Quick answer
You can amend a Self Assessment return online up to 12 months after the 31 January filing deadline. Sign in to your HMRC account, open the return, correct the figures and resubmit. HMRC recalculates your tax and refunds or bills you the difference.
Updated October 2026 · Figures based on 2026/27 rates
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The deadline for changes
For the 2025/26 tax year the filing deadline is 31 January 2027, so you can amend until 31 January 2028. After that you must write to HMRC, or make an overpayment relief claim, which has its own time limits.
How to make the change
Log in to your Self Assessment account, choose to view or amend the return for the right year, change the figures, and resubmit. You get an updated tax calculation straight away.
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Paper returns
If you filed on paper, you need to send HMRC the corrected pages or a letter explaining the change, so the online route is much quicker.
If the change increases your tax
Pay the extra as soon as possible. Interest runs from the original due date, so acting quickly keeps costs down.
When you need to amend a return
Mistakes happen: a missing invoice, a figure typed in the wrong box, a bank statement that turns up late, or an expense claimed twice. If you spot an error after you have submitted your Self Assessment return, you can normally correct it. For returns filed online, you can amend within 12 months of the 31 January filing deadline for that tax year. For example, for the 2024/25 return, the normal deadline is 31 January 2026, so amendments are generally possible until 31 January 2027. After that window, you would usually need to contact HMRC to ask for a correction, and the rules depend on the type of error, so check current HMRC guidance.
It is far better to correct a known mistake promptly than to hope it is not noticed. An honest correction is treated very differently from an error that is left in place. Our guide on what to do if HMRC opens an enquiry explains how the process works if HMRC does ask questions.
Step-by-step: how to make the correction
- Gather the evidence: collect the records that show the right figure, such as invoices, bank statements and payslips.
- Log in to your Government Gateway account: open the return for the relevant tax year and choose the option to amend it.
- Change only what is wrong: update the specific boxes, then check any totals that flow from them.
- Submit again: keep the confirmation and a copy of the revised figures.
- Check the new tax calculation: HMRC will show whether more tax is due or whether you are owed a repayment.
If you filed a paper return, you generally correct it by sending HMRC the amended pages or a letter explaining the change, so keep copies of everything you send.
Worked example: income left out by mistake
This is an illustrative example using 2026/27 rates. A sole trader filed a return showing profit of £30,000. Weeks later, they find a £2,000 invoice that was never recorded, so profit should have been £32,000. All of the extra income falls within the basic-rate band.
| Item | Working | Extra tax |
|---|---|---|
| Income tax | £2,000 x 20% | £400 |
| Class 4 National Insurance | £2,000 x 6% | £120 |
| Total extra due | £520 |
If the same trader were already a higher-rate taxpayer, the extra income would instead cost 40% income tax plus 2% Class 4, or £840. Paying the extra before the 31 January payment date avoids late payment interest and penalties. If the deadline has passed, interest runs from the date the tax was originally due, at Bank Rate plus 4%, so it is sensible to settle promptly. You can read more in our guide to late payment penalties.
What if the change reduces your tax
Amendments are not only for underpayments. If you left out an allowable expense, forgot a pension contribution, or claimed too little for mileage, a correction can create a refund or reduce your balance. The same 12-month window applies. Have the supporting records ready, because HMRC may ask to see them. Our guide on how long to keep tax records explains the retention periods.
Common mistakes when amending
- Leaving it until after the 12-month window has closed.
- Changing one figure without checking the totals that depend on it.
- Forgetting that more tax means paying the extra, not just submitting the form.
- Not keeping a copy of the original return and the amended one.
- Starting a repayment claim without the records to back it up.
Checklist before you amend
- Identify exactly what was wrong and in which tax year.
- Collect the supporting documents.
- Work out the effect on tax and National Insurance.
- Submit the amendment within the 12-month window.
- Pay any extra tax straight away and keep the confirmation.
How SATR can help
SATR, a trading style of Accotax, uses qualified UK accountants to check your figures and file online for a fixed fee from £199 + VAT. If you run a business, see Self Assessment for sole traders, or read how to appeal an HMRC penalty if a charge has already been issued.
This article is general information, not personal tax advice.
Frequently asked questions
Does amending trigger an HMRC enquiry?
Not by itself. HMRC opens enquiries for many reasons, and correcting a genuine error promptly is better than leaving it.
Can I amend a return from earlier years?
Yes within the time limits, and for older errors you may need to contact HMRC directly.
How do I know if I have the right to amend or need to contact HMRC instead?
If you filed online and are within 12 months of the 31 January filing deadline, you can usually amend it online. Outside that period, contact HMRC; check current HMRC guidance for the route that applies.
Will I be charged a penalty if I correct an error?
Penalties depend on the reason for the error and whether it was reported promptly. Telling HMRC about a mistake you have found is generally treated more favourably than leaving it, but check current HMRC guidance.
Do I need to amend if I forgot to claim an expense?
Not required, but you may want to. An amendment within the 12-month window can reduce your tax or produce a refund.
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The Bottom Line
You can amend a Self Assessment return online within 12 months of the 31 January filing deadline — after that window closes, you must contact HMRC and the options narrow. Correct mistakes promptly, pay any extra tax straight away to stop interest, and keep copies of both the original and amended returns.
Spotted an error and want it fixed properly? SATR’s qualified UK accountants will check your figures, make the amendment and file online for a fixed fee from £199 + VAT. Start your tax return today or get an instant quote.
