Answers · UK 2025/26
I made a mistake on my tax return - how long do I have to correct it?
You normally have 12 months from the 31 January filing deadline to amend a Self Assessment return online. For 2025/26 (deadline 31 January 2027), you can amend until 31 January 2028. After that window you must claim 'overpayment relief' in writing, generally within 4 years of the tax year end.
Full answer
Spotting an error after you file is common, and HMRC gives you a built-in correction period. For online returns, log back into your HMRC account, select the return and choose 'Amend'. The deadline is 12 months after the filing deadline. Worked example: Aisha filed her 2025/26 return on 20 January 2027 but later realised she forgot GBP 3,000 of allowable expenses, overstating her profit. The filing deadline was 31 January 2027, so she has until 31 January 2028 to amend online. Removing GBP 3,000 of profit at the 20% basic rate cuts her Income Tax by GBP 600, plus 6% Class 4 NI saving of GBP 180 if the profit sits in the GBP 12,570 to GBP 50,270 band - around GBP 780 back. HMRC recalculates the bill and refunds any overpayment or adjusts payments on account. If the 12-month window has passed, you cannot amend online. Instead you make a separate claim for 'overpayment relief' (to reclaim tax) or notify HMRC of additional tax owed. Overpayment relief must usually be claimed within 4 years of the end of the relevant tax year and made in writing with specific wording. If you owe more after amending, pay promptly to limit interest (around 8.75%). The self-employed tax calculator helps you recheck the corrected figures before you resubmit. Full amendment rules are at gov.uk/self-assessment-tax-returns/corrections.
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This answer is informational only and does not constitute financial, tax or legal advice. Figures are for the 2025/26 UK tax year. See our methodology and sources.