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Change a Self Assessment Return After Filing: 12-Month Rule
General information, not tax advice. We are not HMRC and are not affiliated with it. Check GOV.UK. Last checked against GOV.UK: 17 October 2026. Anything marked expected should be re-checked. Examples use made-up figures. This page contains no affiliate links. Never enter your UTR, National Insurance number or other identifiers on this site.
Short answer: GOV.UK says you can correct a tax return within 12 months of the Self Assessment deadline, online or by sending another paper return. GOV.UK's example: for the 2024 to 2025 tax year you'll usually need to change your return by 31 January 2027. By the same rule, the corrections deadline for the 2025-26 return is expected to be 31 January 2028 (Monday), our calculation, not a published GOV.UK date.
How (GOV.UK)
- Online: wait 3 days (72 hours) after filing, sign in, go to your Self Assessment account, choose the tax year, correct the return and file it again.
- Paper: send the corrected pages to the address on your Self Assessment paperwork, writing "amendment" on each page and including your name and UTR (do not enter these on any website other than HMRC's).
- Commercial software: contact the provider; contact HMRC if the software can't make corrections.
What changes
An online correction updates your bill straight away; within 3 days your statement shows the difference and any interest. If you owe more, the updated bill shows the payment deadline and the effect on payments on account (payments on account). If you're owed tax you may be able to claim a refund.
Provisional figures
If you don't know your profit for the whole year before the deadline, GOV.UK says to include provisional figures, tell HMRC you've used them, and change the return within 12 months once you know. Interest on extra tax runs from the original due date.
After the 12 months
GOV.UK says you need to write to HMRC to report income you left out (stating the tax year, why you underpaid, the amount and your signature) or to claim overpayment relief, available up to 4 years after the end of the relevant tax year, with a signed declaration.
FAQ
Can an agent sign an overpayment claim? GOV.UK says no one else, including a tax agent, can sign on your behalf.
Does correcting avoid a penalty? Not covered on this page.
What we could not verify
- Whether and when penalties apply to errors corrected within the 12 months.
- The 31 January 2028 date is our arithmetic.
- Rules for amending returns for other forms (partnerships, trusts).
Sources (official)
- GOV.UK, Self Assessment tax returns: If you need to change your return — https://www.gov.uk/self-assessment-tax-returns/corrections (read 17 October 2026)
- GOV.UK, Self Assessment tax returns: Sending a return — https://www.gov.uk/self-assessment-tax-returns/sending-return (read 17 October 2026)
Deadlines calendar → Penalty estimator →
Not tax advice. See the disclosure.