How to fill in a Self Assessment Tax Return online

This quick guide provides step-by-step instructions on how to fill in a Self Assessment Tax Return online.

How to fill in a Self Assessment Tax Return online

Do I need to file a Self Assessment Tax Return?

If you’re self-employed, you’re responsible for calculating and paying your income tax. This means completing an annual Self Assessment Tax Return.

Even if you’re employed, you might still have to complete a Self Assessment Tax Return if you receive other income not taxed at source. This can include rental income and investment income from dividends. You can check if you need to file a return on the website.

When does self-employed tax have to be paid?

The UK tax year runs from 6 April to 5 April.

If you are filing your Self Assessment Tax Return online, the deadline for submitting it and paying any tax owed is midnight on 31 January following the end of the tax year.

If you are submitting a paper return, there is an earlier deadline for this of 31 October following the end of the tax year. The deadline for paying your tax is still 31 January.

There are several ways you can pay your self-employed income tax, some of which take a few days to process. Make sure to leave enough time for HMRC to receive the payment by the deadline.

Filing self-employment taxes for the first time

If you’re submitting a return for the first time, you’ll need to register for Self Assessment first.

You need to register for Self Assessment by 5 October after the end of the tax year during which you are required to submit a return. If you miss this deadline, you may have to pay a penalty.

How to register for Self Assessment

You can register online by post or by phone.

Once you’ve registered you’ll be sent a Unique Taxpayer Reference (UTR) in the post. You’ll need your UTR to file a return.

To submit your return online you’ll also need to set up a Government Gateway account. You’ll be given details of how to do this when you are sent your Unique Taxpayer Reference Number.

What information is needed to fill in a Self Assessment Tax Return?

To fill in your Self Assessment Tax Return, make sure you have the following:

  • Your 10-digit Unique Taxpayer Reference (UTR)
  • Your National Insurance number
  • Details of all your untaxed income from the relevant tax year – including from self-employment, property and savings.
  • Your P60, P45 or P11D form if you’ve also been employed or received income where you were taxed at source
  • Details of any contributions to charity or pensions that could be eligible for tax relief
  • Records of expenses relating to your self-employed business

Can my accountant or bookkeeper file my tax return?

Yes, you can appoint an accountant to complete and file your Self Assessment Tax Return. Some bookkeepers also provide this service, but you’ll need to check with them first. What do I need to give my accountant for Self Assessment?

What do I need to give my accountant or bookkeeper for Self Assessment?

Your accountant or bookkeeper will need the same information, documents and records as if you were filling in the return yourself. Please refer to the list above.

Steps for how to fill in a tax return online

If you’re completing your Self Assessment Tax Return online yourself, HMRC has lots of useful webinars, helpsheets and advice to help you prepare.

The main part of a Self Assessment Return is called the SA100. It covers:

  • dividends
  • student loan repayments
  • interest
  • pensions
  • annuities
  • charitable giving
  • claiming tax reliefs and allowances

Fill in the sections that are relevant to you. You can skip those that don’t apply to your circumstances.

Some people will need to fill in supplementary pages to tell HMRC about additional types of income. This includes:

Once you have completed your return, check the information over carefully. Once you have submitted your return, HMRC will tell you how much tax you owe.

Can I pay my Self Assessment tax in advance?

Yes — in some cases HMRC may require you to make advance payments made toward your next tax bill. These are called Payments on Account and are paid in two instalments, due by 31 January and 31 July. Each payment is half of your previous year’s tax bill.

When you become self-employed, your first payment on account will be due at the same time as your tax payment from your first Self Assessment. This means your first self-employed tax bill may be higher than you expect.

Payments on account are based on your previous tax bill. HMRC is assuming you will earn the same the following year. You will need to make a ‘balancing payment’ if your tax bill ends up being more. Or, if you know your tax bill is going to be lower than last year you can ask HMRC to reduce your payments on account.

Do I need accounting software to file Self Assessment Tax Returns?

At the moment, you can submit a paper return to HMRC or use HMRC’s gateway to file Self Assessment Tax Returns. However, as part of the government’s plan to digitalise the UK tax system, Self Assessment is changing soon.

In 2026, Making Tax Digital for Income Tax Self Assessment (MTD for ITSA) rules will come into effect. It means that landlords and self-employed people earning over £50,000 annually will need to use MTD-compatible software to keep digital records and submit returns to HMRC. Those earning above £30,000 will have to follow MTD for ITSA rules from April 2027.

Instead of one annual Self Assessment Return, you will need to submit quarterly updates and a Final Declaration.

Find out more about how Self Assessment is changing and learn everything you need to know about Making Tax Digital for Income Tax on our resource hub.


Xero does not provide accounting, tax, business or legal advice. This guide has been provided for information purposes only. You should consult your own professional advisors for advice directly relating to your business or before taking action in relation to any of the content provided.

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