HMRC does not have unlimited time to open a Self Assessment enquiry. The 12-month enquiry window under s9A TMA 1970 is one of the taxpayer’s most important procedural protections and understanding it is essential for anyone whose return is at risk of scrutiny.
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The enquiry window under s9A(2) TMA 1970
Section 9A(2) of the Taxes Management Act 1970 provides that HMRC’s right to open a formal enquiry into a Self Assessment return expires 12 months after the date the return was filed or after the date it was due to be filed, if it was filed before that date.
For the vast majority of individuals filing online, the return for the tax year ending 5 April is due by 31 January following the end of that tax year. If you filed on time, HMRC’s window to open an enquiry closes on the next 31 January, exactly 12 months later.
If you filed late, the window runs 12 months from the actual date of filing (s9A(2)(b) TMA 1970). Filing late therefore extends the period during which HMRC can open an enquiry.
Worked examples
These examples illustrate how the window operates in practice.
Example 1, on-time filing
Your 2023/24 Self Assessment return is due by 31 January 2025. You file it on 31 January 2025. HMRC’s enquiry window closes on 31 January 2026. If HMRC has not given you an enquiry notice by that date, it cannot open an enquiry into that return.
Example 2, late filing
Your 2023/24 return is due 31 January 2025. You file it late, on 1 April 2025. The enquiry window closes on 1 April 2026 , 12 months from the actual filing date. Your late filing has given HMRC an additional two months to scrutinise your return.
Example 3, early filing
Your 2022/23 return is due 31 January 2024. You file it early, on 15 December 2023. Because you filed before the due date, the window is measured from the due date, not the filing date, so the enquiry window closes on 31 January 2025, not 15 December 2024. Filing early does not shorten the window.
What happens when the window closes
Once the s9A enquiry window closes, HMRC loses the right to open a formal enquiry into that return. This is a hard statutory boundary, there is no power to extend it and no discretion for HMRC to act late.
HMRC’s only remaining route to challenge the return is a discovery assessment under s29 TMA 1970. A discovery assessment is HMRC’s power to assess additional tax outside the enquiry regime when it has “discovered” that the return is insufficient. Discovery is subject to its own strict conditions and time limits:
- Four-year time limit for assessments arising from innocent errors or careless understatement (from the end of the relevant tax year).
- Six-year time limit for assessments where HMRC alleges the understatement resulted from the taxpayer’s careless conduct.
- Twenty-year time limit for assessments where HMRC alleges deliberate understatement.
Beyond the time limits, discovery requires HMRC to show that a genuinely new fact came to light, not merely that an officer looked at the same information afresh.
The significance of window closure
For taxpayers and their advisers, the closure of the s9A enquiry window has real protective significance. Once closed:
- HMRC cannot issue formal Schedule 36 information notices in the context of an enquiry into that return.
- HMRC cannot amend the return unilaterally through the enquiry process.
- Any challenge must satisfy the higher bar of a discovery assessment, which requires HMRC to establish the jurisdictional conditions.
One critical protection for taxpayers is the concept of “sufficient disclosure.” Where the return itself contains adequate information for a reasonably diligent HMRC officer to identify the issue, for example, a white-space note explaining a contentious claim, HMRC is prevented from raising a discovery assessment on that issue after the window closes. The principle derives from s29(5) TMA 1970 and has been confirmed in numerous tribunal decisions.
Partnership returns
The equivalent provision for partnership returns is s12AC TMA 1970. The partnership return must be filed by 31 January following the end of the tax year, and HMRC has 12 months from the filing date to open a formal enquiry into the partnership return.
An enquiry into the partnership return does not automatically create enquiries into the individual partners’ Self Assessment returns, but HMRC will typically open linked enquiries into the partners’ returns as well, subject to those returns’ own enquiry windows. The interaction between partnership and partner enquiries can be technically complex.
Returns filed by agents
Where a tax return is filed by an accountant or tax adviser on behalf of a taxpayer, the relevant filing date for the purposes of the enquiry window is the date the return was actually filed, not any earlier or later date imposed by agent procedures.
Critically, the filing date is the statutory deadline for the purposes of determining whether a return was filed on time or late. An agent who files on 31 January has filed on time. The enquiry window closes 12 months later. The fact that an agent may have received the information from the client weeks earlier is irrelevant.
Amendments after filing, an important trap
Section 9A(2)(c) TMA 1970 contains a trap that catches many taxpayers and advisers. If you amend your return after it has been filed, HMRC gets a fresh 12-month window to open an enquiry into the amended aspects of the return.
This means that if your original return’s enquiry window is about to close and you amend the return, perhaps to correct a genuine error, you inadvertently extend HMRC’s ability to scrutinise the amended items by up to 12 months.
Discovery assessment interaction
The relationship between the s9A enquiry window and the s29 discovery assessment regime is one of the most technically significant areas of UK tax procedure. Practitioners advising clients on residual exposure, particularly after the window has closed, need to consider:
- Whether the statutory time limit for discovery has itself expired.
- Whether the return contains “sufficient disclosure” to prevent a discovery on the relevant point.
- Whether any discovery HMRC might seek to make is genuinely “new” or merely a re-examination of known information.
Where these conditions are not met, HMRC’s ability to act after the enquiry window closes is far more limited than many taxpayers fear.
For a full picture of the enquiry process from start to finish, see our main guide: HMRC Self-Assessment Enquiry, What Happens and What to Do.
Frequently asked questions
Can HMRC open an enquiry after the 12-month window?
No. Once the s9A TMA 1970 enquiry window has closed, HMRC cannot open a formal enquiry into that return. Its only remaining route is a discovery assessment under s29 TMA 1970, which requires HMRC to have made a qualifying “discovery” and is subject to its own time limits, typically four years for careless understatement and six years for deliberate understatement, with a 20-year limit for fraud.
What if I amend my return, does HMRC get a new chance to enquire?
Yes. Under s9A(2)(c) TMA 1970, if you amend your return, HMRC has a fresh 12-month window from the date of the amendment to open an enquiry into the amended aspects. This is a significant trap: if the original enquiry window has nearly closed, amending the return resets the clock for those aspects. Always take specialist advice before amending a return where the enquiry window is close to expiry.
Does the enquiry window apply to companies?
The s9A TMA 1970 window applies specifically to individuals and partnerships filing Self Assessment returns. Companies fall under the Corporation Tax Self Assessment regime governed by Schedule 18 FA 1998. The enquiry window for companies is broadly similar, 12 months from the filing date of the company tax return, but the detailed rules differ and specialist advice should be sought for corporate matters.
What is “sufficient disclosure” and why does it matter?
Where a return makes sufficient disclosure of a potentially contentious item, for example, by including a white space note explaining an unusual position, HMRC’s ability to raise a discovery assessment after the s9A window closes is severely restricted. Under s29(5) TMA 1970, a discovery cannot be made if a reasonably diligent officer would have been aware of the position from the information in the return. Proactive disclosure in the return is therefore a valuable protective mechanism that every taxpayer taking unusual positions should consider.
Related guides
- HMRC Self-Assessment Enquiry, What Happens and What to Do
- HMRC Aspect Enquiry vs Full Enquiry, What’s the Difference?
- Forcing HMRC to Close an Enquiry, Closure Notice Applications
- What Documents Must You Give HMRC in a Self-Assessment Enquiry?
- HMRC Schedule 36 Information Notices
- Income Tax Investigation Service