Filing a self-assessment tax return in the UK can be a daunting task, especially if you're new to it. Understanding who needs to complete a tax return, how to register, and what the deadlines are is crucial to avoid penalties. In this blog post, we'll provide a comprehensive guide to self-assessment tax returns in the UK, and how Zmartly, our accounting company, can assist you in the process.
Do you need to complete a Self Assessment tax return?
You need to complete a Self Assessment tax return if you are self-employed and earned more than £1,000, are a partner or company director, receive untaxed income from property, savings, investments or dividends, or have other income HMRC cannot collect through PAYE. The online filing deadline is 31 January after the tax year ends, and missing it triggers an automatic £100 penalty (per HMRC).
Who Needs to Submit a Self-Assessment Tax Return?
Self-assessment tax returns are required for individuals who are self-employed, freelancers (earning more than the £1,000 trading allowance), or have additional sources of income beyond their regular employment. Other situations that may require a tax return include earning income from property, savings, investments or dividends, receiving a P800 form from HMRC (which can also flag overpaid tax you may be able to claim back as an HMRC tax rebate), or being a partner or director of a limited company. You can check whether you need to file using HMRC's online tool.
When Do You Need to Submit Your Self-Assessment Tax Form?
The deadline for submitting your online self-assessment tax return is 31st January following the end of the tax year. It's important to file your return early — in fact, you can submit it as soon as the new tax year begins, as we explain in our guide to the earliest you can file your Self Assessment tax return — to ensure you have enough time to pay any tax owed. If you choose to file by post, the deadline is 31st October. For a closer look at the key dates, see our guide to the Self Assessment tax return filing deadline.
| Task | Deadline |
| Register for Self Assessment (first time) | 5 October after the tax year ends |
| Paper tax return | Midnight 31 October |
| Online tax return | Midnight 31 January |
| Pay the tax you owe (balancing payment) | Midnight 31 January |
Deadlines are confirmed on the GOV.UK Self Assessment deadlines page.
How to Register for Self-Assessment?
If you're new to self-assessment, you'll need to register with HMRC by 5th October following the end of the tax year. You'll receive an activation code and a Unique Taxpayer Reference (UTR) number by mail, which you'll need to set up your account.
How Can You Pay the Tax on Your Self-Assessment?
HMRC will calculate the amount of tax you owe based on your tax return. You can pay your tax bill online using a debit card or by mail-order cheque. If you're employed, you may be able to pay your tax through PAYE if you owe less than £3,000.
What Happens if You Fail to File Your Self-Assessment on Time?
Failure to file your tax return on time can result in penalties, even if you pay your taxes late. You get an automatic £100 penalty as soon as you miss the deadline, even if you have no tax to pay, with further daily penalties of £10 a day (up to £900) after three months, and more after six and twelve months (per HMRC). Interest is also charged on tax paid late. Penalties increase the longer you delay, so it's important to file your return promptly to avoid additional charges.
For more info, here’s an overview by HMRC.
Frequently asked questions about Self Assessment
Do I need to file a Self Assessment if I earn under £1,000 from a side hustle?
No. If your total income from self-employment or a side hustle is £1,000 or less in the tax year, the trading allowance usually means you do not need to register or file (per HMRC). Above £1,000 you must report it.
What is a UTR and how do I get one?
A Unique Taxpayer Reference (UTR) is a 10-digit number HMRC issues when you register for Self Assessment. You'll receive it by post within about 10 working days of registering, and you need it to file your return.
Can I pay my Self Assessment tax through PAYE?
Sometimes. If you owe less than £3,000 and are also employed or receive a pension, HMRC can collect the tax through your tax code, provided you file online by 30 December.
Related reading: how to pay yourself smartly as a director in 2025/26, allowable expenses for limited companies, and if you're weighing up handling it yourself, whether you can do your own tax return in 2025/26.
How Zmartly Can Help
At Zmartly, we understand that navigating self-assessment tax returns can be overwhelming. Our team of expert accountants is here to help you every step of the way. From registering for self-assessment to filing your tax return and ensuring compliance with HMRC regulations, we'll handle it all, leaving you to focus on what you do best. Contact us today to learn more about our services and how we can help you with your tax obligations.
Filing a self-assessment tax return in the UK is a legal requirement for many individuals, including the self-employed and those with multiple sources of income. By understanding who needs to file, how to register, and what the deadlines are, you can ensure you meet your tax obligations and avoid penalties with the assistance of Zmartly, your trusted accounting company.
