Key takeaways
- You can amend a return within 12 months of the 31 January filing deadline, so the 2024/25 return can be amended until 31 January 2027.
- Most returns filed online can be amended online through your HMRC account; paper returns and some others are amended by post.
- After the window closes, you can claim back overpaid tax with an overpayment relief claim within four years of the end of the tax year.
- If you owe extra tax, pay it promptly: interest runs from the original due date.
- Telling HMRC about errors before it finds them reduces, or removes, any penalty.
Mistakes happen on tax returns: a forgotten dividend, an expense missed, a pension contribution entered at the net figure. The good news is that correcting a return is usually simple if you act within the time limits. This guide explains how.
The amendment window
You can change a Self Assessment return within 12 months of the 31 January filing deadline for that year.
| Tax year | Original online deadline | Amendment deadline |
|---|---|---|
| 2023/24 | 31 January 2025 | 31 January 2026 (now closed) |
| 2024/25 | 31 January 2026 | 31 January 2027 |
| 2025/26 | 31 January 2027 | 31 January 2028 |
The window runs from the deadline, not from the date you filed. A return filed early in May still has until 12 months after the following 31 January.
How to amend
Returns filed online through HMRC
- Sign in to your HMRC online account.
- Go to your Self Assessment and choose the option to view or amend your return for the relevant year.
- Make the changes and resubmit. HMRC recalculates your tax.
You need to wait about 72 hours after the original submission before you can amend it online.
Returns filed with commercial software
Use the same software to submit an amended return, if it supports amendments. Otherwise write to HMRC.
Paper returns
Download a new return for that year, write “amendment” on each page, and include only the pages that change, with your name and UTR. Send it to the address on the return.
If you owe more tax
Pay the extra as soon as possible. Interest is charged from the original due date (usually 31 January after the tax year) to the date you pay, at the Bank of England base rate plus 4%. Late payment penalties can also apply if the tax was due more than 30 days earlier.
If the error was careless, HMRC can charge an inaccuracy penalty, but it is substantially reduced, potentially to nil, if you tell HMRC before it has reason to believe it might find the mistake (an unprompted disclosure). Errors made despite taking reasonable care are not penalised. See HMRC penalties.
If you paid too much
An amendment that reduces your tax produces a repayment or a credit on your account, which you can claim through your online account. If the change affects your payments on account for the following year, those are adjusted too.
After the amendment window: overpayment relief
If the 12-month window has passed and you find you have paid too much tax, you can make an overpayment relief claim. You must claim within four years of the end of the tax year concerned; for 2022/23, the deadline is 5 April 2027.
To claim, write to HMRC (or use the online route where available) stating:
- that you are claiming overpayment relief
- the tax year and the reason the tax was overpaid
- the amount of tax overpaid and how you calculated it
- that you have not already appealed against the assessment
- a signed declaration that the details are correct and complete
Overpayment relief is not available in some situations, for example where the error results from a mistake about how to make a claim or election, or where you could have appealed.
If you have paid too little after the window closes
You must still tell HMRC. Write to HMRC or use the online disclosure route. HMRC can assess tax for four years (six for careless errors and 20 for deliberate ones), so the liability does not disappear when the amendment window closes.
HMRC corrections and enquiries
- HMRC corrections: within nine months of receiving your return, HMRC can correct obvious errors, such as arithmetical mistakes. You can reject the correction within 30 days.
- Enquiries: HMRC can open a formal compliance check into a return within 12 months of the date you filed it, if filed on time.
Frequently asked questions
How long do I have to amend my tax return?
12 months from the 31 January filing deadline for that tax year. The 2024/25 return can be amended until 31 January 2027.
Can I amend my tax return online?
Yes, if you filed online through HMRC. Sign in to your account and choose the option to amend the return for that year.
What if I find a mistake after the amendment deadline?
If you overpaid, make an overpayment relief claim within four years of the end of the tax year. If you underpaid, tell HMRC as soon as possible.
Will I be fined for amending my return?
Not if the original error was made despite reasonable care. If you were careless, telling HMRC before it finds the error keeps any penalty low or removes it.
Sources
Rates, thresholds and rules in this guide were checked against HMRC and GOV.UK guidance in October 2026. Tax rules change, so check the current position on GOV.UK or with a qualified adviser before acting. This guide is general information, not personal advice.