How to amend Self Assessment tax return after filing it

PHS Team

20 Nov, 2025

If you’ve submitted your tax return and later realised something is wrong, don’t worry. You can amend 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 the 2024/25 return by 31 January 2026, you have until 31 January 2027 to make any 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

If you originally filed your tax return on paper, the amendment needs to be made on paper as well. The process is simple, but each step must be done carefully to make sure HMRC accepts the correction.

Steps to Amend a Paper Self-Assessment Return

1. Get the correct form for the tax year you want to amend (e.g., SA100 form).
2. Write “Amended” clearly 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.
5. Add a short note explaining:
   • What are you changing
   • Why the change is needed

Where to Send the Amended Return

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

Self Assessment
HMRC
BX9 1AS
United Kingdom

HMRC will then review your updated return and confirm whether your tax position changes, which may result in a refund or an extra amount to pay.

Tip: Always keep copies of invoices, receipts, business bank statements, and accounting records. These are helpful if HMRC requests evidence.

Amending After 12 Months

If the 12-month amendment deadline has passed, you cannot update your return through your HMRC online account or by re-filing the form. Instead, you must write directly to HMRC to request the change.

Your letter should include:

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

If you are asking for a refund, also include:

• Evidence of the original tax paid
• A statement confirming you haven’t already claimed the refund
• Your preferred refund method (e.g., bank details)

This approach is more formal because HMRC will review the information and decide whether the change can be made. It can take some time, and in some cases HMRC may request additional documentation.

Tip: If you are not sure how to present the correction or calculate the revised figures, professional support can help avoid errors and delays.

What Is the Time Limit to Amend a Tax Return?

If you need to amend a tax return, it’s important to know how far back you can amend a tax return in the UK, because HMRC only allows changes for a set period after the original filing deadline.

You have 12 months from the Self Assessment deadline to make changes. For example:

• The 2024/25 tax return deadline is 31 January 2026
• You have until 31 January 2027 to amend that return

If more than 12 months have passed, you cannot simply amend online. However, there may still be options:

In more complex cases, such as disputes or old tax years, professional help may be necessary.

When Will I Hear Back from HMRC?

Once you submit an amendment, HMRC usually takes up to 4 weeks to process the changes, but it can take longer during busy periods (especially January). If your amendment results in a refund, it may take a further 1-2 weeks for the money to be issued after the update is processed. 

You can check the progress anytime through your HMRC online account or by contacting HMRC directly if there are delays.

If HMRC Finds an Error

Sometimes HMRC may contact you if they notice something unusual in your tax return. This does not automatically mean you did something wrong. It may simply mean they need clarification.

However, if HMRC believes you underpaid tax, they can:

• Recalculate your bill
• Charge interest
• Add penalties (if the mistake was careless or deliberate)

If you find a mistake yourself and amend the tax return quickly, penalties are often lower. HMRC encourages voluntary correction.

Conclusion

Amending a Self Assessment tax return is simple if done within the allowed time. Correcting errors accurately helps avoid penalties and keeps your tax records accurate.

How We Help You

If you need an accountant to help amend a Self Assessment tax return after filing it, our team can assist you. We can handle the process of amending your Self Assessment tax return for you, whether online or on paper. We make sure all corrections are accurate and submitted on time. Call us at 0208 8611685 or email info@phs-uk.co.uk for expert support.

Frequently Asked Questions