Common Mistakes People Make With Their Self-Assessment Tax Return

7 December 2020

With January fast approaching, the Self-Assessment Tax Return deadline is nearly here. Leaving your tax return until the last minute can be a very expensive mistake. There are many penalties for not completing it on time.

Wrong calculations can lead to incorrect tax payments. This means you may have to pay out more money in HMRC penalties.

If your circumstances have changed this year or if you are unsure whether you need to complete a self-assessment tax return, our checklist will help you. If you would like an experienced team of tax experts to help you with your tax return this year, please contact us to arrange a free no-obligation consultation.


Do I need to file a self-assessment tax return in January?

Tax Return post-it note next to a calculator


This checklist indicates whether you should file a tax return in January 2021. If you answer ‘yes’ to any of the questions below from April 2019 to March 2020, you must complete and submit a tax return:

  • Were you self employed and earned more than £1,000?
  • Were you a partner in a business at any time in the year?
  • Did you receive income of over £100,000?
  • Have you received £10,000 or more before tax from taxed savings and investments, including dividends?
  • Did you receive more than £2,500 in untaxed income?
  • Have you received income from letting out a property?
  • Did you receive foreign income liable to UK tax?
  • Are you an employee claiming expenses or professional subscriptions of £2,500 or more?
  • Did you, or your partner, receive child benefit and have an individual “adjusted net income” of over £50,000 per annum?
  • Was your State Pension more than your Personal Allowance and was it your only source of income?
  • Have you received a letter from HMRC requiring you to complete a Self Assessment Tax Return 2019-20 and you meet at least one of the other criteria?
  • Have you made profits from selling assets such as shares, a second home or other chargeable assets, and need to pay Capital Gains Tax?
  • Did you live or work abroad? (You should check your residency status)

Are you filing a self-assessment tax return for the first time? Avoid this mistake

If you are filing a self-assessment tax return for the first time, you need to ensure that you have a ten-digit Unique Taxpayer Reference (UTR) code. This can usually take 10 working days to receive. So, make sure you give yourself plenty of time to receive the number and complete your submission by the deadline.

An error can lead to incorrect submissions and tax payments. Here at Maple, we have put together a list of five common mistakes to avoid- saving you from having to pay out more money.

Common mistakes made on self-assessment tax returns 

Where do I find my tax code and what is it?


Using the wrong tax code 

Using the wrong tax code is one of the most common mistakes people make. It may seem simple but to ensure you are not over or underpaying tax payments, check that your tax code is correct. You can do this on the government website. Your tax code is usually 3 numbers followed by an L. For example, 1250L is the tax code currently used for most people who have one job or pension. It is important that the correct tax code is issued. As it will determine how much tax-free income you get in that tax year.


To claim or not to claim

It is important to stay up to date on what can and can’t be claimed. It is easy to fall behind if you are not preparing tax returns daily. Ideally, taxpayers should be utilising all the available allowances. However, it is important to be aware that there can be costly penalties for incorrect claims. Therefore, speaking to your accountant or requesting information from HMRC will ensure you are claiming correctly.

Alternatively, if you are a higher-rate taxpayer and you have made any donations to charity through Gift Aid during the tax year don’t forget to record these donations. They will generate tax relief.


Not including interest on income

Taxpayers commonly forget to include interest that they have received from a bank or building society account. And this can be substantial. To avoid this common mistake, ensure that before you start, you have all documents relating to your savings and investments. This way you can accurately state the interest earned. Interest earned from loans, online peer-to-peer lending, credit unions, friendly society accounts and dividends from UK companies, should all be included.

Large one-off payments, such as a PPI compensation that has generated interest, should be checked to see whether this should be included in the tax return. However, interest from ISA doesn’t need to be included and shouldn’t be declared.


Incorrect pension contributions 

When paying into your pension, you need to make sure that your pension contributions are entered in your tax return. Getting them correct is important. Too little and you will miss out on tax relief, but too much and HMRC could charge interest on any underpayment.

In other cases, employers may deduct your pension contributions through your salary. If this is the case you won’t need to enter these contributions separately on your self-assessment tax return, as the available tax relief will have been applied through your net salary.



Missing the deadline

Avoid leaving your self-assessment tax return to the last minute as penalties could occur. The deadline for online tax return submission is midnight 31 January 2021. HMRC penalty fines range from £100 to 100% of your tax bill, so you must make sure your tax return is filed correctly and on time.


How can Maple Accountancy help you with your self-assessment tax return? 

At Maple Accountancy, we work closely alongside our clients to support them with their tax requirements. We can help you find and access the right tax relief schemes, to ensure that you never pay any more tax than you absolutely have to.

Our team at Maple Accountancy will help you keep up with the ever-changing tax legislation and relief schemes.

We can provide advice on how you can modify your business model or processes so you can claim back more in the future. Our tax experts have an in-depth understanding of HMRC rules and regulations so we can help prepare and file your self-assessment tax return for you. If any issues do occur, we can even handle correspondence with HMRC for you, saving you the hassle.

If you need advice on your self-assessment tax return this year, arrange your free consultation on 011 341 820 78 or email us at

Maple Accountancy is a firm of expert Business Advisors offering accountancy services, tax and business advice to owner-managed and family-owned businesses.

All clients are individual, and we tailor our service to your needs. Use the site to find out what makes us different and understand why you should appoint us.


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