How to amend Self Assessment tax return after filing it

PHS Team

01 Sep, 2026

If you’ve submitted your tax return and later realised something is wrong, don’t worry. You can amend the Self Assessment tax return online or by paper within 12 months of the original filing deadline if you made a mistake, missed income, claimed the wrong expenses, or entered the wrong figures. 

You can correct your tax return online through your HMRC account, through your accounting software, or by sending an amended paper form if you filed by post. It is a simple process, and in this blog, we will explain each step clearly, how long you have to make changes, and what to do if HMRC has already identified an error.

Why You May Need to Amend a Tax Return

You may need to amend a tax return for some reasons. Common situations include:

• You forgot to include some income
• You claimed the wrong expenses
• You added the wrong figures by mistake
• You later found receipts or invoices that were missed
• HMRC contacted you about a difference in records
• You used the wrong tax code or allowance

Mistakes happen to everyone. What matters is correcting them as soon as possible. HMRC has clear errors and mistakes guidance to help people understand what needs to be fixed. Correcting your return early can help avoid penalties and interest charges.

How to Amend a Self-Assessment Tax Return

If you’ve already submitted your Self Assessment tax return and later realise something needs correcting, you can amend it within 12 months of the original filing deadline. For example, if you filed your 2025/26 tax return by 31 January 2027, you have until 31 January 2028 to make any necessary changes. 

To amend your return online:

1. Log in to your HMRC online account
2. Open the Self Assessment section
3. Select “More Self Assessment details”
4. Select “At a glance” on the left side of the screen.
5. Go to “Tax return options”
6. Choose the tax year you want to update
7. Click “Amend Return” and enter the correct details

Once you’ve made the updates, simply submit the amended return. Keep a copy of the confirmation message or PDF for your records.

Amending a Paper Return Within the 12-Month Period

If you originally filed your tax return on paper, you generally need to make the amendment on paper as well. The process is straightforward, but each step should be completed carefully to ensure HMRC can process the correction.

Steps to Amend a Paper Self Assessment Return

1. Obtain the correct form for the tax year you want to amend, such as the SA100 form.
2. Clearly write “Amended” at the top of each page of the form.
3. Include your:
   • Full name
   • Unique Taxpayer Reference (UTR)
   • National Insurance number
   • Tax year being corrected
4. Enter the corrected figures in the relevant sections of the form.
5. Include a brief note explaining:
   • What you are changing
   • Why the change is necessary

Where to Send the Amended Return

If no other address is provided, send your amended tax return to:

1. Self Assessment
2. HMRC
3. BX9 1AS
4. United Kingdom

HMRC will review your amended return and update your tax position where necessary. This may result in a refund if you have overpaid tax or an additional amount to pay if your tax liability increases.

Amending After 12 Months

If the 12-month amendment deadline has passed, you generally cannot update your return through your HMRC online account or by simply submitting the form again. Instead, you need to write to HMRC to request the correction.

Your letter should include:

• The tax year you want to correct
• Details of the mistake and why you believe your tax position is incorrect
• The corrected figures and an explanation of how you calculated them
• The amount you believe has been overpaid or underpaid
• Your signature (typed names are not accepted)

If you are requesting a refund, you should also include:

• Evidence of the original tax payment
• A statement confirming that you have not already claimed the refund
• Your preferred refund method, such as your bank details

This process is more formal because HMRC will review the information provided and decide whether the correction can be made. It may take some time, and HMRC may request additional information or supporting documents in some cases.

What Is the Time Limit to Amend a Tax Return?

If you need to amend a tax return, it is important to understand how far back you can amend a tax return in the UK. HMRC allows you to make changes within a specific period after the original filing deadline.

For the 2025/26 tax year, the online Self Assessment tax return deadline is 31 January 2027. You generally have until 31 January 2028 to amend your return.

For example:

• The 2025/26 tax return deadline is 31 January 2027.
• You have until 31 January 2028 to make an amendment.

If more than 12 months have passed since the amendment deadline, you cannot simply make the change through your online Self Assessment account. 

However, depending on the circumstances, other options may be available:

When Will I Hear Back from HMRC?

Once you submit an amendment, HMRC usually processes the changes within four weeks. However, it may take longer during busy periods, particularly around the Self Assessment deadline in January. If your amendment results in a refund, it may take a further one to two weeks for the money to be issued after the amendment has been processed.

You can check the progress of your amendment through your HMRC online account or contact HMRC directly if there are significant delays.

If HMRC Finds an Error

HMRC may sometimes contact you if they identify something unusual in your tax return. This does not necessarily mean that you have made a mistake. They may simply require further information or clarification.

However, if HMRC determines that you have underpaid tax, it may:

• Recalculate your tax bill
• Charge interest on the outstanding amount
• Apply penalties if the error was careless or deliberate

If you identify a mistake yourself and correct it promptly, any applicable penalty may be lower. HMRC generally encourages taxpayers to make voluntary disclosures and correct errors as soon as possible.

Conclusion

Amending your Self Assessment tax return is usually straightforward if you identify the mistake within the permitted 12-month amendment period. Acting promptly can help you avoid unnecessary complications, ensure your tax position is accurate and reduce the risk of penalties or interest where applicable.

However, correcting tax returns can become more complicated when dealing with older tax years, significant errors or changes that affect your overall tax liability. This is where professional support can make the process easier.

PHS Associates can help you review your tax return, identify errors, calculate the correct figures and guide you through the amendment process. Our experienced team can also help ensure your Self Assessment obligations are handled accurately and efficiently, giving you greater confidence that your tax affairs are in order.

Need help amending your Self Assessment tax return? Contact Us today for professional accounting and tax support.

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