Quick answer
If HMRC opens an enquiry into your return, you will get a letter explaining what it is checking and what information to provide. Respond by the date given, keep copies of everything, and consider asking an accountant to deal with HMRC on your behalf.
Updated October 2026 · Figures based on 2026/27 rates
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Why returns are checked
Enquiries can be random, triggered by figures that look unusual compared with similar taxpayers, or based on third-party data such as bank interest or property records.
What to do first
Read the letter carefully and note the deadline. Gather the records HMRC asked for and answer only what is asked, accurately and on time.
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How long enquiries take and how they end
Simple enquiries may close in a few months. HMRC ends them by issuing a closure notice, which confirms your return or amends the tax due.
Getting help
A qualified accountant can liaise with HMRC, check your figures and negotiate if extra tax or penalties arise.
How the enquiry process normally works
An HMRC enquiry, sometimes called a compliance check, starts with a letter. HMRC may open an enquiry into a return within 12 months of the date you filed it. Some enquiries cover the whole return and some are limited to a single item, such as a particular expense or a figure that looks out of line. Some are random, others follow information HMRC has received from banks, platforms or other third parties.
The letter will say what HMRC wants to look at and by when you should respond. It may ask for records such as bank statements, invoices, mileage logs or rental agreements. Opening an enquiry does not mean HMRC believes you have done something wrong, but it does mean you should take it seriously and respond on time.
How far back can HMRC look?
If HMRC finds that tax has been underpaid outside the normal enquiry window, it can issue a discovery assessment. The usual time limits are four years for an innocent error, six years where the error was careless and 20 years where the behaviour was deliberate. This is why keeping complete records matters, and why it is wise to fix errors you spot rather than leaving them.
A worked example of an adjustment
This is an illustrative example, not a real client. Maria is a freelance photographer with a taxable profit of £30,000. HMRC opens an enquiry and finds that she left out £3,000 of cash income and claimed £800 of personal clothing as a business cost. Her taxable profit is therefore understated by £3,800.
| Item | Working | Amount |
|---|---|---|
| Corrected profit | £30,000 + £3,000 + £800 | £33,800 |
| Extra Income Tax | £3,800 x 20% | £760 |
| Extra Class 4 NI | £3,800 x 6% | £228 |
| Extra tax and NI | £760 + £228 | £988 |
Maria would owe the extra £988, plus interest from the original due date. HMRC may also charge a penalty. How large it is depends on whether the error was careless or deliberate and on how much help you give HMRC, so check current HMRC guidance on penalties for inaccuracies. Telling HMRC about errors promptly and cooperating fully generally leads to better outcomes than waiting to be found.
Common mistakes during an enquiry
- Ignoring the letter or missing the response deadline. If you need more time, ask for it in writing before the date passes.
- Sending unorganised paperwork without explanation. Label documents and match them to the figures on the return.
- Providing information you have not checked. Wrong answers can create new problems.
- Responding in anger. Keep your tone calm and factual.
- Handling a complex enquiry alone when the sums are large or the facts are complicated.
An enquiry checklist
- Note the date on the letter and the response deadline.
- Identify which tax year and which part of the return it concerns.
- Gather the records behind those figures.
- Prepare a short, factual covering reply and keep a copy of everything you send.
- Consider asking an accountant to review your response or act as your agent.
Related reading
You may find what to do if HMRC sends a compliance check letter, HMRC letters explained, how long to keep your Self Assessment records and how to amend a Self Assessment return helpful. Our sole trader service page shows how we support self-employed clients.
SATR can prepare accurate returns for a fixed fee from £199 + VAT, using qualified UK accountants who file online. A properly documented return is the best protection if HMRC ever asks questions.
This is general information, not personal tax advice. Check current HMRC guidance for your situation.
What to expect after you reply
After you send your information, HMRC reviews it and may ask follow-up questions, request further documents or ask for a meeting. Enquiries vary in length. A simple one may close within a few months, while a complicated one can take considerably longer. When HMRC is satisfied, it issues a closure notice that states its conclusion and whether your return needs to change.
If HMRC concludes that more tax is due, you will receive a revised calculation. If you disagree, you normally have a right to ask for a review or to appeal within the time limit shown on the decision letter. Our guide on how to appeal an HMRC Self Assessment decision sets out the usual steps. Do not let the appeal deadline pass while you consider your options.
Keep a clear paper trail
Throughout an enquiry, keep a log of every letter, phone call and email, with the date and what was said. If HMRC calls you, it is perfectly acceptable to ask for the request in writing so you can give an accurate answer after checking your records. Send important documents by a method that gives proof of delivery, or use the secure message service in your online account where available.
Frequently asked questions
Is an enquiry the same as an investigation?
An enquiry is a check of a return. Serious cases can become formal investigations, but most enquiries are routine.
Can HMRC go back several years?
Yes. HMRC can look back up to 4 years for innocent errors and longer where there has been carelessness or deliberate behaviour.
Can I ask for more time to answer an enquiry?
Yes. Write to the officer before the deadline, explain why you need longer and say when you will respond. Do not simply let the date pass.
Should I use an accountant for an enquiry?
You are not required to, but it can help if the sums are large or the questions are technical. An accountant can act as your agent once you authorise them.
Will an enquiry always lead to extra tax?
No. Many enquiries close without a change. Others result in an adjustment. The outcome depends on the records and the facts.
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The Bottom Line
An HMRC enquiry starts with a letter, not an accusation. Read it carefully, note the deadline, gather only what was asked for and reply factually and on time. HMRC can usually open an enquiry within 12 months of filing, and discovery assessments can reach back 4, 6 or 20 years depending on the behaviour. Most enquiries close without drama when the records are complete and the response is organised.
If the letter feels daunting, let a specialist deal with HMRC for you. Our fixed-fee Self Assessment service starts from £199 + VAT — prepared and filed online by qualified UK accountants. Order online now or see our penalty appeal service.
