7 most common Self Assessment tax return errors revealed

More than 12m people in the UK file a Self Assessment tax return each year and although most do so when required, about 1.8m miss the online filing deadline, which results in late-filing penalties. Mistakes in…

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Last Updated: 23rd March 2023

More than 12m people in the UK file a Self Assessment tax return each year and although most do so when required, about 1.8m miss the online filing deadline, which results in late-filing penalties.

Mistakes in Self Assessment tax returns are another problem and HMRC can also take action if it believes you’ve been careless when completing yours. So, what common Self Assessment tax return mistakes should you avoid?

What are the most common Self Assessment tax return errors?

1 Missing/incorrect UTR or NI number  

Your UTR (Unique Taxpayer Reference) is the unique ten-digit reference number that enables HMRC to identify you. You’ll need it when completing your Self Assessment tax return (here’s what to do if you’ve lost your UTR number). Be careful when entering it. If you haven’t completed a Self Assessment tax return before, you need to register as self-employed with HMRC, when you’ll also get your UTR number.

Your Self Assessment tax return should also include your correct National Insurance number and you can find this online by logging into your personal tax account or by digging out an old payslip or P60. Failing that, contact HMRC.   

2 Incorrect figures or incomplete information

Whether through lack of due care and attention or simply poor arithmetic, sometimes people get their figures wrong when adding up their income and/or costs, so that incorrect data is entered into their Self Assessment tax return. Double-check your calculations, to ensure that this doesn’t happen.

In other cases, people fail to answer all relevant questions or provide the requested information/data. You can’t say that you don’t have it, but you’ll supply it later. If you do, HMRC will simply request the information when it notices any omissions in your tax return and it only leads to unnecessary delays. If you’re asked for information, include it.

3 Ticking the wrong box(es)

This can happen if you lack knowledge or don’t pay enough attention when completing your Self Assessment tax return. If you’re in any doubt, read HMRC’s guidance on filling out your Self Assessment tax return or seek tailored expert advice. Always be sure to check the right boxes.  

4 Over-claiming allowable expenses

As a sole trader, landlord or individual, you can claim a range of allowable expenses for some costs and expenses. However, some Self Assessment tax returns include claims for expenses that aren’t allowable or they claim for costs generated by personal rather than business use. Expenses must be generated “wholly and exclusively for trade” to be allowable.

Whether made through lack of knowledge or deliberately to try to reduce tax liability, HMRC action could result if your Self Assessment tax return expenses are not totally accurate and provable.       

5 Under-claiming allowable expenses

In other cases, lack of knowledge/experience leads some to fail to claim allowable expenses to which they’re entitled, which means their Income Tax bill is higher than it should be. If necessary, do some research into allowable expenses or get tailored professional advice about what you claim. That will help to ensure that the expenses/costs entered within your Self Assessment tax return are accurate, so that you don’t pay too much tax.  

6 Not including all income

If done deliberately to conceal earnings and lessen tax liability, it’s called underreporting, which is tax evasion – an offence that can have serious consequences. In other cases, other sources of income are more innocently left out of Self Assessment tax returns. This can include:

  • all pay, bonuses, benefits and tips from work as an employee if applicable
  • rental or holiday let income
  • savings interest
  • investment income
  • pensions and earnings from overseas
  • state benefits, such as maternity/paternity pay.

Be sure to include all sources of taxable income in your Self Assessment tax return.

7 Missing supplementary pages

In some cases, HMRC needs more information to be able to assess your circumstances for tax purposes, usually linked to income that can’t be sufficiently explained within the main Self Assessment tax return (SA100). An example might be a lump sum from an overseas pension scheme or property, or a life insurance pay out, etc. In such cases, you’ll need to file supplementary pages with your SA100.

What if you make a mistake in your Self Assessment tax return?

You can correct mistakes in your Self Assessment tax return within the year after the filing deadline for the tax year to which the return refers. So, for the 2021/22 tax year, you need to amend your tax return before 31 January 2024. You’ll need to write to HMRC if you miss this deadline. When you change any figures, HMRC will update your tax bill accordingly.

If HMRC notices any minor errors in your Self Assessment tax return, it may simply correct them or contact you with queries. HMRC may ask you to pay more tax, due to the error contained within your Self Assessment tax return, or give you a refund if you’ve overpaid.

If you’ve taken reasonable care with your return and have made a genuine, minor mistake, a penalty is unlikely. More serious issues can result in HMRC investigating your tax affairs and issuing you with higher tax bills and harsher penalties, especially if you knowingly and deliberately conceal earnings or fraudulently inflate your costs.

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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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