How to Amend a Self Assessment Tax Return After You Have Filed It (2026/27)

This guide is general information, not advice.

This is for anyone who has filed a Self Assessment return and then spotted a mistake: income left out, an expense missed, a box filled in wrongly. The fix depends on how long ago the deadline was and how you filed.

By the end you will know whether you are still inside the amendment window, which route to use (online, paper or software), what happens to your bill, and what to write to HMRC if the window has closed.

Before you start

Have these to hand:

• your Unique Taxpayer Reference (UTR) and Government Gateway sign-in details
• the tax year of the return you are correcting, and a copy of the return or your SA302 and tax year overview
• the corrected figures and the records behind them
• your bank details, if you expect a refund

Step-by-step

1. Check you are inside the window. GOV.UK’s change your return page says you can correct a return within 12 months of the Self Assessment deadline, online or by sending another paper return. Its example: a 2024/25 return can usually be changed until 31 January 2027. HMRC’s manual at SAM124165 says the filing date for this purpose is 31 January after the tax year, or 3 months from a late-issued notice to file if later, whether you filed on paper or online. If you are outside the window, skip to step 5.

2. Online returns: wait 72 hours, then amend. GOV.UK says you must wait 3 days (72 hours) after filing before updating. Then sign in to your account, and from “Your tax account” choose “Self Assessment account”, then “More Self Assessment details”, then “At a glance” from the left-hand menu, then “Tax return options”. Choose the tax year, go into the return, make the corrections and file it again.

3. Paper returns: send corrected pages. Download the SA100 and any supplementary pages you need, or call HMRC for them. Write “amendment” on each page, add your name and UTR, and send them to the address on your Self Assessment paperwork. If you cannot find it, GOV.UK gives Self Assessment, HM Revenue and Customs, BX9 1AS.

4. Commercial software: go to the provider. GOV.UK says to contact the software provider for help correcting the return, and to contact HMRC if the software cannot make corrections.

5. After the window: under-declared tax. If it is more than 12 months after the deadline, the corrections page says to write to HMRC with the tax year you are correcting, why you think you paid too little, how much you think you underpaid, and your signature. HMRC reviews it and may update what you owe.

6. After the window: overpaid tax. You claim overpayment relief in writing, up to 4 years after the end of the tax year it relates to. GOV.UK lists what the letter must include, or the claim is rejected:

   • that you are making a claim for overpayment relief
   • the tax year you think you overpaid for
   • why you think you paid too much
   • how much you think you overpaid
   • whether or not you have previously appealed about the same payment
   • a declaration that the details are “correct and complete to the best of your information and belief”, signed by you (or a company officer for a company). No one else, including a tax agent, can sign it.

   You can say how you want to be repaid, and you must keep evidence of the overpaid tax. HMRC’s manual at SACM12150 says the claim cannot be made on a tax return form, and gives the address for Income Tax and Capital Gains Tax claims as PAYE Self-Assessment, HM Revenue and Customs, BX9 1AS.

Deadlines and what happens next

Online amendments show the amended bill straight away. Within 3 days your statement also shows the difference from the old one and any interest; see it under “View statements” in your account. If you owe more, the statement shows the deadline for paying and the effect on any payments on account. If you filed a paper amendment, HMRC sends an updated bill and pays any refund into your bank account, provided your bank details are on the return.

If you are owed money, the claiming a refund page warns you may not get it paid out if tax is due in the next 45 days, for example a payment on account: it is set against that instead.

If you owe more, HMRC’s manual at SALF302 says extra tax from an amendment made after the filing date is due 30 days after it is notified to you, but interest runs from the original due dates for the year. CH143240 puts it the same way: the interest start date is when the tax would have been due had the original return been right. HMRC’s interest rates page sets late payment interest at base rate plus 4% from 6 April 2025, and repayment interest at base rate minus 1% with a 0.5% floor.

Overpayment relief is not a second amendment window. HMRC’s manual at SACM12080 says you are not entitled to it while you can still amend the return. For a 2026/27 return, the amendment window runs to 31 January 2029 (12 months after the 31 January 2028 online deadline), and the 4-year overpayment relief limit ends on 5 April 2031.

Common mistakes

• Counting 12 months from the day you filed. The window runs from the filing deadline, so an early filer gets no extra time and loses none.
• Amending within 72 hours. The online service needs 3 days after filing.
• Using an overpayment relief letter while the window is open. Amend the return instead; the claim will not be accepted while you still can.
• Leaving an item out of the claim letter. GOV.UK says a claim missing any of the listed items is rejected, and an agent’s signature does not count.
• Waiting to pay extra tax. Interest runs from the original due date, so the 30-day payment date does not stop it building.

Worked example

You are a sole trader outside Scotland with 2026/27 profits of £30,000, so all of your taxable profit sits in the basic-rate band. You file online on 10 May 2027. Your amendment window runs to 31 January 2029, 12 months after the 31 January 2028 deadline.

Amending down. In June 2027 you find £1,000 of allowable expenses you missed. Your profit falls to £29,000. At the 20% basic rate of Income Tax and the 6% main Class 4 rate for 2026/27 (per the self-employed NI rates page), your bill falls by £200 plus £60, so £260. Class 2 does not change: your profits are still over £7,105, so it is treated as paid.

Amending up. You also find £500 of sales you left out. At the same rates that adds £100 plus £30, so £130. Both changes go in one amendment, a net reduction of £130. Because you amended before the 31 January 2028 due date, no interest has arisen; had you amended only for the missing sales, in March 2028, interest on the £130 would run from 31 January 2028.

Related reading

• Self Assessment filing walkthrough 2026/27 - the filing process from start to submit.
• How to pay your Self Assessment tax bill - paying extra tax after an amendment, and the reference to use.
• How to get your SA302 and tax year overview - checking the corrected calculation once the amendment is processed.