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HMRC errors and mistakes guidance: how to amend a tax return

4-minute read

Simply Business Editorial Team

17 December 2021

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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 amend your tax return even after it’s been submitted. Read on to find out how.

What to do if you need to amend your tax return

A Self Assessment error or mistake can be frustrating, but there are things you can do. Perhaps you inputted the wrong figures? Or maybe an HMRC error means you’ve over or underpaid your tax? Read on for what to do now.

When can I file an amended tax return?

It’s possible to make changes if you’ve made a mistake on your Self Assessment tax return after filing it, but there is a time limit. Any changes must be made within 12 months of the normal 31 January filing deadline. For example:

  • for the 2019-20 tax year, you’ll need to make any changes by 31 January 2022
  • for the 2020-21 tax year, you’ll need to make any changes by 31 January 2023

If you miss the deadline, you may still be able to make the changes but you’ll have to write to HMRC.

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How to amend a tax return

How you amend your tax return will depend on how you filed it in the first place. Most people now file their Self Assessment returns online.

If you filed online, you can make a tax return correction by following these steps:

  1. Sign into your account using your Government Gateway user ID and password
  2. From ‘Your tax account’, choose ‘Self Assessment tax account’ (ignore this step if you don’t see this option)
  3. Select ‘More Self Assessment details’
  4. Select ‘At a glance’
  5. Select ‘tax return options’
  6. Choose the tax year for the return you’re updating
  7. Make your corrections in the return, and file it again

If you filed your return on paper, you’ll need to download a new tax return and send HMRC the pages that need correcting.

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 Self Assessment paperwork.

If you used third-party software to file your return, you should contact the software provider for help on what to do. If you can't make changes using your chosen software, contact HMRC directly.

What if I’ve missed the deadline for making changes?

If you want to make a tax return correction 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

How to claim a refund for HMRC errors and mistakes

If you think you’ve paid too much tax because HMRC made a mistake, you can usually claim a full rebate. You’ll need to claim within four years of the end of the tax year you’re claiming for. This is called ‘overpayment relief’.

Overpayment relief claims must be made in writing and must:

  • clearly specify that you are making a claim for overpayment relief
  • identify the tax year for which you think you overpaid
  • include the amount you think you’ve overpaid
  • state why you think overpayment has happened
  • state if you’ve previously made an appeal with the payment or assessment
  • include a signed declaration that the information you’ve given is “correct and complete to the best of your knowledge”

You must also have documentary proof of tax deducted as you may need to provide this later.

If your claim is successful, a refund will be sent to your bank account within four weeks.

How to claim a refund if you’ve made a mistake

If you’ve paid too much tax because you made a mistake on your tax return, you’ll need to make any amendments within the first year of filing it.

You won’t have to pay a penalty if you’ve taken ‘reasonable care’ to fill out your tax return correctly. However, HMRC can issue penalties if you’ve understated tax or misrepresented your tax liability because you’ve been careless. explains how penalties are calculated:

  • if a penalty arises because of a lack of reasonable care, the penalty will be between 0 and 30 per cent of the extra tax due
  • if the error is deliberate, the penalty will be between 20 and 70 per cent of the extra tax due
  • if the error is deliberate and concealed, the penalty will be between 30 and 100 per cent of the extra tax due

Make sure you keep on top of bookkeeping for your business or hire an accountant if you’re not sure of anything.

What if I need to pay more tax?

Making changes to your tax return after filing may mean you’ve underpaid tax. If you’ve made your changes online, you should be able to see the new amount of tax you owe on your Self Assessment tax account within three days.

You’ll also see a deadline for paying the remaining tax. Remember that you may be charged interest.

Your new tax bill may also affect any payments on account you need to pay.

Updating personal information

It’s important to tell HMRC about changes to your personal details for Self Assessment as soon as they’ve changed.

If you’ve failed to do this and you need to change the address for your tax return after filing it, make sure you make any corrections following the process above (up to a year after the filing deadline).

You can update your address online if you’ve set up a personal tax account.

Do you have any unanswered questions about changing your tax return? Let us know in the comments.

Photograph 1: Paul Maguire/

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We create this content for general information purposes and it should not be taken as advice. Always take professional advice. Read our full disclaimer

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