Filing a Self-Assessment return and then spotting a mistake is one of the most common situations taxpayers face, and in almost all cases it is fixable. The correct route depends on two things: what kind of error it is, and how long ago the return was filed. Within twelve months of the filing deadline you can amend the return directly. Beyond that window the mechanism changes to overpayment relief or a formal disclosure, and the rules are stricter.
This piece walks the amendment window, what to do once it closes, the SA303 used to reduce Payments on Account, and how to handle under-declared income that owes HMRC money. It sits in [the Self-Assessment hub](/guide/definitive-uk-self-assessment-guide/) alongside the companion pieces on [paper versus online filing](/blog/paper-vs-online-filing-digital-only-path/) and [the tax-free allowances that reduce your bill](/blog/tax-free-allowances-personal-to-trading-allowance/).
The 12-month amendment window
The standard way to correct a Self-Assessment return is to amend it. A taxpayer has until twelve months after the 31 January filing deadline to amend a return. For the 2024-25 tax year, the online filing deadline was 31 January 2026, so the return can be amended up to 31 January 2027. An online return is amended by logging back into the Government Gateway, opening the submitted return, and changing the relevant figures. A paper return is amended by writing to HMRC or submitting a corrected paper return clearly marked as an amendment.
It is worth being precise about the deadline, because it catches people out. The window runs from the filing deadline, not from the date the return was actually filed. A taxpayer who files a 2024-25 return early, in May 2025, still has until 31 January 2027 to amend it, not twelve months from May. Equally, a taxpayer who filed late does not get an extended amendment window; the clock still runs from the statutory 31 January deadline. Knowing the exact end date matters because once it passes the amendment route closes and the slower, more formal mechanisms take over.
How an online amendment works in practice
- Log in to the Government Gateway personal tax account.
- Select Self-Assessment and choose the relevant tax year.
- Open the already-submitted return and edit the figures that were wrong.
- Resubmit the return; HMRC recalculates the tax automatically.
- The revised calculation shows the new balance owed or repayable.
Once an amendment is submitted, HMRC recalculates the tax and adjusts the account. If the amendment reduces the tax due, any overpaid amount can usually be repaid or set against future liabilities. If it increases the tax due, the additional amount becomes payable, potentially with interest from the original due date.
When the amendment window has closed
Once the twelve-month amendment window has passed, the return can no longer be amended in the normal way. If the closed-window error means too much tax was paid, the route is a claim for overpayment relief. If it means too little tax was paid, the route is a disclosure to HMRC. The two routes are not symmetrical, and the time limits differ, so it matters which way the error runs.
Overpayment relief for tax paid that was not due
Overpayment relief is the mechanism for reclaiming tax that was overpaid where the amendment window has closed. A claim for overpayment relief must generally be made within four years of the end of the relevant tax year. The claim must be in writing, state that it is a claim for overpayment relief, identify the tax year, explain the grounds and the amount, and include a signed declaration that the details are correct. HMRC will not accept a late ordinary amendment in place of a properly made overpayment relief claim.
There are situations where overpayment relief is not available even within the four-year window, and they are worth knowing before relying on it. Relief is generally refused where the overpayment arose from a mistake in a claim or election that could have been corrected by other means, or where the taxpayer knew, or ought reasonably to have known, that the basis of the original assessment was wrong but did not act. The practical message is that overpayment relief is a genuine safety net for honest mistakes discovered late, not a way to revisit a deliberate choice once it has proved unfavourable.
Under-declared income: making a disclosure
Where the error means income was under-declared and too little tax was paid, and the amendment window has closed, the taxpayer should make a disclosure to HMRC rather than wait to be found. HMRC operates digital disclosure facilities for exactly this situation. Coming forward voluntarily (an unprompted disclosure) generally attracts lower penalties than waiting for HMRC to open an enquiry (a prompted disclosure). Interest runs on the unpaid tax from the original due date in either case, so disclosing sooner reduces the interest as well as the penalty.
Penalties for inaccuracies depend on whether the error was careless or deliberate, and on the quality of disclosure. A genuine, carefully-explained mistake disclosed unprompted typically attracts a far lower penalty than a deliberate omission found by HMRC. The key principle is that the taxpayer who comes forward first is treated more favourably than the one who is caught.
Reducing Payments on Account with an SA303
Payments on Account are advance payments towards the next year's tax, based on the current year's liability. If a taxpayer expects their next year's tax bill to be lower (for example, income has fallen or a one-off gain will not repeat), they can apply to reduce their Payments on Account using form SA303 or the equivalent online claim. This is not an amendment to a return; it is a forward-looking adjustment to the advance payments. Reducing Payments on Account too aggressively, so that the actual liability turns out higher, results in interest being charged on the shortfall, so the claim should be a realistic estimate rather than an optimistic one.
The SA303 route is included here because taxpayers frequently confuse reducing Payments on Account with correcting an error, and they are different things. Correcting an error fixes a figure on a return that has already happened. An SA303 claim adjusts an estimate of tax that has not yet been finally determined. A taxpayer whose income has genuinely fallen does not have an error to amend; they have an over-estimated payment to reduce. Using the wrong mechanism (trying to amend a prior return to lower a Payment on Account, for instance) does not work and wastes time.
Errors HMRC corrects itself
HMRC can correct obvious mistakes or arithmetical errors in a return within nine months of the date the return was filed. This is separate from the taxpayer's own twelve-month amendment right. If HMRC makes such a correction and the taxpayer disagrees, the taxpayer can reject the correction. Most HMRC corrections are minor (a transposed figure, an obvious slip), but a taxpayer should always check a corrected calculation rather than assume HMRC's version is right.
A useful way to picture the different correction routes is to think of them as a sequence in time. While the return is open and before the deadline, the taxpayer simply refiles. For twelve months after the deadline, the taxpayer amends. HMRC has its own nine-month correction power running in parallel. Once the amendment window closes, the routes split by direction: overpayment relief for tax overpaid, and disclosure for tax underpaid, each with its own time limit and its own evidence requirements. Identifying which stage applies is the first step in fixing any post-filing mistake.
Interest and penalties on extra tax
| Situation | Interest | Penalty |
|---|---|---|
| Amendment within 12 months, more tax due | Interest from original due date | Usually none if simply corrected |
| Overpayment relief claim (tax overpaid) | Repayment interest may apply | None |
| Unprompted disclosure of under-declared income | Interest from original due date | Reduced, based on behaviour |
| Prompted disclosure after HMRC enquiry | Interest from original due date | Higher, based on behaviour |
| Deliberate concealment found by HMRC | Interest from original due date | Highest penalty band |
I made a mistake but the return is not yet due. Can I just refile?
Yes. If the return has been submitted but the 31 January deadline has not yet passed, simply log back in, correct the figures, and resubmit before the deadline. HMRC treats the latest submission before the deadline as the return. There is no penalty for correcting a return you filed early, which is one of the advantages of filing well before the deadline: it leaves time to spot and fix mistakes.
How long should I keep records in case of a correction?
Self-employed taxpayers and landlords should keep records for at least five years after the 31 January filing deadline; other taxpayers should keep them for at least 22 months after the end of the tax year, and longer if a return is filed late or amended. Because overpayment relief claims and HMRC enquiries can reach back several years, keeping records for the full statutory period protects the taxpayer's ability to evidence any later correction or claim.
Will amending a return trigger an HMRC enquiry?
Amending a return does not automatically trigger an enquiry, and the right to amend within twelve months is a normal part of the system. HMRC can open an enquiry into a return (or an amendment) within statutory time limits, but a careful, well-evidenced correction is not in itself a red flag. Trying to hide an error is far riskier than correcting it openly: a voluntary, accurate amendment or disclosure is treated more favourably than an omission later discovered.
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Self Assessment Tax Returns in HarrowThis article reflects current HMRC guidance as of April 2026. Key references: HMRC Self Assessment overview, HMRC SA returns collection. Tax rules change annually. Always verify deadlines and thresholds at gov.uk or with a qualified accountant.
Our editorial team includes ACCA-qualified accountants and tax writers with experience across self-employment, rental income, and HMRC compliance. All articles are reviewed annually against current HMRC guidance and updated where rules change.