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A beginner's guide to self-assessment tax returns

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If you’re self-employed, completing your self-assessment tax return can be a real chore. But it’s crucial you get it right and file on time. Here’s how to go about it.
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Filing your self-assessment can be a daunting task. Fortunately, it doesn’t have to be as difficult as it might first seem, as we explain. 

What is self-assessment?

Self-assessment is a way of reporting your income and paying tax to HM Revenue and Customs (HMRC).

If you are employed, your income tax is usually automatically deducted from your wages by your employer.

But if you are self-employed or you receive any other income, you will need to submit a self-assessment tax return each year to pay income tax and National Insurance. 

There are two ways you can submit your tax return:

Who needs to file a self-assessment?

You will probably have to file a tax return if:

  • You’re self-employed and your income was more than £1,000

  • You’re a company director

  • You earned £100,000 or more 

  • You had savings or investment income of more than £10,000 before tax

  • You received income from abroad

  • You earned more than £50,000 and claim child benefit

  • You earned £2,500 or more in untaxed income, e.g. renting out a property

If you have received self-assessment forms from HMRC through the post then you will need to complete and submit them, or complete your tax return online.

You can find out exactly who needs to complete a self-assessment tax return on the GOV.UK website.

Tax return deadlines

Tax returns are submitted for tax years rather than calendar years, and you do this in arrears. The self-assessment deadlines for the 2020/21 tax year are as follows:

  • Registering for self-assessment for the first time: 5 October 2021

  • Paper tax returns: Midnight 31 October 2021

  • Online tax returns: Midnight 31 January 2022

  • Pay your tax bill: Midnight 31 January 2022

If you make advance payments towards your tax bill (known as ‘payments on account’) you also need to be aware of a second payment deadline on 31 July. 

To check if this applies to you, log into your personal tax account using your Government Gateway ID and you can view your latest self-assessment return. Doing so will enable you to see when your next deadline is and how much is due towards the bill. 

What are the penalties if you are late?

If you miss the deadline for submitting your self-assessment tax return, you will be charged a penalty of £100. If it is more than three months late, you could be charged an extra £10 a day up to a maximum of £900. 

On top of this, if you are late paying your tax, you’ll be charged:

  • 5% of the tax unpaid after 30 days

  • Another 5% of the tax unpaid after 6 months

  • Another 5% of the tax unpaid after 12 months

You can estimate your penalty using this GOV.UK calculator if you have submitted your tax return late.

How to submit your tax return

Online applications

If you have not filed an online tax return before, you need to register with gov.uk first. This can take up to 20 days, so make sure you do it in plenty of time before the deadline.

There are different ways to register depending on whether you are:

When you have registered, HMRC will send you your Unique Taxpayer Reference (UTR) number and a letter giving you instructions on how to set up your Government Gateway account. Once you’ve done this, you’ll be sent an activation code to finish setting up the account. 

Then you can use the free HMRC Self Assessment online service on the GOV.UK website to submit your tax return.

Postal applications

If you choose to send your application by post, you will need to download form SA100 from gov.uk website.

You can also download a help form that explains how to complete your paper tax return.

Once you have filled out your form, return it to the HMRC before the deadline of 31 October.

What address should you send the tax return to?

Return your completed forms to

HM Revenue & Customs

Self Assessment

BX9 1AS

United Kingdom

What you need to complete your tax return

You need to gather all your paperwork and documents to enter the correct information when you fill out your self-assessment. These may include:

  • Your 10-digit Unique Taxpayer Reference (UTR)

  • Your National Insurance number

  • A P60 from your employer (if you have one) showing your income and the tax you have already paid

  • A P45 if you have left a job in the current tax year

  • A P11D or P9D which shows any benefits and expenses

  • A summary of any rental income and expenses

  • Savings and investment statements showing how much you have earned in interest and other income like dividends

  • Documents detailing your self-employment income, including receipts, bank statements and accounts

  • Documents showing any contributions to charities or pensions that might be eligible for tax relief

How can you pay your tax bill?

You must pay your tax bill by 31 January, and you can send what you owe to HMRC by bank transfer, for example, CHAPS or Faster Payments, or by debit card payment.

You can no longer pay your tax bill by credit card.

You can also send a cheque through the post, or pay at your local bank branch by using a paying-in slip from HMRC which they will have sent you.

If you want to spread your payments over time, you use a budget payment plan. Here is how they work, and how to set one up.

If you end up paying more tax than you need to, here is how you can reclaim it.

Help stretch your budget a little further by making the most of your savings.