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  • Writer's pictureSara Moores

The Complete Guide to Tax Self-Assessment

Every year, millions of people in the UK complete their tax self-assessment. Understanding how to submit this kind of return is essential. 

This guide walks you through everything you need to know about the process.

What is Tax Self-Assessment?

Self-assessment is the system HM Revenue and Customs (HMRC) uses to collect income tax.

Who must submit it?

You should submit a self-assessment if in the last tax year:

  • You were self-employed as a sole trader and made more than £1,000

  • You were a business partner

  • You made more than £150,000

  • You paid Capital Gains Tax

  • You paid the High Income Child Benefit Charge

  • You had untaxed income (such as tips or foreign income)

This tax form also applies to you if you’d like to claim Income Tax Relief, prove you’re self-employed for any purpose, such as to claim Tax-Free Childcare, or to pay voluntary National Insurance contributions.

You can check if you need to submit a tax return here.

When to submit by

You must file this return and pay the tax you owe by the end of the tax year, 5 April, every year it applies to you.

If you’re using paper, you must submit it by 31 October. Online returns are due on 31 January.

How to Submit Tax Self-Assessment


If you’ve never filed a tax return before, you must register with HMRC by 5 October. You will then receive a letter telling you what to do next.

Send your return

You can either complete the form online or call HMRC for a paper copy and mail it in.

Frequently Asked Questions

What if I don’t know my income for the entire year?

If you are unsure of your income, you can work out provisional figures, or estimates. When you file, you must inform HMRC that your numbers are not exact.

Once you know, you have to change your return within 12 months. You can do this on HMRC’s website or by paper mail.

If you’re owed money, you should complete the “Request a repayment” form. If you owe money, it will show in an updated bill.

What if I forgot to declare some of my income?

If you realise you did not report all of your income, you should tell the HMRC about it immediately. You are more likely to face penalties if the HMRC has to contact you first.

What happens if I don’t file on time?

If you miss the deadline, you'll have to pay a late filing penalty of £100. After three months, this figure goes up.

You will also be charged interest on late payments.

What if I think I don’t have to send a return anymore?

If the criteria for those who must send a self-assessment no longer apply to you, you must tell HMRC. If you are correct, you will receive a letter confirming it.

Reasons you may not have to send a return anymore include if you’re no longer self-employed, you no longer pay the High Income Child Benefit Charge, or your income is below £150,000.

Make the Process Easier

Submitting a self-assessment return is complicated. There are penalties for getting it wrong. For ease in tax saving, you can hire a bookkeeper. This way, you can be sure your taxes are completed accurately and efficiently.

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