How do I amend my Self Assessment tax return?

You can amend a filed tax return within 12 months of its 31 January deadline, online or on paper. For 2024-25 the last day is 31 January 2027. After that, you write to HMRC instead.

Mistakes on tax returns are common: a missed CIS statement, a receipt found in the van door in March, a month's rent left off. Fixing one is routine, and the bill is recalculated either way, so you may owe more or be due money back. The rules below follow GOV.UK as checked on 16 October 2026.

How long you have

The law sets the window in section 9ZA of the Taxes Management Act 1970: an amendment cannot be made more than 12 months after the filing date, which is 31 January after the end of the tax year. The window is the same whether you filed online or on paper. If HMRC sent you the notice to file after 31 October, your filing date was 3 months from that notice, and the 12 months run from then.

The right-hand column is for tax you paid but did not owe. A claim can be made up to 4 years after the end of the tax year, so a 2022-23 overpayment can still be claimed until 5 April 2027. While you can still amend, the law expects you to amend, because HMRC does not have to give overpayment relief where you can get the money back another way.

Online, on paper, or through software

Online. Wait 3 days (72 hours) after filing. Then sign in to your HMRC online account, open your Self Assessment account, choose More Self Assessment details, then At a glance, then Tax return options. Pick the tax year, make the corrections and file the return again.

Paper. Download or ask HMRC for the SA100 and any supplementary pages you need to change. Write "amendment" on each page with your name and Unique Taxpayer Reference (UTR), and send the corrected pages to the address on your Self Assessment paperwork. GOV.UK gives Self Assessment, HM Revenue and Customs, BX9 1AS if you cannot find it.

Commercial software. Ask the software provider how to correct the return, and contact HMRC if the software cannot do it.

After the 12 months

Once the window has closed, you cannot amend the return itself. You write to HMRC.

• You paid too little. Write with the tax year, why you think you underpaid, how much, and your signature.
• You paid too much. Claim overpayment relief in writing. Your letter must say it is a claim for overpayment relief, and give the tax year, why you overpaid, how much, and whether you have appealed the same payment before. It must end with a declaration that the details are "correct and complete to the best of your information and belief", signed by you. GOV.UK says no one else, including a tax agent, can sign it, and that a claim missing any of this is rejected. The law also lists cases where HMRC does not have to pay, for example a mistake in a claim or election, or where you could have amended in time and knew, or should have known, before the window closed that the relief was there.

Keep the evidence behind the claim, because HMRC may ask for it.

Under Making Tax Digital

If you are in Making Tax Digital for Income Tax (MTD), you change a submitted tax return through your compatible software, and the deadline is the same: within 12 months of the submission deadline. Your HMRC online account then shows an updated tax calculation.

If the mistake is in your digital records and you have not yet submitted the return, correct the records instead. A correction made during the year flows into your next quarterly update. One more catch: changing a return can change your MTD start date, because MTD is decided by income on past returns. If an amendment takes you below the threshold, GOV.UK says you can opt out or carry on voluntarily.

What this means for you

If you spot a mistake on your 2024-25 return, you have until 31 January 2027 to fix it online, and the corrected bill appears straight away. Within 3 days your statement shows the difference and any interest. If you owe more, the statement shows when to pay and how your payments on account change. If you are owed money, GOV.UK says you may be able to claim a refund.

For a CIS subcontractor who left out a statement, an amendment is how the missing deduction gets counted. Our guide to claiming back CIS from earlier years covers older years.

Steps to amend your return

1. Find the original return and your UTR, and work out which figures were wrong.
2. Check the dates: is today within 12 months of that year's 31 January deadline?
3. If it is, and you filed online, wait 3 days after filing, then amend through your HMRC online account or your software.
4. If you filed on paper, send the corrected pages marked "amendment", with your name and UTR.
5. If the window has closed, write to HMRC, with a signed overpayment relief claim if you paid too much.
6. Check your updated statement and pay any extra tax by the date it shows.

The free MTD check shows whether an amended figure changes your MTD date. There is more in the trades section and the landlord section.