If you have submitted a self-assessment return and later realise you need to make changes, there are specific rules to follow. This situation might arise if, for instance, you entered a number incorrectly or omitted certain information from your self-assessment return.

If you filed your return online, you could amend your return online as follows:

1. Sign in to your personal tax account using your Government Gateway user ID and password.
2. From ‘Your tax account’, choose ’Self-Assessment account’ (if you do not see this, skip this step).
3. Choose ‘More Self-Assessment details’.
4. Choose ‘At a glance’ from the left-hand menu.
5. Choose ‘Tax Return options’.
6. Choose the tax year for the year you want to amend.
7. Access the tax return, make the corrections, and file it again.

You must wait 3 days (72 hours) after filing before updating your return. If you opted to file your return on paper, you would need to download a new return and fill in the pages that you wish to change and write ‘amendment’ on each page. You must also include your name and Unique Taxpayer Reference on each page and then send the corrected pages to the address where you sent your original return.

If you used commercial software to submit your self-assessment return, then you should contact your software provider in the first instance. If your software provider cannot help, contact HMRC.

The deadline for making changes for the 2022-23 tax year using the methods mentioned above is 31 January 2025. If you miss this deadline, you will need to write to HMRC. For example, if you discover an error in your 2021-22 return after 31 January 2024. Your letter should specify the tax year you are correcting, explain why you believe you have overpaid or underpaid tax, and state the amount involved. You can request a refund up to four years after the end of the relevant tax year.