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

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If you’ve made an error or mistake, you’ll want to know how to amend a tax return. Thankfully, it’s possible to make changes even after you’ve submitted your Self Assessment.

Here’s the HMRC errors and mistakes guidance that self-employed people should know.

Can I amend my tax return?

Perhaps you inputted the wrong figures, or an HMRC error means you’ve over or underpaid your tax.

Errors and mistakes are frustrating, but the good news is yes – you can amend your tax return after filing.

You’ll need to wait 72 hours after filing before making a change, plus there’s a deadline for making changes.

This deadline is fairly generous however, so as long as you spot the change you want to make in good time, you should be OK.

Your amended tax return will then lead to an amended bill, which will show whether you owe tax or are due a refund.

How to amend a tax return

The method for amending a tax return depends on whether you filed online or on paper.

How to amend a tax return online

Most people now submit digitally, though. So if you filed online, you can make a tax return amendment 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

How to amend a tax return on the HMRC app

You can also amend your tax return using the HMRC app. If you’re using the app for the first time, you’ll need to sign in using your Government Gateway user ID. After this point, you’ll be able to sign in using:

  • your phone’s fingerprint or facial recognition software
  • a 6-digit PIN number

Once you sign in to the app, there’s a section titled ‘Self Assessment’ which is where you’ll find any previous or upcoming payments.

Other ways to amend a tax return

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.

How far back can you amend a tax return?

While amending a tax return is certainly possible, there is a time limit. If you notice a mistake on your tax return, you need to make any changes within 12 months of the normal 31 January Self Assessment deadline. For example:

  • for the 2022-23 tax year, you’ll need to make any changes by 31 January 2025
  • for the 2023-24 tax year, you’ll need to make any changes by 31 January 2026

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

It’s possible to claim a tax refund up to four years after the end of the relevant tax year. You’ll need to write to HMRC, saying that you’re making a claim for overpayment relief. There’s more on this below.

What if I’ve missed the deadline for amending a tax return?

As said above, if you want to amend your Self Assessment but have missed the relevant deadline, you need to 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’re claiming a tax refund, you need to say that you’re claiming ‘overpayment relief’ and include:

  • the tax year for which you think you overpaid
  • the amount you think you’ve overpaid
  • why you think overpayment has happened
  • that you’ve not tried to claim this refund before
  • a signed declaration that the information you’ve given is “correct and complete to the best of your knowledge”

You may need to give documentary proof of tax deducted, so be sure to have this ready.

If your claim is successful, you’ll get a refund to your bank account within four weeks.

What happens if you make a mistake on your tax return?

By following the steps in this article, you should be able to fix any mistakes in your tax return. The important thing is to do this in the allotted timed and be up front about what happened.

Failing to amend an incorrect tax return could result in fines or even an investigation from HMRC.

HMRC errors and mistakes that aren’t your fault

Of course, HMRC can make mistakes too, whether these involve computer problems or human error. This makes it important to check your tax calculations.

It’s also important to keep accurate tax records, so that you can prove the mistake isn’t on your end.

If for example HMRC is investigating you for tax underpayment but you’re sure you’ve paid correctly, your tax returns, receipts, invoices, and bank statements will be needed as evidence.

You can ask the tax authority to cancel the tax you owe if there was a delay in them asking you for information or they didn’t act on information they had.

What does ‘reasonable care’ mean when amending a tax return?

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.

Gov.uk 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.

More tax information for small business owners

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

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Sam Bromley

Sam has more than 10 years of experience in writing for financial services. He specialises in illuminating complicated topics, from IR35 to ISAs, and identifying emerging trends that audiences want to know about. Sam spent five years at Simply Business, where he was Senior Copywriter.

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