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The guide to filling in and filing your Self Assessment tax return

4-minute read

Sam Bromley

26 November 2019

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It’s important to file your tax return before the deadline or you could get a fine. Here’s how to do a tax return ready for 31 January.

This guide runs through how to fill in a tax return, so get all your information ready and we’ll take you through the process step-by-step.

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How to fill in a tax return: before you start

When do I need to file my tax return by?

The deadline for filing your tax return online is 31 January each year.

Keep in mind that the process is slightly different if you file paper tax returns. The deadline for filing your paper tax return is 31 October – so filing an online return does give you more time. Read more about tax return deadlines.

Of course, the 31 January deadline doesn’t mean you should leave completing your tax return until the day before. It’s easier to make mistakes when you’re rushing – and you should leave enough time to gather all the right information.

What information do I need to fill in a tax return?

You're likely to need details of:

  • employment income (if you’re also employed)
  • dividends
  • partnership income
  • interest
  • rental income
  • foreign income
  • pensions contributions
  • Gift Aid
  • pension income
  • payment on account
  • redundancy lump payment or unemployment benefit
  • P11D
  • capital gains

It’s really important to keep good records throughout the year so you don’t come unstuck when doing your tax return. And if HMRC checks it after you’ve filed, they may ask to see your documents.

For the self-employed, the key information is likely to be your income and expenditure details, so you should have all your invoices and receipts to hand.

If you need to ask third parties (like banks and building societies) for information, make sure you leave enough time for them to give it to you.

You’ll also need your unique taxpayer reference number, which you get when you register as a Self Assessment taxpayer.

If you need help with your records, you might want to consider hiring an accountant – also keep in mind that there’s accounting software available that can make keeping good records a breeze.

How to register

Remember that you do need to register for Self Assessment at the website so you can log in, if you need to send a tax return and you didn’t send one last year. You’ll get sent an activation code in the post, which can take up to 10 days to receive.

The deadline for registering for Self Assessment is 5 October in your business’s second tax year.

How to do a tax return: step-by-step

Filling in your tax return

You’ve gathered all the information you need, you’ve logged in to the website, and you’re ready to fill in your tax return.

The good news is that as long as if you’ve kept good records, the next steps shouldn’t be too difficult. HMRC’s system reacts to your details as you enter them and gives you reminders on where to find information if you get stuck. Here’s what to do:

1. Check your personal details

HMRC should be kept up to date with any changes to your address or your name, for example. You can check and update them during the Self Assessment process.

2. Fill in the sections that apply to you

HMRC’s system reacts to your details as you enter them. This means that as you fill in the form, it may remove sections if they’re not relevant.

3. Report on what you’ve earned

This is where you need your sales invoices to hand. Remember that you might also need to enter other income, like property income or gains on investments.

4. Add your tax-deductible expenses

Use your expense receipts when filling in this section. Our guide to what you can claim as self-employed tax deductible expenses has more details.

5. Double check your return

If you need to you can save everything you’ve entered and come back to your tax return, which can be useful if you want to check your numbers. But if you notice an error, it’s possible to change your tax return after filing.

What happens next?

When you submit, you should get a confirmation message and a reference number. HMRC will calculate the tax you owe, as well as the National Insurance contributions you need to pay. The deadline for paying your tax return is the same day as the deadline for filing – 31 January. If you file your tax return late, you’ll get a £100 penalty (if it’s up to three months late, and more if it’s later).

Remember that most self-employed people usually need to make a payment on account too, which can catch newly self-employed people out – you should make sure you have enough set aside.

If you need more help

There’s lots of guidance on the website and you can also call the Self Assessment helpline on (0300) 200 3310. But in previous years HMRC’s phone lines have crashed so make sure you leave enough time to get in touch with them if you need to.

How are you getting on with filling in your tax return? Let us know in the comments below.

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We create this content for general information purposes and it should not be taken as advice. Always take professional advice. Read our full disclaimer

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