Made a Mistake on Your Tax Return? Here’s How to Fix It (and Avoid Extra Trouble)

Let’s be honest, tax returns can be fiddly things. With dozens of figures, forms and shifting rules, it’s no wonder that mistakes occasionally slip through. Maybe you forgot to include some income. Maybe you accidentally claimed for something you shouldn’t have. Or maybe you’ve only just realised that you missed out on a valuable tax relief.

Whatever the reason, the good news is: you can correct a previously submitted tax return. But it’s important to understand the right way to do it and what the potential consequences might be if HMRC think the error wasn’t entirely innocent.

When and How to Amend a Tax Return

If you filed your Self Assessment tax return online, HMRC gives you 12 months from the original 31 January filing deadline to go back and amend it.

So, if you submitted your 2024/25 return by 31 January 2026, you’ve got until 31 January 2027 to make any changes online through your HMRC portal or by using tax software.

Amendments can be made for all sorts of reasons, including:

• Adding income you forgot to include
• Correcting an expense you shouldn’t have claimed
• Claiming a relief you were eligible for but missed

Once amended, HMRC will automatically recalculate your tax position. If you’ve overpaid, you’ll usually get a refund (often with a small amount of repayment interest). If you’ve underpaid, you’ll be told how much you owe and interest will be added from the original due date.

But what if you’ve missed the amendment deadline?

Enter: Overpayment Relief (for refunds only)

If the 12-month window has passed and you’ve discovered you paid too much tax, there’s still a way to fix it using Overpayment Relief.

This is a formal claim process to request a refund of tax you shouldn't have paid. It applies where:

• You made a mistake in your original tax return
• The mistake led to an overpayment of tax
• You are not trying to re-open a settled tax dispute or take advantage of hindsight

Conditions for Overpayment Relief

To claim overpayment relief:

• You must write to HMRC (there’s no online form for this).
• You must explain the mistake and show the correct tax position.
• You must make the claim within 4 years of the end of the tax year in question.

So, for a mistake in the 2021/22 return, the deadline is 5 April 2026.

What If You Underpaid Tax?

If your mistake meant you underpaid tax, and the amendment deadline has passed, HMRC can still raise an enquiry or make a discovery assessment.

This depends on the type of error:

• Careless error: HMRC has up to 6 years to correct it.
• Deliberate error: HMRC has up to 20 years to raise an assessment.

In these cases, you won’t be the one voluntarily correcting your return, HMRC will raise the underpayment themselves.

Penalties and Interest

If you’re correcting a return after the deadline and it results in more tax being due, you may face:

• Interest: Charged from the date the tax was originally due.
• Penalties: If HMRC believes the error was careless or deliberate.

Penalties are based on behaviour:

When to Get Help

Correcting tax returns isn’t just about typing in new figures, it can raise questions about time limits, penalties and your standing with HMRC.

If you’re unsure:

• Whether you’re still within time to amend
• Whether overpayment relief applies
• What the potential interest or penalty might be

…it’s a good idea to get advice from an accountant or tax adviser.

Final Thought: Mistakes Happen - What Matters is How You Handle Them

Making a mistake doesn’t mean you’ve done anything wrong. But once you know there’s an error, there’s a responsibility to put things right, sooner rather than later.

By taking action early and understanding your options, you’ll often find that HMRC take a reasonable and proportionate approach. Just don’t bury your head in the sand. The longer you leave it, the fewer your options and the bigger your potential bill.

Need help?

Why not book a meeting with us to discuss your circumstances and see how we can help.