There are special rules to follow if you have submitted a Self-Assessment return and subsequently realise you need to change it. This can happen if for example you made a mistake like entering a number incorrectly or missing information from the return.

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

1. Sign into your personal tax account using your 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. Go into the tax return, make the corrections and file it again.

If you opted to file your return on paper, you will 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, then you should contact HMRC.

The deadline for making changes for the 2019-20 tax year using any of the methods outlined above is 31 January 2022.

If you have missed the deadline, you will need to write to HMRC instead. For example, if you found a mistake in your 2018-19 return after 31 January 2021. In the letter, you will need to say which tax year you are correcting, why you think you have paid too much or too little tax and by how much. You can claim a refund up to 4 years after the end of the tax year it relates to.