should I keep records for MTD?
Keep your MTD records for at least 4 years from the end of the tax year they relate to, matching the standard HMRC enquiry window.