A guide to completing a self-assessment tax return online

21 November 2014 ( 15 March 2017 )

Your guide to self assessment online: explaining how registration for self assessment works and what you need to do your tax return online.



If you need to complete a self-assessment form in order to do your tax return, doing so online is likely to be the simplest option.

It also means you can submit your tax return later: under current rules, self assessment online tax returns have to be filed by January 31 following the end of the relevant tax year.

Paper tax returns, on the other hand, have to be received by HM Revenue & Customs by October 31 -  three months earlier.

But if you haven’t used the HMRC’s web service before, time is of the essence. It can take a matter of weeks to register for self-assessment online, so it is important that you get started well in advance of the January deadline.

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How to register for online self-assessment

HMRC says that you should register for self-assessment by October 6 following the end of the tax year (which runs from April 6 to April 5) in question.

To register, go to this page on the Government website: Register for and file your Self Assessment tax return

Here you will be asked the reason you need to file an online self-assessment form, for example if you’ve just started as a sole trader, or if you have to pay capital gains tax.

Do you have a Unique Taxpayer Reference (UTR) number for your self-assessment online form?

If you have been sent a self-assessment form already or have filled one in in the past, you should already have a Unique Taxpayer Reference (UTR) number: this will make the process quicker. 

If not, you will have to wait for a UTR to be issued by post – this happens as part of the registration process above – which will slow things down significantly, as it can take up to 10 working days to come through if you're in the UK.

Armed with your UTR, you can register as a user on the HMRC website, where you’ll be given a username and password.

You will then be sent an activation code for the site in the post. 

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How to fill in your tax return for self-assessment

Once you are up and running on the HMRC website, you can start to fill in your tax return online.

You will need some or all of the following information:

  • Your P60 or P45 showing income from employment.

  • P11D form showing workplace benefits.

  • Income from property rental.

  • Pension payments or contributions.

  • Capital gains from investments.

  • Dividends from shareholdings.

  • Bank account interest.

You won’t need to send any original documentation off when you fill in your form online, but keep hold of it in case HMRC have any further questions.

The form can be filled in over the course of a few days: you can save your progress as you go.

You will be shown a calculation of how much tax you owe or are owed, after which you can submit your tax return. An email will confirm your self-assessment form has been successfully submitted.

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The opinions expressed are those of the author and are not held by Saga unless specifically stated.

The material is for general information only and does not constitute investment, tax, legal, medical or other form of advice. You should not rely on this information to make (or refrain from making) any decisions. Always obtain independent, professional advice for your own particular situation.