by Asim Abbas
11 November 2026 
How to amend a tax return and claim back overpaid tax
How to amend a tax return and claim back overpaid tax
Spotted a mistake after filing your tax return? You’re not alone. Missed expenses, wrong figures and forgotten income are all common.
- Amend within 12 months of the 31 January deadline
- For 2024/25, the deadline is 31 January 2027
- Older overpayments: overpayment relief for 4 years
- Owe more? Pay quickly to limit interest
You can usually amend a return online within 12 months of the filing deadline, and older mistakes can still be fixed through other routes.
This guide explains the deadlines, how to amend and what happens if you owe more or less tax.
Amendment deadlines
| Tax year | Online filing deadline | Amend by |
|---|---|---|
| 2023/24 | 31 January 2025 | 31 January 2026 (passed) |
| 2024/25 | 31 January 2026 | 31 January 2027 |
| 2025/26 | 31 January 2027 | 31 January 2028 |
If you filed late, the amendment window still runs to 12 months after the normal deadline, not 12 months after you filed.
How to amend online
- Sign in to your HMRC online account
- Choose the tax year and select the option to amend your return
- Change the figures and resubmit
- Check the new tax calculation
If you used commercial software, amend through that software. Paper returns are amended by sending corrected pages to HMRC.
Self Assessment tax calculator
Check what you should have paid.
Common reasons to amend
- Allowable expenses you forgot to claim
- Pension contributions or Gift Aid you left out
- Interest, dividends or rental income you missed
- A P60 or P11D figure entered incorrectly
- Capital gains you didn’t report
Leah filed her 2024/25 return in January 2026, then realised she forgot £1,800 of business mileage. As a basic rate taxpayer with Class 4 NI at 6%, amending saves her £468 (26% of £1,800). She amends online before 31 January 2027 and HMRC repays the difference or reduces her balance.
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Get a fixed-fee quoteCall 0161 531 4179Mistakes older than 12 months
Once the amendment window closes:
- You overpaid: claim overpayment relief in writing within four years of the end of the tax year. For 2022/23, that’s by 5 April 2027
- You underpaid: tell HMRC in writing. Coming forward yourself keeps penalties lower
If you owe more tax
Pay the extra as soon as possible. Interest runs from the original due date. Penalties may apply if HMRC thinks the error was careless, but an unprompted correction usually reduces or removes them. If you can’t pay all at once, you can set up a Time to Pay arrangement online for debts up to £30,000.
When HMRC corrects your return
HMRC can correct obvious errors within nine months of you filing. You can reject a correction if you disagree. HMRC can also open a compliance check, usually within 12 months of filing.
Read HMRC’s guide to correcting a tax return. Our Self Assessment team can review past returns for missed reliefs, and our disclosure guide covers bigger undeclared amounts.
What should I do next?
Gather the right figures.
12 months after 31 January.
Or in your software.
Frequently Asked Questions
How long do I have to amend a tax return?
12 months from the 31 January filing deadline. For 2024/25, that is 31 January 2027.
Will I get a penalty for amending?
Not usually if you correct the error yourself promptly. Interest is charged on any extra tax.
Can I amend a return more than a year later?
Not online. Use overpayment relief within four years if you overpaid, or write to HMRC if you underpaid.
How long does a refund take after amending?
Often a few weeks, although HMRC may run checks first.
Can my accountant amend my return?
Yes. An authorised agent can amend it for you.
Need help with this? Wingate Accountants offers fixed-fee online accountancy support across the UK. Call 0161 531 4179 or request a quote at wingateaccountants.co.uk.
About the Author
Stephen Reynold
I’m Stephen Reynold, an accountant at Wingate Accountants. I work with sole traders and small companies, turning tax rules into clear, practical next steps.
