How do I change my Self Assessment tax return?

Self Assessment tax returns are stressful enough at the best of times - but what if you’ve filed yours and realise that you’ve made a mistake?

Thankfully, it’s possible to make a correction to your tax return even after it’s been submitted. Read on to find out how.

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When can I change my Self Assessment tax return?

If you’ve made a mistake on your Self Assessment tax return, you can make an amendment or correction. You must do this by the year after the filing deadline for the tax year you’re amending. For example:

  • For the 2016-17 tax year, you must make the change by 31 January 2019
  • For the 2017-18 tax year, you must make the change by 31 January 2020

If you miss this deadline you may still be able to make the change in some circumstances. In these cases you’ll have to write to HMRC. You can find out how to contact the Revenue here.

How do I update my tax return?

The way you make an amendment to your tax return will depend on how you filed it in the first place. Most people now file their Self Assessment returns online. If this the case for you, you can update your tax return by following these steps:

  1. Log in to your HMRC Self Assessment account

  2. Click ‘More Self Assessment details’

  3. Click ‘At a glance’

  4. Click ‘tax return options’

  5. Choose the relevant return

  6. Make the corrections in the return, and file it again

If you filed your return on paper, you’ll need to download a new version of the return and send HMRC the pages on which you’re making corrections. You can download a return here. Write ‘Amendment’ on each corrected page, and make sure that you include both your name and your Unique Taxpayer Reference (you can find your UTR on previous correspondence from HMRC). Send it to HMRC at the address found on your previous Self Assessment correspondence.

If you used third-party software to file your return, you should contact the software provider for guidance. If you can’t make amendments using your chosen software, contact HMRC directly.

What if I’ve missed the deadline?

If you want to make a change to a tax return but have missed the relevant deadline, you should write to HMRC. When writing to HMRC, make sure you include the following details:

  • The tax year for which you want to make a correction
  • The reason why you think you’ve paid an incorrect amount of tax
  • The amount you think you’ve either over or underpaid

If you think you’ve paid too much tax, you can claim a refund up to four years following the end of the relevant tax year. This is called ‘overpayment relief’. In these cases, you should also do the following:

  • Specify that you are making a claim for overpayment relief
  • Include proof that you paid tax through Self Assessment in the relevant period
  • Give details of how you wish to be repaid, for example by bank transfer
  • Specify that you’ve made no previous attempts to claim this refund
  • Include a signed declaration that the information you’ve given is “correct and complete to the best of your knowledge”

What if I need to pay more tax?

If you updated your tax return online, you’ll see an amended figure in your online Self Assessment account within three days. If you’re owed a refund, you can arrange payment to your bank account through the same online service. This can take up to four weeks.

If, as a result of your tax return changes, you owe more tax, you’ll also be given a deadline for payment. Remember that you may be charged interest. In addition, where applicable you may have to make additional payments against your payment on account.

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