Insights

Understanding HMRC Enquiry Timelines for Self Assessment

Sara Purnell · Posted on: December 16th 2025 · read

The Self Assessment filing deadline, 31 January 2026 is well known, but many taxpayers overlook the equally important timelines for HMRC enquiries. While most assume HMRC has 12 months from the filing date to enquire into a return, the rules are different for late or amended submissions.

In these cases, HMRC can open an enquiry for any reason up to 12 months from the end of the quarter (Jan, Apr, Jul, Oct) in which the return was filed with HMRC.

Investigations can also be opened after this point in certain circumstances, such as if the taxpayer did not take reasonable care or HMRC has reason to suspect that there may have been a loss of tax.

Share this article

Related tags

Services

This distinction is subtle but significant. Many taxpayers miss the fact that amended or late returns do not follow the “standard” 12-month window. Understanding these timelines is essential for planning and avoiding unexpected compliance notices or delays in resolving queries.

A series of consultations and tax changes were announced in the recent Autumn Budget, solidifying the Government’s aim to continue closing the tax gap, and therefore we don’t expect to see a slowdown in the 316,000 compliance checks HMRC opened in 24/25. There is a noticeable spike in enquiries in December and January, right before the enquiry window closes for those “late filers”. This pattern shows the importance of being proactive rather than reactive. Waiting until the last minute can leave you scrambling to provide information or address queries.

"Whether filing on time, late, or amending a return, knowing the precise enquiry timeline allows you to plan, respond, and stay in control. Don’t let assumptions about deadlines leave you exposed, understanding the system is your best safeguard."

Sara Purnell, Tax Senior Manager