How to file your self-assessment tax return

Jan 3, 2024

If you’ve never had to complete a self-assessment tax return, the first time can be daunting. But don’t worry, you’re in the right place! In this blog we’ll demystify the process, and if you have completed one before, this guide might still teach you a thing or two about doing your self-assessment better.


What is self-assessment?

As the name suggests, self-assessment is all about the taxpayer assessing their own tax liabilities by telling HMRC about their financial activities and income via form SA100. HMRC then uses that reported income to work out how much tax and National Insurance contributions (NICs) you need to pay.
This is different to employees, who have their income tax and NICs automatically deducted through the PAYE system — this doesn’t happen for self-employed workers, or for some other sources of income, such as dividends, pensions or income from savings and investment, which is again where self-assessment comes in.


Who has to register for self-assessment?

In general, self-assessment is due for anyone who receives income that is not taxed at source. So, in the case of a sole trader, because your income received through invoices does not have NICs or income tax subtracted, you must tell HMRC about your income, even if it turns out that you don’t owe any tax.

Income from abroad, income from rental properties, investment income, dividends from your limited company — it all must be reported via self-assessment.

Employees who earn over £100,000 also must register for self-assessment. This is because once you make above this amount, your personal allowance changes. HMRC requires people in this category to file a self-assessment tax return so they can ensure the correct tax has been paid.


When to register?

Before you can file a tax return, you need to register for self-assessment with HMRC by 5 October following the tax year you’re filing for. As an example, if you need to file for the 2023/24 tax year for the first time, you should register by 5 October 2024. If you miss the deadline, you may have to pay a fine. The good news is that you’ll never have to register again — unless you tell HMRC you no longer need to file a tax return.


What you need to register

Registering is relatively straightforward. You’ll just need to supply some personal information, like your full name and date of birth, a phone number, email address, and National Insurance number. In return, you’ll get a unique taxpayer reference (UTR) number through the post that HMRC will use to identify you.


Filing your return

Once you’ve registered for self-assessment, it’s time to file your tax return. Watch out for these common mistakes:

  1. Missing or incorrect UTR/National Insurance number
  2. Incorrect figures and incomplete information
  3. Ticking the wrong boxes
  4. Over- or under- claiming allowable expenses5. Missing some sources of income
  5. Leaving your tax return until last minute

For more information on self-assessment mistakes and how to avoid them, check out our blog! 


Speak to us

Self-assessment is complicated, especially for the uninitiated and those with particularly complex and numerous revenue streams. Don’t get caught out: hire an accountant for a fraction of the cost of what you might have to pay if you get your tax return wrong.


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