What happens if you discover a mistake on your tax return? The good news is that errors can usually be corrected, but it is important to take action as soon as possible to avoid paying the wrong amount of tax or missing out on a possible refund.

If you realise that you have made an error after submitting your self-assessment tax return, you can normally amend your return within 12 months of the self-assessment filing deadline. The amendment can be made online or by submitting a revised paper return. For example, your self-assessment for the 2024-25 tax year can usually be amended up to 31 January 2027.

If you amend your return online, your tax calculation will be updated immediately and will show whether you owe additional tax or are entitled to a repayment. Any changes may also affect payments on account.

If the 12-month amendment period has passed, you will need to contact HMRC in writing. This applies if you need to report income that was missed from your return or if you believe you have paid too much tax and want to claim overpayment relief.

Overpayment relief claims can generally be made up to four years after the end of the relevant tax year. You must explain why you believe the tax has been overpaid, provide details of the amount involved, and confirm that the information provided is correct and complete.