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10 common tax self-assessment mistakes

03 January 2022

A guide to the common mistakes people make when they fill in a tax self-assessment form - and how to help you avoid making those mistakes.

Taxes, spelt out in wooden blocks
How to avoid making errors on a tax self-assessment form

There are a number of common errors that people make when filling in their tax self-assessment forms.

Avoid these tax self-assessment mistakes to ensure you don’t pay too much or too little tax, and so you aren’t hit with any additional penalties.

1. Registering too late

If HM Revenue & Customs doesn’t realise you should be registered for self-assessment for a particular tax year, it won’t issue you with a Unique Taxpayer Reference (UTR) number.

Applying for one can take weeks so make sure you do so well in advance of the filing deadline to avoid any late-submission penalties.

2. Failing to submit the form

If you’re filing your return online, make sure you complete the process and hit the “submit” button. This comes after the calculation of any tax you owe or are owed.

Once you have submitted your return successfully, you’ll get a confirmation email.

3. Forgetting to pay your tax

As well as filing your return ahead of the January 31 online deadline, you have to pay any tax outstanding by then. This page on the Goverment website tells you how.

If you don’t pay in time you could be fined and charged interest.

Read our guide to the things you might have to pay tax on.

4. Omitting income

All taxable income needs to be declared on your return: as well as earnings or pension payments.

This includes bank account interest – even if it's not very much – or rent from buy-to-let property

5. Ignoring pension contributions

Don’t forget to state any extra pension contributions you have made: these could qualify for tax relief and reduce your overall bill.

6. Not being aware of payments on account

As well as your current tax bill, HMRC may ask you to pay some of the next year’s bill upfront – this is known as a payment on account.

These payments are charged at the end of January and the end of July.

As a result, your first January payment could be 50% bigger than you were expecting.

7. Getting your tax code wrong

Your tax code shows how much tax you should be paying and can be found on your payslips. 

Make sure you input it correctly, otherwise you could end up paying too much or too little tax.

8. Failing to declare charitable giving

If you have given money to charity, include it on your return as it could help reduce your tax bill.

9. Getting numbers wrong

If any of the figures on your return are incorrect, you have until the following January 31 to correct them and amend your return.

10. Expenses errors

If you are self-employed you can cut your tax bill by claiming expenses such as the cost of equipment. 

Bookmark this page on the Government website for further details on allowable expenses.

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The opinions expressed are those of the author and are not held by Saga unless specifically stated. The material is for general information only and does not constitute investment, tax, legal, medical or other form of advice. You should not rely on this information to make (or refrain from making) any decisions. Always obtain independent, professional advice for your own particular situation.