How to Amend a Self Assessment Tax Return

To amend a Self Assessment tax return, you sign in to your HMRC online account, open the return for that year and file a corrected version, provided you are within 12 months of the 31 January deadline. For 2024/25, that window closes on 31 January 2027, the same day your 2025/26 return is due. This guide shows which of the three routes applies to your mistake and what each one costs you in time, interest and paperwork.

Key takeaways

• You can usually amend a return online or on paper within 12 months of its filing deadline.
• After that, you write to HMRC: to report missing income, or to claim overpayment relief.
• Overpayment relief can reach back four years from the end of the tax year concerned.
• Amendments that increase your tax usually bring interest and may change your payments on account.

What does "amending a return" mean?

An amendment is a correction you make to a return you have already filed, by submitting a new version that replaces the old one. HMRC then recalculates your tax and updates your account. It is different from HMRC correcting your return itself, and different from a formal claim once the amendment window has closed. GOV.UK sets out the rules on its page about correcting a tax return.

Which route fits your mistake?

The overpayment relief route is stricter. GOV.UK says the letter must state that you are making a claim, give the tax year, explain why you think you overpaid, say how much, and state whether you have already appealed. You must sign it yourself; an agent cannot sign on your behalf. Claims missing these details are rejected.

Steps to amend online

1. Pin down the change. Work out the corrected figure and keep the evidence: an invoice you missed, a P60 you misread, a revised mileage log.
2. Wait 72 hours after the original filing. GOV.UK says the online option is not available before then.
3. Sign in to your HMRC online account. Go to your Self Assessment account and find the return for the relevant year.
4. Change only what is wrong. Edit the affected boxes, then check the new calculation against what you expected.
5. File the amended return and save the confirmation. Keep a short note explaining what changed and why.
6. Check your statement. Look at the updated balance and your next payments on account, then pay anything extra promptly to limit interest.

If you filed on paper, GOV.UK says to send the corrected pages to the address on your Self Assessment paperwork, marking each page as an amendment and adding your name and Unique Taxpayer Reference.

Illustrative example: two corrections, two outcomes

This is an illustrative example with an invented profile and figures, not a real client. Jordan is a self-employed electrician who filed a 2024/25 return in December 2025. In November 2026, while gathering papers for 2025/26, Jordan finds two problems:

• A missed tool purchase. A £600 drill set bought in March 2025 was left off. As a basic rate taxpayer, Jordan's tax for 2024/25 should fall by about £120 in income tax, plus a smaller National Insurance saving. Jordan amends online and the overpayment is credited to the Self Assessment account.
• A missed invoice. A £1,500 job carried out and paid in late April 2025 belongs in 2025/26, not 2024/25, so it simply goes on the next return. No amendment needed.

Jordan also checks 2022/23. A £400 van repair was missed that year. The 12-month window for 2022/23 closed on 31 January 2025, so Jordan would need an overpayment relief claim by letter, reaching HMRC by 5 April 2027. Jordan weighs the time involved against the likely refund before deciding. The lesson: check older years now, while some routes are still open.

Common mistakes

• Leaving it until the last week of January. The 2024/25 amendment deadline lands on the busiest filing day of the year. Use our 31 January countdown to plan.
• Moving income into the wrong year. Some errors are timing issues, not amendments. Check which tax year the income or cost really belongs to.
• Not paying the extra tax quickly. Interest runs from the original due date, so delay costs money.
• Ignoring the new calculation. Compare the revised tax calculation with your own figures before you file.
• Keeping no record of why. If HMRC asks later, a one-paragraph note and the supporting receipt answer the question quickly. Our record keeping guide explains how long to keep them.

How we can help

We review filed returns, make amendments within the window and prepare overpayment relief claims where older years are affected. Personal tax returns start from £180/year on a fixed fee. See our personal tax service, check our pricing, or contact us with the year you want to correct.