Being told HMRC is opening a compliance check or enquiry into your tax return can be worrying, but the process follows a defined structure with clear rights along the way. This guide explains what typically happens at each stage for 2026/27.
How an Enquiry Starts
HMRC can open an enquiry into a Self Assessment return within a set time limit after it is filed, without needing to give a specific reason, though enquiries are often triggered by inconsistencies in the return, a nudge letter that was not acted on, or third-party data that does not match what was declared.
You will receive a formal notice confirming an enquiry has been opened and which return (or aspect of it) is being checked — this could be a narrow "aspect" enquiry into one specific item, or a wider "full" enquiry looking at the whole return.
During the Enquiry
HMRC can request specific information and documents relevant to the areas under enquiry, and you (or your adviser) are expected to respond within reasonable timescales. You have the right to know why particular information is needed and to challenge a request you consider unreasonable through the appropriate channels.
You can appoint an accountant or tax adviser to deal with HMRC on your behalf at any point, which many people find reduces stress and ensures the right information is provided in the right format.
How Enquiries Conclude, and Your Rights
An enquiry ends when HMRC issues a closure notice, either confirming no changes are needed or setting out any amendments, additional tax, interest and penalties due. You generally have the right to ask the tribunal to direct HMRC to close an enquiry if it has been open an unreasonably long time without progress.
If you disagree with the conclusion, you can ask for an internal review and, if still unresolved, appeal to the First-tier Tribunal — throughout the process you retain the right to representation, to see the basis for any figures HMRC proposes, and to be treated professionally and fairly.
Frequently Asked Questions
What is the difference between an aspect enquiry and a full enquiry?
An aspect enquiry focuses on one specific area of your return, such as a particular expense claim or income source, while a full enquiry examines the whole return in detail — HMRC decides which type to open based on the concerns identified.
How long does an HMRC tax investigation usually take?
It varies enormously depending on complexity, the volume of information requested, and how quickly it is provided — straightforward aspect enquiries can conclude in weeks or a few months, while complex full enquiries covering several years can take considerably longer.
Do I have to respond to every HMRC information request?
You are generally expected to respond to reasonable and relevant requests connected with the enquiry, but you have the right to question a request you believe is not properly connected to the matters actually under enquiry, ideally with professional advice if there is a dispute about scope.
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Can I get an enquiry closed if it drags on too long?
Yes — if HMRC has not made reasonable progress or a decision within what you consider a reasonable time, you can apply to the tribunal for a direction requiring HMRC to issue a closure notice, bringing the enquiry to a formal end.
What happens if HMRC finds an error during an enquiry?
HMRC will typically propose an amendment to your return, additional tax due, interest on the late-paid tax, and potentially a penalty depending on the reason for the error (careless, deliberate, or with a reasonable excuse) — you can challenge the figures through review or appeal if you disagree.
Should I get an accountant involved once an enquiry starts?
It is generally advisable, particularly for a full enquiry or anything involving significant sums, since an experienced adviser can help ensure information is presented clearly, deadlines are met, and your rights throughout the process are properly protected.
What actually triggers an HMRC tax investigation?
Common triggers include figures that look inconsistent year-on-year, expense claims that seem high for your industry, a nudge letter you did not respond to, third-party data (banks, online platforms, other agencies) not matching what was declared, or simply random selection as part of HMRC’s routine compliance activity.
How far back can HMRC investigate my tax affairs?
For an ordinary careless error HMRC can generally go back further than the standard enquiry window, and where it considers the underpayment was deliberate, it can assess considerably further back still — the exact time limit depends on why the error occurred, so it is worth getting advice on your specific facts.
What is the difference between a Code of Practice 8 and Code of Practice 9 investigation?
Code of Practice 8 (COP8) is used where HMRC suspects a significant loss of tax but not necessarily fraud, while Code of Practice 9 (COP9) is used specifically where HMRC suspects deliberate tax fraud and offers a route to full disclosure in exchange for HMRC not pursuing a criminal prosecution — COP9 cases carry much higher stakes and should always involve specialist advice.
Will an HMRC investigation affect my credit rating or ability to get a mortgage?
An enquiry itself is not reported to credit reference agencies and will not show on your credit file, but if it results in a tax debt that goes unpaid and leads to formal debt-recovery action, that could indirectly affect your finances, so it is best to keep any additional tax due paid or arranged as soon as it is confirmed.
Disclaimer: This guide reflects UK rules as they generally apply in 2026/27. This guide is for general information only and is not professional advice. Consult a qualified adviser and refer to gov.uk for current official guidance before relying on any treatment.
