Tax Return Mistakes That Lead to Penalties: Common Errors to Avoid

Filing a Self Assessment tax return may seem simple, but a minor mistake can lead to penalties and interest. You must understand tax return mistakes that lead to penalties, as it can help you avoid unnecessary costs and ensure your tax return is accurate.

This comprehensive blog explains the common Self Assessment tax return mistakes that may lead to penalties and evaluates how HM Revenue and Customs (HMRC) calculates fines. It also discusses HMRC penalties for incorrect tax returns and what you need to do to ensure compliance with current UK rules.

Why Do Self Assessment Tax Return Mistakes Occur?

Self Assessment mistakes can happen when you miss income, enter incorrect figures, misunderstand allowable expenses, or fail to keep complete records.

Indeed, tax rules can also be complex, particularly when you have multiple income sources or business and personal expenses. Careful record-keeping and reviewing the return before submitting it to HMRC can help reduce errors.

What Are the Common Tax Return Mistakes That Lead to Penalties?

Before getting into HMRC tax return penalties, you need to understand that not every mistake on a tax return automatically results in a penalty. HMRC evaluates and considers whether the mistake resulted from reasonable care, carelessness, deliberate behaviour, or deliberate and concealed behaviour.

If you took reasonable care but the return was incorrect, HMRC will not normally charge an inaccuracy penalty. It may charge a penalty when an inaccurate return results in tax being unpaid or understated, or a loss or repayment being incorrectly stated, and the inaccuracy was careless or deliberate.

Here are some tax return mistakes that lead to penalties:

Missing Self Assessment Deadlines

If you fail to submit a Self Assessment return on time, it can result in penalties, even if there is no tax to pay.

Self Assessment Late Filing Penalties

HMRC charges an immediate £100 fixed penalty for being 1 day late. For 3 months late, the penalty is £10 per day for up to 90 days, to a maximum of £900.

Moreover, for 6 months late, the penalty is £300 or 5% of the tax due, whichever is greater. For 12 months late, a further £300 or 5% of the tax due, whichever is greater. Higher penalties can apply in cases involving deliberate withholding of information.

Visit the official HMRC website to learn more about the Self Assessment penalties and deadlines.

Entering Incorrect Income Figures

One of the rookie tax return mistakes that lead to penalties is reporting the wrong amount of income.

For example, you might accidentally skip income from self-employment, property, investments or another taxable source. An incorrect figure can also occur from entering information from records incorrectly.

As mentioned, an inaccurate return does not automatically mean that HMRC will charge a penalty. HMRC assesses the circumstances and behaviour behind the error and charges a penalty accordingly.

Claiming Expenses Incorrectly

Expenses can be an important part of calculating taxable profits, particularly when you are self-employed. However, you can claim only allowable business expenses for tax purposes. Claiming expenses that are not wholly for business purposes can be one of the tax return mistakes that lead to penalties.

Important: You cannot claim personal expenses, but an identifiable business proportion of certain mixed-use expenses may be deductible.

Using Estimates Without Keeping Proper Records

When completing a Self Assessment, accurate records are essential. You need to keep accurate business records for at least 5 years after the 31 January submission deadline for the relevant tax year.

If records have genuinely been lost or cannot be replaced, you should do your best to provide figures and inform HMRC when filing if they are using estimated or provisional figures. You should not guess figures casually when accurate information is available.

Keeping organised records throughout the year makes it much easier to prepare an accurate return and may help you avoid the risk of tax return mistakes that lead to penalties.

Forgetting to Report Additional Income

Self-employed individuals or businesses sometimes focus on their main source of income and skip another source that also needs to be considered. Depending on your circumstances, this could include income from self-employment, investment, property, savings, or other taxable sources.

If you fail to include relevant income, it can result in an incorrect tax calculation. If omitting an income source leads to an understatement of tax and the relevant conditions for an inaccuracy penalty are met, HMRC may charge a penalty.

You can reduce the risk of this mistake by making a list of all your income sources before starting your return, and compare it with your invoices, payslips, bank statements, and other records.

Failing to Amend Mistakes

One of the major tax return mistakes that lead to penalties is failing to correct a mistake after finding it. If you have identified an error even after submitting your tax return, you should fix it as soon as possible.

HMRC allows you to correct a Self Assessment tax return within 12 months of the Self Assessment deadline, subject to the applicable rules. Correcting a mistake promptly can help ensure your tax position is accurate. If you missed the normal amendment period, different procedures may apply.

Paying Your Tax Bill Late

Submitting your return on time is important, but it is only one part of the process. You also need to pay the tax you owe by the relevant deadline. Missing the deadline is one of the common tax return mistakes that lead to penalties.

For the 2025–26 tax year, the deadline to pay any tax due is 31 January 2027. You may also have payments on account to consider. Therefore, filing early can be useful because it gives you more time to understand the amount you need to pay and plan for the payment deadline.

How Does HMRC Calculate Penalties for Incorrect Tax Returns?

HMRC works out a fine based on how much tax you did not pay, called Potential Lost Revenue (PLR). Also, it looks at your behaviour that caused the error. For example, it checks whether you made an honest mistake or tried to hide it.

Careless behaviour can have a maximum penalty of 30% of the PLR. However, a deliberate but not concealed inaccuracy can have a penalty up to 70%. Whereas the penalty for deliberate and concealed inaccuracy is up to 100%.

Keep in mind that your final penalty can be reduced depending on factors such as the quality of the disclosure.

What Should You Do If You Discover a Tax Return Mistake?

If you realise that your tax return contains an error, you should fix it promptly. Do not simply ignore it. The first thing you need to do is identify exactly what is wrong and gather the relevant records. Then check whether you can amend the return through your Self Assessment account.

You can generally correct online returns within 12 months of the statutory filing date. If correcting mistakes on a tax return UK means you owe additional tax, you need to check the updated calculation and payment requirements.

However, if you are unsure about the appropriate correction or the error could have significant tax consequences, you should consider professional tax advice.

How Can You Avoid Self Assessment Penalties?

The best way to reduce the risk of tax return mistakes that lead to penalties is to build a reliable process rather than waiting until the filing deadline.

Consider these practical points to avoid self-assessment penalties:

• Keep accurate records throughout the year rather than trying to rebuild everything at the last minute.

• List all sources of income before completing your return.
• Check expense claims carefully. You may need supporting evidence before claiming expenses.
• Reconcile important figures against bank statements, invoices and other records and identify discrepancies.
• Submit your return early when possible. Filing early does not remove the need to pay the tax by the relevant payment deadline, but it gives you more time to identify errors and plan for the amount due.
• Check the tax calculation and Self Assessment statement to confirm the amount you owe before making your payment.
• Correct errors promptly if you discover them after filing.
• Keep records for the required period in case HMRC needs to check your figures.

The Bottom Line

Understanding tax return mistakes that lead to penalties can help UK taxpayers manage their tax affairs more effectively.

Late filing, late payment and certain inaccuracies can result in penalties, depending on the circumstances. You need to keep accurate records, take reasonable care, and check your return before submission, as it can help reduce the risk of errors.

Additionally, if you find a mistake after filing, fix it as soon as possible. For more complex situations, it is best to seek professional advice, as it may help you determine the appropriate action.

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Disclaimer: The information provided on AccountingFirms.co.uk is for informational purposes only and should not be considered as financial advice. Always consult with a professional accountant to ensure compliance with UK laws and regulations.