When it comes to self-assessment tax returns, HMRC tends to be “process now, check later”; so, they assume that it is correct, but they reserve the right to later make enquiries about it! There is a deadline for taxpayers to amend their tax returns or HMRC to start an investigation.
For personal tax returns HMRC must deliver an enquiry notice within 12 months from the filing date of the return (usually, the filing date will be 31st January!). If you submit your tax return after the normal filing date (or submit on time and amend later) the enquiry deadline is extended to the next quarter-end date (31st January, 30th April, 31st July and 31st October).
For companies, a notice of enquiry must be delivered within 12 months from the normal filing date of a corporation tax (CT) return (the normal filing date is twelve months from the end of your company’s accounting period). Like the personal tax returns, this deadline is extended for late or amended returns (e.g if a return wasn’t submitted until 10th May 2025, HMRC has until 31st July 2026 in which to start an enquiry).
However, if HMRC miss the enquiry deadline, they have a secret weapon to use called “discovery rules”. These rules mean that they can make a limited enquiry into your or your company’s tax affairs. If they suspect that tax has been underpaid (due to carelessness or by deliberate action) and they could not have been expected to be aware of the circumstances by the time the enquiry deadline has passed, HMRC will also issue a discovery assessment.
This assessment is usually up to 4 years from the end of the tax year in which the underpayment occurred, or the assessment was used. This will be extended to 6 years if the error/omission from your tax return was deliberate and to 20 years if it was deliberate and you tried to hide the error.