How far back can HMRC look? Enquiry windows, discovery time limits and how long to keep records

By Matthew Hunt FMAAT, founder of Digivolve Accountants, Haywards Heath. Last reviewed 5 October 2026 for the 2026/27 tax year.

Once a tax return is filed, most people want to know when they can stop worrying about it. The honest answer is that it depends on two separate clocks: the enquiry window, which gives HMRC a short period to check a return for any reason, and the assessment time limits, which let HMRC go back much further if tax has been underpaid. Knowing how both work tells you how exposed you are and how long you need to keep your paperwork.

The short version

HMRC can usually open an enquiry into an on-time Self Assessment return for 12 months from the date it receives it.
After that, HMRC can still assess underpaid tax up to 4 years after the end of the tax year in ordinary cases.
The limit stretches to 6 years if the underpayment was careless, and 20 years if it was deliberate.
Failing to tell HMRC you need to pay tax can also bring in the 20 year limit.
Filing early starts the enquiry clock sooner, so the window closes sooner.

The enquiry window

An enquiry, now usually called a compliance check, is HMRC’s formal way of checking a return. HMRC does not need a reason to open one within the enquiry window, and it can look at anything on the return.

For individuals, partnerships and trustees, the window on an on-time return runs for a full 12 months from the date HMRC receives it. If the return is late, the window stays open until the next quarter day after the first anniversary of filing. The quarter days are 31 January, 30 April, 31 July and 31 October. If you amend a return, HMRC gets a separate window for the amendment, again running to the quarter day after the first anniversary of the amendment.

For companies, the rules are similar. A single company or a member of a small group that files on time can be enquired into for 12 months from the day the return was delivered. Companies in larger groups are measured from the filing date instead. Late company returns follow the same quarter day rule as individuals.

Discovery: how far back HMRC can really go

Once the enquiry window has closed, HMRC cannot open a routine check. But if it discovers that tax has been under-assessed, it can still raise an assessment within strict time limits. These run from the end of the tax year for individuals, or the end of the accounting period for companies.

Situation	How far back HMRC can assess
Return checked within the enquiry window	12 months from filing (on time returns)
Ordinary case, no carelessness	4 years after the end of the period
Careless error	6 years after the end of the period
Deliberate error, or failure to notify	20 years after the end of the period

There is also a 12 year limit for offshore matters in income tax, capital gains tax and inheritance tax. For VAT, the 6 year careless extension does not apply, so the ordinary 4 year limit is the starting point unless the behaviour was deliberate.

Worked example: a 2024/25 tax return

The 2024/25 tax year ended on 5 April 2025 and the online filing deadline was 31 January 2026. Take a self-employed client who filed online on 15 June 2025.

Enquiry window: closed on 15 June 2026, 12 months after HMRC received the return. Had the return gone in on 31 January 2026, the window would stay open until 31 January 2027.
Ordinary time limit: 5 April 2029.
Careless error: 5 April 2031.
Deliberate error: 5 April 2045.

Filing in June rather than January closed the enquiry window more than seven months earlier, with nothing else changed. That is one of the simplest reasons we encourage clients to get their records to us early.

How long to keep your records

Individuals who are not self-employed: keep records for at least 22 months after the end of the tax year if you filed on time. If you filed late, keep them for at least 15 months after you sent the return.
Sole traders and partners: keep records for at least 5 years after the 31 January filing deadline. For 2024/25 that means at least until 31 January 2031. If a return is sent more than 4 years after the deadline, the period is 15 months from the date it was sent.
Limited companies: keep records for 6 years from the end of the last financial year they relate to. Keep them longer if a transaction covers more than one accounting period, the company bought an asset expected to last more than 6 years, the return was sent late, or HMRC has started a compliance check.

These are minimums. Because a careless error opens up a 6 year window, we suggest keeping business records for at least 6 years, and longer for property and share purchases.

If you get a letter from HMRC

A compliance check letter is not an accusation, but how you respond matters.

Read the letter carefully and note any deadline. If you need more time to gather documents, say so early.
Do not ignore it. HMRC takes your level of co-operation into account when working out penalties, and helping it complete the check can significantly reduce them.
Get an adviser involved. HMRC confirms you can ask a professional adviser to deal with it for you. You can also ask for contact in writing rather than by phone.
Keep filing and paying any returns and tax that fall due while the check is running.
Watch appeal deadlines. If you disagree with a decision, you normally have 30 days from the date of HMRC’s letter to act.

Fee protection insurance

Even a check that ends with no extra tax can take months of correspondence and accountancy time. Fee protection insurance covers the professional fees for dealing with an HMRC enquiry, within the policy terms, so you are not paying to prove your return was right. It does not cover tax, interest or penalties.

Common mistakes we see

Throwing records away once the enquiry window closes. The 4, 6 and 20 year limits still apply.
Filing on 31 January out of habit. It keeps the enquiry window open for the longest possible time.
Assuming an innocent mistake is never careless. Not taking reasonable care, such as guessing figures without records, can be treated as careless and extend the limit to 6 years.
Amending a return without thinking it through. An amendment opens a fresh window on the amended figures.
Replying to HMRC alone. Answers given without advice can widen a check that could have stayed narrow.

Questions and answers

How long does HMRC have to open an enquiry into my tax return?

For an on-time Self Assessment return, 12 months from the date HMRC receives it. For a late return, until the next quarter day (31 January, 30 April, 31 July or 31 October) after the first anniversary of filing.

Can HMRC go back more than 4 years?

Yes. HMRC can go back 6 years if an underpayment was careless and 20 years if it was deliberate or you failed to notify HMRC that you needed to pay tax. There is also a 12 year limit for offshore matters.

How long should a limited company keep its records?

At least 6 years from the end of the last financial year they relate to, and longer if the return was late, HMRC has opened a check, or the records relate to long-life assets or transactions spanning more than one period.

Does filing my tax return early reduce the risk of an HMRC enquiry?

Filing early does not change whether HMRC chooses to check your return, but it starts the 12 month enquiry window sooner, so the window also closes sooner.

Worried about an HMRC check?

Whether you have just received a letter or simply want your returns filed early and your records in order, we can help. Book a free consultation, no obligation: book a free consultation. Prefer to talk? Call or WhatsApp us on 01444 300981.

Find out more about our HMRC tax investigation support and how our fee protection service covers the cost of dealing with an enquiry.

Sources

This guide is general information based on the rules for the 2026/27 tax year and is not personal advice. Your own position depends on your full circumstances.