How to Correct a Self Assessment Tax Return After Submission

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Key takeaways

  • You can amend a Self Assessment tax return within 12 months of the Self Assessment deadline.
  • If you filed online, you must wait 72 hours (3 days) after submission before you can amend it.
  • You can amend by sending another paper return (within the correction time limit).
  • If you miss the amendment deadline (or need to change an earlier tax year), you’ll need to write to HMRC.

Why you might need to correct a return

It’s common to spot an issue after you’ve filed — for example, a figure entered incorrectly or something missed.

When you amend your return, your Self Assessment bill will be updated based on the corrected information. This may mean you owe more tax, or you may be due a refund.

Note: Amending a return is different from filing late — it’s about correcting what you’ve already submitted.

What you’ll need

Before you start, make sure you have:

  • The tax year you need to correct.
  • Access to your Self Assessment online account (if you’re amending online).
  • Your Unique Taxpayer Reference (UTR) if you’re sending paper corrections (your UTR is on previous tax returns or letters from HMRC).

How long you have to make a correction

You can correct a tax return within 12 months of the Self Assessment deadline, either:

  • online, or
  • by sending another paper return.

Worked example (using an HMRC example)

For the 2023 to 2024 tax year, you’ll usually need to change your return by 31 January 2026.

Method 1: Amend an online Self Assessment return (step-by-step)

You must wait 3 days (72 hours) after filing before you can update an online return.

Steps

  1. Sign in to your account.
  2. From “Your tax account”, choose “Self Assessment account”.
  3. Choose “More Self Assessment details”.
  4. Choose “At a glance” from the left-hand menu.
  5. Choose “Tax return options”.
  6. Choose the tax year for the return you want to amend.
  7. Open the return, make the corrections, then file it again.

Method 2: Amend by paper (within the time limit)

If you’re correcting by paper, HMRC’s guidance is to send the corrected pages and clearly mark them as an amendment.

Steps

  1. Get the return and any supplementary pages you need.
  2. Complete the corrected pages.
  3. On each corrected page, write “amendment”.
  4. Include your name and UTR.
  5. Send the corrected pages to the address on your Self Assessment paperwork.

If you cannot find the address, HMRC’s guidance says you can send corrections to:

Self Assessment
HM Revenue and Customs
BX9 1AS

What happens after you amend

If you amend online

  • You’ll see your amended bill straight away.
  • Within 3 days, your statement will also show:
    • the difference from the old bill (so you can see whether you owe more or less)
    • any interest

If you amend by paper

HMRC will send you an updated bill.

Pitfalls and troubleshooting

“I submitted online and can’t amend it yet”

If it’s been less than 72 hours since you filed, you’ll need to wait before the online amendment option becomes available.

“It’s been more than 12 months since the Self Assessment deadline”

If you miss the deadline — or if you need to change a return from an earlier tax year — HMRC’s guidance is that you’ll need to write to HMRC.

HMRC says your letter must include:

  • the tax year you’re correcting
  • why you think you’ve paid too much or too little tax
  • how much you think you’ve overpaid or underpaid
  • your signature (no one else can sign on your behalf)

If you’re making an overpayment relief claim, HMRC also requires additional information in your letter, including a signed declaration.

“I used commercial software”

HMRC’s guidance is to contact your software provider for help correcting your return. If your software cannot make corrections, you’ll need to contact HMRC.

Quick checklist

  • Confirm you’re within 12 months of the Self Assessment deadline for the return.
  • If amending online, check it’s been at least 72 hours since submission.
  • Select the right tax year and make the corrections.
  • Re-file the amended return (online).
  • If sending paper corrections, write “amendment” on each corrected page and include your name and UTR.
  • If you’re outside the amendment window, prepare a letter to HMRC including all required details.

Summary

If you need to correct a Self Assessment tax return after submission, you normally have 12 months from the Self Assessment deadline to do it. Online amendments require a 72-hour wait after filing, while paper corrections can be made by sending updated pages marked as an amendment (within the time limit). If you’re outside the amendment window or changing an earlier tax year, HMRC’s guidance is to write to them with the required information.