Everyone makes mistakes on a tax return: a missed bank interest statement, a mileage figure keyed twice, an invoice dated in the wrong tax year. The good news is that a filed Self Assessment return is not set in stone. There is a built-in correction window, and a separate route once it closes. This guide follows gov.uk's corrections to a Self Assessment return page.
The 12-month window
You can usually change a return within 12 months of the Self Assessment filing deadline. The deadline is 31 January after the tax year, so:
| Tax year | Filing deadline | Correct it by |
|---|---|---|
| 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. If you filed early, you still have until 12 months after the deadline. Section 9ZA of the Taxes Management Act 1970 is the legal source for the taxpayer's right to amend, and the 12 months run from the filing date, normally 31 January after the tax year. The four-year overpayment relief claim comes from paragraph 3 of Schedule 1AB to the same Act, which also says the claim cannot be made inside a return.
Amending online
After you file online you must wait 72 hours before you can update the return. Then:
- Sign in to your Self Assessment account.
- Go to More Self Assessment details, then At a glance, then Tax return options.
- Choose the tax year, make the corrections and file again.
If you used commercial software, ask the provider to help. If it cannot make the correction, contact HMRC.
Once you refile, your amended bill appears straight away online, and within three days your statement shows the difference from the old bill and any interest.
Amending a paper return
Get the SA100 (and any supplementary pages that changed) and send the corrected pages to the address on your Self Assessment paperwork. Write amendment on each page along with your name and Unique Taxpayer Reference. If you cannot find the address, gov.uk gives Self Assessment, HM Revenue and Customs, BX9 1AS.
What happens to your bill
- You owe more. You must pay by the deadline shown on your amended calculation, and the change may affect your payments on account. Interest runs on any underpaid tax.
- You overpaid. You may be able to claim a refund. If you filed on paper, HMRC pays refunds into your bank account if you included your bank details.
Because later payments depend on the bill, correct errors before the second payment on account if you can.
Provisional figures
If you did not know a figure when you filed, such as full-year profit for a new business, you enter an estimate and amend later when the final numbers are known. Note that the figure is provisional where the return allows, then amend inside the 12 months. Our first-year self-employed guide shows when that happens.
After the 12 months
Once the window has closed, you cannot simply refile. You write to HMRC instead.
- Missed income. Give the tax year, why you think you underpaid, how much, and sign the letter. HMRC may contact you for more detail.
- Overpaid tax. Make an overpayment relief claim. The claim can be made up to four years after the end of the relevant tax year. The letter must set out the claim, the tax year, the reason, the amount and whether you have previously appealed the same payment. The claim needs a declaration that the details are correct and complete to the best of your information and belief, and you must sign it yourself. A tax agent cannot sign for you. Incomplete claims are rejected.
Keep the evidence of the overpaid tax, because HMRC may ask for it.
gov.uk's page does not cover penalties for errors; it links to a separate penalties section. Tell HMRC as soon as you spot a mistake, and see the Self Assessment late filing penalty guide for the filing and payment penalties that are described elsewhere.
Before you refile: a quick checklist
- Identify exactly which boxes and supplementary pages change.
- Recalculate the tax so you know the new bill.
- Check the effect on payments on account and on any benefits that depend on income.
- Refile within the 12 months, and pay any extra tax by the date shown.
- Keep a note of what changed and why, alongside your records.
How ac-co helps
ac-co drafts the SA100 from your bank, books and HMRC data, so you see the full computation and approve it before it goes, which catches many errors before they reach HMRC. If you need to correct a return after filing, follow the gov.uk steps above for your HMRC account, or ask your software provider to make the change.