Making Corrections to Your Tax Return

Making mistakes on your tax return happens — the good news is that you can correct errors yourself within certain time limits. Whether you've spotted an error after hitting submit or HMRC has contacted you about your return, understanding the correction process can help you put things right and avoid penalties.

How to correct your tax return yourself

If you discover a mistake after you've submitted your Self Assessment tax return, you can amend it yourself within 12 months of the Self Assessment deadline. For a return covering the 2023/24 tax year (which had a deadline of 31 January 2025), you have until 31 January 2026 to make corrections.

To amend your return online, log in to your HMRC online account and update the relevant sections. The system will recalculate your tax bill based on the changes you make.

If you filed a paper return, you'll need to write to HMRC with details of the corrections. Include your Unique Taxpayer Reference (UTR) and clearly explain what needs to be changed.

What happens if your correction changes your tax bill

When you correct your return, your tax bill may increase or decrease depending on the nature of the error.

If the correction means you owe more tax, you'll need to pay the additional amount. HMRC will also charge interest on the extra tax from the original payment deadline (31 January) until you pay. This applies even if you make the correction well before the 12-month deadline — interest runs from the date the tax was originally due, not from when you discovered the error.

If you've overpaid tax, HMRC will refund the difference and pay you interest on the amount you overpaid.

Using provisional figures and correcting them later

You might not know your exact profit for the whole tax year before the Self Assessment deadline. This can happen if your accounting period (the period your business accounts cover) ends at a different time to the tax year, or if you're waiting for a valuation.

In these situations, you can submit your return using provisional figures — your best estimate of what the final numbers will be. You must tell HMRC that you've used provisional figures when you submit your return.

Once you know the actual figures, you have 12 months from the Self Assessment deadline to amend your return with the correct information. The same rules about interest apply: if you owe more tax based on the actual figures, you'll pay interest from the original deadline. If you overpaid, you'll receive interest.

If HMRC contacts you about errors

HMRC may write to you if they believe there's an error on your return or if they need more information. This doesn't automatically mean you've done something wrong — they may simply need clarification.

If HMRC identifies an error and you agree with their assessment, they'll adjust your tax bill accordingly. You'll need to pay any additional tax due, plus interest from the original deadline.

If you disagree with HMRC's view, you can provide evidence to support the figures you submitted. Respond promptly to any letters from HMRC to avoid complications.

Correcting returns older than 12 months

The 12-month correction window is a firm deadline. After this period, you can no longer amend your return yourself through the standard process.

If you discover an error after the 12-month deadline has passed, you'll need to contact HMRC directly. They have discretion to accept late corrections in certain circumstances, but there's no guarantee they'll do so. You may need to make a formal "overpayment relief" claim if you've paid too much tax, or HMRC may open an enquiry if you've underpaid.

Keeping accurate records

To minimise the need for corrections, keep thorough records throughout the tax year. You should retain bank statements, receipts, invoices, and other financial documents that support the figures in your return.

Good record-keeping helps you complete your return accurately the first time and provides evidence if HMRC queries anything. You're legally required to keep records for at least five years after the 31 January submission deadline.

Penalties and deliberate errors

Making an honest mistake and correcting it promptly won't result in penalties. HMRC distinguishes between genuine errors and deliberate attempts to underpay tax.

If HMRC believes you've been careless or deliberately submitted incorrect information, they may charge penalties on top of the tax and interest due. The size of the penalty depends on the nature of the error and whether you've cooperated with HMRC.

Correcting errors yourself within the 12-month window demonstrates that mistakes were genuine, which works in your favour.

When to seek professional help

While straightforward corrections can be handled yourself, consider getting professional advice if:

• The error is complex or involves multiple areas of your return
• You're not sure whether something needs correcting
• HMRC has opened a formal enquiry into your return
• The correction involves a significant amount of tax
• You've missed the 12-month deadline and need to negotiate with HMRC

An accountant can review your situation, communicate with HMRC on your behalf, and ensure corrections are made properly.

Sources

• Self Assessment tax returns

This article provides general guidance based on current HMRC rules. For advice specific to your situation, speak to your accountant.