How far back HMRC can go depends on why the tax was unpaid, not simply how long ago it was. For an honest mistake, the time limit is shorter than for a careless error, and shorter again than for something HMRC considers deliberate. HMRC's own internal guidance sets out the framework directly, and it is worth understanding which category you are likely to fall into before you assume the worst, or the best.
What are HMRC's time limits for going back?
HMRC's ability to raise a "discovery assessment", an assessment made after your original filing window has closed, is governed by fixed time limits set out in legislation and explained in HMRC's own manual. The table below sets out the framework as confirmed in HMRC's Self Assessment manual, SALF411.
| Category | Time limit | What it covers |
|---|---|---|
| Ordinary case | 4 years after the end of the relevant tax year | Incomplete disclosure without careless or deliberate conduct |
| Careless | 6 years after the end of the relevant tax year | A loss of tax brought about carelessly |
| Deliberate, or failure to notify | 20 years after the end of the relevant tax year | Loss of tax brought about deliberately, or a failure to notify a tax liability |
| Offshore matters | 12 years after the end of the relevant tax year | Income Tax, Capital Gains Tax or Inheritance Tax involving an offshore matter or offshore transfer, where this significantly harder for HMRC to identify |
These time limits run from the end of the tax year the assessment relates to, not from the date HMRC opens its enquiry. A discovery assessment for the 2019/20 tax year on a careless basis, for example, would generally need to be raised by 5 April 2026.
What decides which time limit applies to me?
The category HMRC applies is a judgement about your behaviour, not the amount of tax involved. An honest mistake, made despite taking reasonable care, falls into the ordinary 4-year window. A mistake HMRC considers you should have caught with reasonable care is "careless" and gets 6 years. Anything HMRC considers a knowing, intentional understatement is "deliberate" and gets 20 years.
This distinction matters enormously in practice, because the difference between careless and deliberate is not always obvious, and HMRC and the taxpayer do not always agree on which applies. If you are asked to self-assess your own behaviour, for example as part of a disclosure, getting this categorisation right affects both how many years HMRC can go back and the size of any penalty.
The offshore matter extension to 12 years applies separately, and can combine with a careless or ordinary error, extending the normal window specifically because offshore income or gains are harder for HMRC to trace without additional information-sharing.
Does this mean HMRC will actually go back that far?
Having the legal power to assess further back is not the same as HMRC choosing to use it in every case. In practice, HMRC's compliance checks often focus on the years where it has a specific concern, and the length of an enquiry tends to reflect the complexity of what it finds, not an automatic reach for the maximum time limit.
That said, once HMRC opens an enquiry and starts finding irregularities, particularly ones that look deliberate, the scope can widen. This is one reason getting early advice on how a disclosure is framed matters: how your behaviour is categorised affects how far back HMRC can properly go, not just the penalty rate.
HMRC's Compliance Handbook sets out the broader framework behind these time limits in more technical detail at CH51000, for readers who want the fuller picture beyond the Self Assessment-specific summary above.
What this means in practice
If you are worried about a specific past year, start by working out honestly which category your situation is likely to fall into: an oversight, something you should have caught, or something you knew was wrong at the time. This shapes everything that follows.
Gathering records for the years you think may be affected is something you can do yourself. Deciding how your own behaviour should be categorised, and how that interacts with any disclosure you make, is not something to guess at. HMRC's guidance is explicit that if you are unsure how to categorise your own behaviour, you should get professional advice before submitting anything.
If HMRC has already written to you, whether a general enquiry letter, a nudge letter, or a Code of Practice letter, our tax investigations page and our Code of Practice 9 page cover what those processes involve and how the categorisation question fits into each.
Frequently Asked Questions
Yes, where the loss of tax is judged to have been brought about deliberately, or where someone failed to notify HMRC of a tax liability at all. This is the longest of the standard time limits and is reserved for the most serious category of behaviour.
Careless means a mistake was made that reasonable care would have avoided; deliberate means HMRC considers the understatement was intentional. The distinction is a judgement call, and it directly affects both the time limit and the penalty, so it is worth taking seriously rather than assuming.
The time limits run from the end of the tax year the assessment relates to, not from when the error happened or was discovered. This means the practical deadline is always tied to a specific tax year end date.
Yes. Income Tax, Capital Gains Tax and Inheritance Tax involving an offshore matter or offshore transfer that is significantly harder for HMRC to identify can be assessed up to 12 years back, regardless of whether the underlying error was careless.
Coming forward voluntarily, even for an honest 4-year-window error, is usually treated more favourably than waiting for HMRC to find it. It also gives you the chance to set out your own account of what happened, rather than HMRC forming a view without you.
Talk to us before you decide
If you are unsure how far back an old error might reach, or how your own situation would be categorised, speak to Ghulam Alahi, Managing Director, who leads our work on HMRC enquiries and disclosures. The first conversation is about understanding your specific years and behaviour calmly, before you approach HMRC or respond to a letter. Call 020 8554 2135 or email info@visionconsulting.co.uk, or get in touch via our contact page.
By the Vision Consulting team.
This is general information, not advice. Your position depends on your circumstances.