You can normally amend a Self Assessment return within 12 months of its usual filing deadline. The route depends on how it was filed and the year concerned. If that window has closed, use the appropriate HMRC correction or claim process rather than submitting a duplicate return.
How long you have
You can correct a return within 12 months of the Self Assessment deadline, online or by sending another paper return. That window is tied to the normal filing deadline for the year, not to the day you happened to submit early. Example: for a return whose online deadline was 31 January 2026, the usual amendment cut-off is 31 January 2027. Always check GOV.UK for the year you are correcting.
Online amendments (HMRC account)
GOV.UK says you must wait 3 days (72 hours) after filing before you can update the return online. Then:
- Sign in to your HMRC account
- Open your Self Assessment account
- Choose More Self Assessment details → At a glance → Tax return options
- Select the tax year, open the return, correct it and file again
If you filed through commercial software, contact the software provider first; GOV.UK says to contact HMRC if the software cannot make corrections.
Paper amendments
If you need to amend on paper, GOV.UK tells you to mark “amendment” on each page and include your name and Unique Taxpayer Reference (UTR), then send the corrected pages to the address on your Self Assessment paperwork.
If the 12-month window has passed
You cannot use the normal online or paper amendment route. GOV.UK says you must write to HMRC explaining what is wrong. That is a slower, more formal path — get your figures and evidence straight before you write. An amendment is not the same as a voluntary disclosure for income you never reported at all; older undeclared rental income, for example, may belong on a different HMRC process (see undeclared rental income).
After you amend
Your tax calculation can change. You may owe more (with a new payment date and possible interest) or be due a refund. Pay any extra on time; if you cannot, read I cannot pay my Self Assessment tax bill rather than ignoring the revised balance.
Common mistake
Amending only the final tax figure “to make it look right” without fixing the income and expense boxes that drive the calculation. Correct the source figures and let HMRC’s calculation rebuild.
TaxSimba’s Self Assessment service can help rebuild a correct return and decide whether an amendment or another HMRC route fits. We cannot promise HMRC will accept every correction without follow-up questions. Start Self Assessment with TaxSimba if you want an accountant to handle the change with you.
Keep the original and corrected figures side by side
Illustrative preparation example.
Record the affected year, box or schedule, original amount, corrected amount and evidence. Confirm the method used for the original filing and retain the amendment confirmation. Changing your working spreadsheet alone leaves the submitted return unchanged.
Check: What exactly changed, how was it submitted and what calculation now appears at HMRC?
| Check | Practical action |
|---|---|
| Is the normal amendment window still open? | Check the tax year's statutory deadline rather than counting a year from early submission. |
| Which source figures need correction? | Keep the original, corrected values and evidence for the change. |
| Was the correction actually submitted? | Retain confirmation and check the revised calculation and payment position. |
Can I amend in HMRC's portal after filing through software?
Check the software provider's correction route first. Follow HMRC guidance if that route cannot make the required amendment.
What if the ordinary window has closed?
Use the appropriate written HMRC process for the correction or claim. Time limits and required statements differ, so identify the exact issue before sending it.
Download a checklist for this guide
- Record tax year and original submission reference.
- Check the applicable amendment or claim deadline.
- List original and corrected values with evidence.
- Submit through the appropriate route and retain confirmation.
- Check the revised tax, interest and payment dates.