The short answer: you can correct a Self Assessment tax return within 12 months of the filing deadline, online or on paper. For the 2024 to 2025 tax year, that usually means by 31 January 2027. After that you must write to HMRC, and if you paid too much you can claim overpayment relief up to 4 years after the end of the tax year.
Correcting a return online
Wait 3 days (72 hours) after filing before you update your return. Then sign in to your HMRC online account, choose the tax year and file the corrected return. Your bill updates straight away.
Correcting a paper return
Download a new tax return (SA100), write “amendment” on each page with your name and Unique Taxpayer Reference, and send it to the address on your Self Assessment paperwork. HMRC will send you an updated bill.
The time limit
| Tax year | Filing deadline (online) | Usual deadline to amend |
|---|---|---|
| 2024 to 2025 | 31 January 2026 | 31 January 2027 |
| 2025 to 2026 | 31 January 2027 | 31 January 2028 |
After the deadline has passed
If more than 12 months have passed since the filing deadline, you need to write to HMRC to correct the return. If you paid too much tax, you can claim overpayment relief up to 4 years after the end of the tax year it relates to.
If you owe more tax
Your updated bill shows the extra tax and when it must be paid. A correction can also change your payments on account for the next year. Correcting a mistake yourself, before HMRC finds it, can also reduce any penalty if the mistake was careless.
Common reasons to amend
- Income left out, such as a second job, interest, dividends or rental income
- Expenses you were entitled to but did not claim (see allowable expenses)
- Pension contributions or Gift Aid donations missed
- Figures entered in the wrong box, or estimates that can now be replaced with final figures
How we help
We review your return, work out the correct figures and file the amendment for you, or write to HMRC where the 12-month window has passed. See our Self Assessment service.
Sources: gov.uk, Self Assessment tax returns: corrections; Self Assessment tax returns: deadlines; Penalties for inaccuracies in returns or documents (CC/FS7a). Checked 6 October 2026.
Common questions
How long do I have to amend my tax return?
Usually 12 months from the 31 January filing deadline. For the 2024 to 2025 tax year, that is usually 31 January 2027.
Can I amend my return straight after filing?
You need to wait 3 days (72 hours) after filing before updating it online.
What if the 12 months have passed?
Write to HMRC. If you paid too much tax, you can claim overpayment relief up to 4 years after the end of the tax year.
Will amending my return trigger an HMRC check?
Correcting a genuine mistake is the right thing to do and can reduce any penalty. HMRC can check any return, including an amended one, within its time limits.
General information only, not personal advice. Tax rules, rates and thresholds change, so check gov.uk or speak to us about your own situation.