The honest answer is four years, six years or twenty — and which one applies depends entirely on how the mistake happened, not on how big it was.
There is also a separate, much shorter window in which HMRC can simply open an enquiry into a return without needing any reason at all. Understanding the difference is the whole subject.
The enquiry window: twelve months
HMRC can open an enquiry into a return for 12 months from the date the return is received, provided you filed on or before the filing deadline. A 2025/26 return received on 20 June 2026 can be enquired into up to 20 June 2027.
Within that window HMRC does not need a reason. It can enquire into any return, and a proportion are selected at random.
Filing late extends the window. If a return is delivered after the filing deadline, the enquiry period runs to the quarter day next following the first anniversary of the day the return was made. The quarter days are 31 January, 30 April, 31 July and 31 October. A return filed on 3 March therefore stays open until the following 30 April — a longer exposure than if it had been filed on time.
Amending a return opens a new window. You can amend within 12 months of the Self Assessment deadline. The amendment does not extend the original enquiry period, but it creates a separate one for the amendment itself, running to the quarter day after the first anniversary of the amendment.
Once the enquiry window closes without HMRC opening an enquiry, the return is final — unless HMRC makes a discovery.
Discovery assessments: 4, 6, 12 or 20 years
A discovery assessment is how HMRC reopens a year that would otherwise be closed. It requires HMRC to discover that tax has been under-assessed, and the time limit turns on behaviour.
| Behaviour | Time limit |
|---|---|
| No careless or deliberate behaviour | 4 years from the end of the tax year |
| Careless behaviour by you or your agent | 6 years from the end of the tax year |
| Loss of tax involving an offshore matter or offshore transfer | 12 years from the end of the tax year |
| Deliberate behaviour by you or your agent | 20 years from the end of the tax year |
The clock runs from the end of the tax year, not from the filing date. For 2025/26, the four-year limit expires on 5 April 2030 and the twenty-year limit on 5 April 2046.
Careless means a failure to take reasonable care. It is not a high bar — an unreconciled bank account, a rental figure taken from memory, a category of income overlooked. Deliberate means you knew the return was wrong when you submitted it. The distinction is worth a decade and a half.
Two further 20-year cases matter for people who never registered at all:
- Failure to notify liability to tax — 20 years, reduced to 4 years where you have a qualifying reasonable excuse and did not delay unreasonably afterwards
- Failure to provide information about an avoidance scheme — the same structure
The 12-year offshore limit applies for 2015/16 onwards whether or not you were careless. For 2013/14 and 2014/15 it required careless behaviour. It does not apply where HMRC already received relevant overseas information from which it could reasonably have identified the lost tax in time.
If you never filed at all, HMRC can also make a determination of the tax due within 3 years of the filing date. You displace a determination by filing the return — the later of three years from the filing date, or twelve months from the date of the determination.
The rule that cuts the other way
Overpaid tax has its own limit. You can claim overpayment relief up to 4 years after the end of the tax year it relates to. Beyond that, the money is gone regardless of how clear the error is.
There is a genuine asymmetry here: HMRC has six years for your carelessness, and you have four for their overpayment. That is a reason to check returns properly rather than assume anything can be fixed later.
How long you must keep records
Record-keeping periods are longer than most people expect, and they differ depending on why you file.
| Who you are | How long to keep records |
|---|---|
| Self-employed (sole trader or partner) | 5 years after the 31 January submission deadline for the relevant tax year |
| Running a property business (landlord) | 5 years after the 31 January submission deadline for the relevant tax year |
| Not self-employed and not a landlord, return filed on time | 22 months after the end of the tax year |
| Not self-employed, return filed late | 15 months after you sent the return |
| Return sent more than 4 years after the deadline | 15 months after you sent the return |
Worked through: a sole trader filing a 2025/26 return by 31 January 2027 must keep the records until at least the end of January 2032. Someone with only employment and savings income filing the same year on time keeps theirs until roughly the end of January 2028.
If records are lost, stolen or destroyed and cannot be replaced, you must do your best to provide figures and tell HMRC on the return whether you are using estimated figures — your best guess where actual figures are unavailable — or provisional figures, which are temporary and must be replaced with actuals when you have them. Flagging them on the return is the point; using estimates silently is what turns a records problem into a carelessness problem.
What actually triggers an enquiry
There is no published list, and HMRC does not explain individual selections. In practice, cases come from a mixture of random selection and risk assessment. HMRC holds a very large amount of third-party data — bank and building society interest, dividends, land registry transactions, information from online marketplaces and payment processors, and information exchanged with other tax authorities — and cross-checks it against returns.
Common practical risk factors:
- Figures that do not match third-party data. A gap between reported interest or dividends and what HMRC already holds.
- Unexplained movements. A large drop in reported profit, or margins well outside sector norms, without explanation.
- A property disposal with no gain reported. UK residential property disposals must be reported and paid within 60 days of completion, and then again on the return.
- Round numbers and estimates. Expenses that look assembled rather than recorded.
- Consistently late filing. It does not cause an enquiry, but it lengthens the window in which one can start.
- Boxes left blank that should not be. Including the tick boxes for estimated or provisional figures.
The controllable part of this is documentation. HMRC cannot assess what you can evidence; it can only assess what you cannot.
Frequently asked questions
How far back can HMRC go into my tax returns?
Four years from the end of the tax year where there was no careless or deliberate behaviour, six years where there was carelessness, twelve years where offshore matters are involved, and twenty years for deliberate behaviour or failure to notify liability.
How long does HMRC have to open an enquiry?
Twelve months from the date the return is received, if you filed on or before the deadline. Filing late extends it to the quarter day following the first anniversary of the day the return was made.
How long do I need to keep my tax records?
Five years after the 31 January submission deadline if you are self-employed or a landlord with property income. If you are neither and filed on time, 22 months after the end of the tax year. If you filed late, 15 months after you sent the return.
Can I claim back overpaid tax from six years ago?
No. Overpayment relief can only be claimed up to four years after the end of the tax year it relates to. After that the claim is out of time.
What is the difference between careless and deliberate behaviour?
Careless means failing to take reasonable care — an unchecked figure or an overlooked source of income. Deliberate means knowing the return was wrong when you submitted it. The time limit moves from six years to twenty.
Filing so it stands up later
Almost every long-running HMRC case starts as a small, undocumented figure on a return filed years earlier. Getting the return right and keeping the working papers is not paperwork for its own sake — it is what shortens the window and keeps behaviour on the right side of “careless”.
SmartFiling files Self Assessment returns for a fixed fee, with every return reviewed and signed off by an ICAEW Chartered Accountant and a three-week turnaround from receiving your records.
