A guide to completing a self-assessment 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.

As well as being quick and easy you can also 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.

Not everyone has to complete a self-assessment form – indeed most people don’t. If you do, you will sometimes get a letter from HM Revenue & Customs informing you of the fact. But you can’t rely on this: instead, it is up to you to work out whether you need to submit an annual return.

If you fail to fill out a return when you should have done so, you could face penalties on top of any tax due. No one enjoys doing their tax return – but the earlier you tackle it, the less stressful it becomes.

Be aware that scammers take advantage of people planning to file their tax paperwork, so watch out for phishing texts, emails, phone calls and letters.

Who needs to complete a tax return?

People who work for themselves (sole traders), and those with more complex tax affairs involving rents or investments above a certain level, will need to complete a return; this must cover any untaxed income from the last tax year.

As a self-assessment taxpayer, remember to include everything you’ve earned over the tax year. This includes income from employment, self-employment, income from property, and interest and gains from your savings and investments.

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. You can recover or reset your HMRC account if you can't remember your details.

If you don't already have a UTR, 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, or 21 days if you are abroad.

Six taxes you can legally avoid

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.
• Information on Gift Aid donations

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.

Check your return

When filing online, you can save your tax return at any time, coming back to it later if you don’t have time to fill in everything in one sitting. Your tax will be calculated automatically as you fill in the return.

Before sending your return, make sure you have double-checked all the information you have provided to ensure there are no mistakes. Once you are happy, press the “submit” button – and wait for your confirmation code. Most importantly, remember to pay any tax due.

What happens if you miss the self-assessment deadline?

Those who file their self-assessment after 31 January will be hit by an automatic one-off fixed penalty of £100 if you file within three months after the deadline. The longer you delay, the more expensive it gets, with daily penalties and the potential for additional tax. Use the HMRC calculator to estimate your penalty.

While penalties may be waived if you can provide a “reasonable” excuse, there are very few excuses that HMRC will accept for late filing. Those it will accept include the recent death of a partner or a serious illness.

As a self-assessment taxpayer, it is worth doing all you can to avoid filing late. Don’t leave everything until the last minute; act now and ensure you submit your online tax return on time.

If you have any problems or queries, you can call HMRC’s online services helpdesk on 0300 200 3600.

What self-assessment is for

The self-assessment system exists to ensure that people pay the right amount of tax on the income they earn. In most cases, tax is deducted “at source” – so for workers, their employer takes off income tax and national insurance contributions, while payments from a pension fund are typically taxed by the pension company itself.

But for people who have significant income from other sources, such as self-employment, rent on buy-to-let property, or gains from the sale of assets such as shares or a second home, self-assessment is the only way to pay the tax they owe.

In the past, workers who were in the higher-rate income tax bracket often had to fill in a return even if all their income was taxed at source, but this is generally no longer the case.

Self-assessment dates to remember

January 31: Deadline for filing self-assessment return online

January 31: Deadline for paying tax due for previous financial year

Whether you file your return online or by post, your tax bill has to be settled by midnight on January 31.

January 31: First payment on account for current financial year

April 5: Financial year ends

The financial year runs from April 6 to April 5 the following year.

July 31: Second payment on account for financial year just ended

This is when the other half of the expected tax bill for the most recent financial year is due. If you expect to pay less tax for this period – for example if your income has fallen, or you have made no capital gains – tell your tax office and your payment on account can be reduced.

October 5: Deadline for registering for self-assessment

If you are not already registered for self-assessment and your tax office does not know you need to file a return for the previous financial year, you should inform HM Revenue & Customs before this date.

If you don’t register by October 5 and you fail to pay any tax owing in time, you could face extra penalties.

October 31: Deadline for filing paper self-assessment forms

If you are filing your return by post, the form needs to have arrived at your tax office by midnight on this date.

Your tax office will also charge you half of the tax it expects you to pay for the current financial year. Unless you tell officials otherwise, this is usually half of your previous year’s bill.

Leaving the self-assessment system

If you have filled out a return in the past but now no longer need to – for example, if you have left self-employment – you can get in touch with HMRC and ask for your return to be withdrawn. Call its helpline on 0300 200 3310.

Subscribe today for just £34.95 for 12 issues...

Subscribe today and save 51%