Correcting mistakes in a Self Assessment tax return after you’ve filed

5 Minute Read

Last Updated: 10th January 2025

We all make mistakes, don’t we? Normally they’re minor mistakes and we’re able to correct them immediately or very soon after we’ve made them, with little harm done. No biggy. That goes for tax returns just as much as anything else.

Every year, people often make mistakes when completing their Self Assessment tax return. Sometimes those with less experience of completing Self Assessment tax returns leave out their UTR (Unique Taxpayer Reference – the unique ten-digit reference number that enables HMRC to identify you) or their National Insurance number.

In other cases people tick the wrong boxes or enter the right/wrong information in the wrong place. People leave out essential information or make mistakes when adding up their figures. They can overclaim or underclaim their expenses or fail to report taxable sources of income. Some people don’t realise that they need to file supplementary pages with their main tax return to report income from a less common source.

How to correct Self Assessment mistakes

HMRC also realises that mistakes happen. That’s why it allows you to correct any mistakes in your Self Assessment tax return within 12 months after the filing deadline. So, if you made a mistake in your tax return for the 2022/23 tax year, filed it before 31 January 2024 as required, you can amend it before 31 January 2025.

You should correct any mistakes as soon as possible. If you file your return online and then you suddenly realise you’ve made a mistake, you must wait 72 hours (ie three days) before you can update your Self Assessment tax return. When you change any figures or information you’ve filed previously within a tax return, HMRC will adjust your tax bill accordingly. You’ll see your amended bill immediately if you update your tax return online. If you’re in any doubt, contact HMRC for guidance.

If you miss this 12-month window or you want to correct a return from a previous tax year, you’ll need to write to HMRC to explain why and request changes to the tax return that you’ve filed. If HMRC notices any minor errors in your Self Assessment tax return, it will either correct them or contact you for clarification. Errors can result in either HMRC increasing or decreasing your tax bill or giving you a refund if you’ve overpaid tax. You can claim overpayment relief up to four years after the end of the tax year it relates to. If you owe more tax as a result, HMRC will tell you when this is payable.

Potential penalties for tax return mistakes

If you’ve taken reasonable care when completing your return and you’ve made a minor mistake in good faith, you’re unlikely to have to pay a penalty. HMRC reacts much more favourably to those who admit genuine mistakes. However, if you’ve deliberately falsified information within your tax return (eg you haven’t reported all of your taxable income or you’ve inflated your tax expenses), HMRC could take a much closer look at your tax affairs. It can launch an investigation and you could be left with a far higher tax bill plus a penalty to pay.

GoSimpleTax offers you an easier and quicker way to complete and file your Self Assessment tax return. Automatic prompts also make basic mistakes much less likely when you’re filling in your tax return. Many thousands of people already use GoSimpleTax to report their taxable income to HMRC. Enjoy a FREE trial to find out how GoSimpleTax makes completing tax returns so much simpler.

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Blog content is for information purposes and over time may become outdated, although we do strive to keep it current. It's written to help you understand your Tax's and is not to be relied upon as professional accounting, tax and legal advice due to differences in everyone's circumstances. For additional help please contact our support team or HMRC.

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