How to amend a Self Assessment tax return after filing it

You’ve religiously followed all the TV adverts reminding you to make sure you file your Self Assessment tax return on time.

Positively inspired, you feverishly submit your tax return ahead of the fast-approaching deadline only to realise—a few days after 31 January—that you made a mistake in it.

The good news is, there’s a quick and easy way for you to amend your tax return even after you’ve sent it to HMRC. This article will show you how.

It will also let you know the deadline to amend your tax return by, when your tax bill will be updated, and what to do if you think the mistake in your Self Assessment makes you liable to pay more tax or entitles you to a refund.

What are the possible reasons for amending a tax return?

You may need to make a change to your Self Assessment tax return for a variety of reasons.

Here’s a few examples:

• You’ve changed your name or address in the same year that your tax return is due
  

• You entered the wrong amount(s) of taxed or untaxed income

• You want to claim relief for overpayment (a refund)
  

Amendments to your Self Assessment won’t always show on your tax bill. It depends on the information you’re correcting.

For instance, if you’re changing your name or updating your address, there won’t be any adjustments to your actual tax bill.

But let’s say you’re adding untaxed income, your tax bill will definitely reflect that.

What is the deadline to amend a tax return?

You can make changes to your Self Assessment within one year of the original filing deadline either online or by sending another paper return to HMRC.

For example, in the 2024 to 2025 tax year, the filing deadline—for online and paper returns—is 31 January 2026. In this case, you need to amend your return by 31 January 2027.

If you fail to meet this deadline or you need to correct a tax return from a previous year, you must contact HMRC in writing.

How to amend a tax return within one year of the filing deadline

If you submitted your Self Assessment online, you have to wait three days after filing before you can make any changes to your tax return.

Once this time has passed, take the following steps:

• Sign in to your HMRC (Government Gateway) account
  
• Select “Self Assessment account”
  
• Select “More Self Assessment details”
  
• Select “At a glance” from the menu on the left-hand side of your screen
  
• Select “Tax return options”
  
• Select the tax year for the return you want to amend
  
• Select the appropriate tax return, then make the necessary changes and re-file it.
  

If you want to amend information on a paper return, here’s what you need to do:

• Either download the Self Assessment tax return form or call HMRC and request the form to be sent out to you.
  
• Write “amendment” on each page of the Self Assessment form and be sure to include your full name and Unique Taxpayer Reference (UTR), which you can find on previous tax returns or in correspondence from HMRC.
  
• Send the amended Self Assessment form to the address listed on it. If you’re unable to find the address, send it to: Self Assessment, HMRC, BX9 1AS. But if you’re sending your letter by courier, send it to this address instead: HMRC, BP8002, Benton Park View, Newcastle Upon Tyne, NE98 1ZZ.
  

If you used an accountant to file the Self Assessment on your behalf, contact them as soon as you’re aware that certain information needs to be changed on your tax return.

How to amend a tax return more than one year after the filing deadline

If you miss the one-year deadline to amend your tax return, all is not lost.

It’s still possible to make changes to your Self Assessment, but you need to write a letter to HMRC to let them know the situation.

In your letter, it’s important to include the following details:

• The tax year you’re amending information in relation to
  
• The information that needs to be corrected
  
• The reason you believe you’ve overpaid or underpaid tax in that tax year (if applicable)
  
• The amount you’ve overpaid or underpaid in tax (if applicable)
  
• Your signature written in black pen (your printed name alone won’t suffice).

If you’re claiming that the mistake on your tax return means you’ve overpaid tax, which may entitle you to a refund, your letter to HMRC must also:

• State that you’re making a claim for a tax refund in that year.
  
• Provide evidence that you’ve already paid the relevant tax bill.
  
• Confirm that you haven’t previously claimed this refund.
  
• Indicate how you want to receive your refund (for example, by bank transfer or cheque) and the information HMRC will need to make the payment.
  
• Submit a signed declaration, which confirms that the information you’ve provided is “correct and complete to the best of your information and belief.”
  

It’s probably best not to skimp on providing any of this information because HMRC has been known to reject claims that have.

If you made changes to your tax return online, you should expect to see your updated tax bill within three days, which will show:

• The difference between your original tax return and the amended one, so you can see if you need to pay more (or less) tax. If more tax is due, HMRC will issue a new deadline for you to pay the outstanding tax bill. If you’re due a refund, you can select the “Request payment” section of your account and request a refund (although it can take up to a month before you receive the funds in your account).
  
• Any interest payable.
  

To access your amended tax return, sign in to your account and select “View statements” from the left-hand side of your screen.

If you amended a paper tax return, you should expect to receive an updated tax bill from HMRC within four weeks.

If you owe more tax, you’ll receive written instructions on how to proceed. And if you’re owed a refund, you’ll receive it with (or shortly after) the letter from HMRC.

Final thoughts

We all make mistakes but, when it comes to filing your Self Assessment, the most important thing is to correct errors as soon as possible (and, where appropriate, pay more tax or claim any refund that may be due to you).

After all, it’s better for you to be proactive in course correcting an innocent mistake rather than HMRC picking up on your error and potentially assuming foul play.

As a sole trader or small business owner, one of your goals is to be tax efficient.

This involves consistently applying the “Goldilocks” rule, paying just the right amount of tax—not too much and not too little—even if you occasionally have to amend your tax return after it’s been filed.