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How to fill in a self-assessment tax return

Find out whether or not you need to fill out HMRC's self-assessment tax return for the 2020-21 tax year. 

In this article
Coronavirus (COVID-19) tax update What is a self-assessment tax return? Who pays self-assessment tax? Step-by-step: how to register for self-assessment How to fill in a self-assessment tax return
Self-assessment tax return deadlines Submit your tax return with Which? Tax returns if your circumstances change Help with self-assessment 

Coronavirus (COVID-19) tax update

The government has introduced a range of measures to support employed and self-employed workers, as well as small businesses that have been affected by the coronavirus crisis.

Keep on top of the latest news and advice related to the COVID-19 pandemic with Which?

What is a self-assessment tax return?

Self-assessment is used by HMRC to calculate tax on your income.

Generally, your tax is deducted automatically from your wages, pensions or savings - known as PAYE. However, if you receive any other income, you need to report this to HMRC by sending a self-assessment tax return once a year.

This can either by filed online or you can send a paper tax return.

  • Get a head start on your 2020-21 tax return with the Which? tax calculator. Tot up your tax bill, get tips on where to save and submit your return direct to HMRC with Which?.

Who pays self-assessment tax?

More than 12.1m people were expected to file a self-assessment tax return for the last tax year.

If you’re self-employed, you’ll need to submit a self-assessment tax return every year, to pay income tax and National Insurance on your profits. You can find out more in our full guide to paying tax when you're self-employed

Other people who need to fill in a tax return include anyone who:

  • earns £100,000 or more last tax year as an employee or pensioner
  • earned £10,000 or more from savings interest, or investment income
  • earned £2,500 or more in untaxed income - for instance, from tips or commission
  • needs to claim tax relief on pension contributions if you're a higher- or additional-rate taxpayer
  • owes capital gains tax from selling assets at a profit
  • claims child benefit, if your or your partner’s income is over £50,000
  • receives taxable income from abroad, or lives abroad but receives an income in the UK
  • receives state pension payments that exceed your personal allowance and it's your only source of income
  • is a business partner, or director of a limited company
  • is a trustee of a registered pension scheme or other trust
  • is a trustee or representative of someone who has died
  • is a 'name' at the Lloyd's of London insurance market
  • is a minister of religion
  • received a P800 form from HMRC saying you didn't pay enough tax last year, and you haven't yet paid the outstanding sum.

In some cases, you may need to complete a self-assessment tax return and also pay via PAYE: for instance, if you receive a private pension or investment income, make a taxable capital gain or run a business on the side of your employment.

If you run a limited company, you'll need to file a company tax return in addition to a tax return on your personal income.

Will I be sent a tax return?

You'll usually be sent a tax return if:

  • you have untaxed income from investment, land or property, or from overseas.
  • you make capital gains above the annual exempt amount (£12,300 for 2020-21 and 2021-22).
  • you were required to fill in a tax return last year.
  • you're a pensioner who gets reduced age-related allowance, though you may be sent a special short version that requires fewer details.

You shouldn't rely on HMRC to contact you before submitting a tax return if you know you owe tax. It's your responsibility to make sure you declare all taxable income each year.

If you receive a tax return, you must return it, regardless of whether you owe tax or not.


Step-by-step: how to register for self-assessment

If you’re looking to submit a tax return for the first time, you’ll need to register for self-assessment first.

The steps are below.

  1. Register with HMRC: The process will vary depending on whether you’re self-employed, registering a partnership or not self-employed - you should click on the option that applies to you. You can register online via HMRC.
  2. Get your Unique Taxpayer Reference (UTR) number: HMRC will send this to you in a letter after you register. The letter will give instructions on how to set up your Government Gateway account.
  3. Use your activation code for your Government Gateway account: Once this is done, you’ll be sent another letter in the post containing your activation code. You’ll need this to complete the set-up of your account - you should do this promptly as the code will expire.
  4. Complete your account setup: It's only once your Government Gateway account is up and running that you'll be able to log in and submit your tax return. 

HMRC warns that the whole process could take up to 20 working days, so make sure you don’t leave it until the last minute.

    How to fill in a self-assessment tax return

    When you submit your tax return online, you’ll just need to fill out the sections that apply to you. We explain the process in our guide to online tax returns.

    For paper tax returns, you’ll need to work out which sections are relevant. Most people will just have to fill out the SA100 form. However, there are several supplementary pages that may apply to your circumstances. We explain more in our guide to paper tax returns.

    Some employees, pensioners and self-employed people with a turnover of under £85,000 can be sent a simplified SA200 return. At four pages long, it's much shorter. Unfortunately, you can’t opt to fill in this shorter form - HMRC will decide and send it out to you.

    Details you may need to include on your tax return include:

    • Income: all taxed and un-taxed income from self-employmenttaxable interest from savings, dividends from shares, or capital gains from selling assets
    • State pension: the total amount of state pension payments you were entitled to receive, plus any lump sum
    • Private pensions: detail the gross amount of any annuities or lump sums
    • Benefits: include anything you’ve received in incapacity benefit and jobseeker’s allowance, plus the total of taxable benefits from bereavement allowance, carer’s allowance or industrial death benefit
    • Other income: this is anything not related to interest or dividends, and you can also include any allowable expenses related to this income
    • Pension contributions: all payments where deductions were made after tax
    • Charitable donations: include the total amount of Gift Aid donations
    • Blind Person’s Allowance: you just need to confirm whether or not you’re claiming this
    • Student loan repayments: detail deductions made by your employer
    • High income child benefit charge: this is only for those receiving child benefit when they, or their partner, earn more than £50,000
    • Marriage allowance: the marriage allowance means you can transfer some of your personal allowance to your spouse if your income is less than the personal allowance (£12,570 in 2021-22, up from £12,500 in 2020-21).

    Before you start filling out your tax return, it’s best to gather all of the information you’ll need.

    Make sure you have records of:

    Self-assessment tax return deadlines

    Self-assessment tax is based on your income from the last tax year - not on the calendar year.

    The tax year runs from 6 April to 5 April, and your tax return will be due the following January.

    The self-assessment deadlines for you 2020-21 return are as follows:

    • 5 October 2021: deadline to register for self-assessment for the first time
    • 31 October 2021: paper tax return deadline
    • 31 January 2022: online tax return deadline (HMRC says you can submit up to 28 February 2021 without getting an instant penalty) 
    • 31 January 2022: tax payment deadline for 2020-21 tax owed, plus any outstanding tax from 2019-20 if you took out a payment arrangement with HMRC. If you pay your tax by payments on account you may have already made payments towards this bill.

    HMRC has the power to charge increasingly expensive penalties if you miss the tax return deadline, which starts with a £100 fine from the first day your return is late. 

    Submit your tax return with Which?

    Our easy-to-use and jargon-free tax calculator offers personalised tax tips, and you can submit the form directly to HMRC. In our video, we explain how to use the Which? tax calculator.

    Tax returns if your circumstances change

    If your circumstances change, and you start earning untaxed income, you must let HMRC know by 5 October following the end of the tax year. It will then decide whether you need to complete a tax return.

    If you used to send a tax return but don't need to any more (for instance, if you're no longer self-employed), contact HMRC to close your self-assessment account.

    Help with self-assessment 

    The UK tax system can be difficult to navigate, so we've rounded up commonly asked questions about filing your self-assessment tax return.



    Where can I request a self-assessment tax form?


    You can call HMRC on 0300 200 3610 to request blank tax return forms or guidance notes. You can also download them online.

    Alternatively, if you register for online self-assessment with HMRC or use the Which? tax calculator, you can submit your information online.


    How do I submit my self-assessment tax return?


    You can submit your return online via the HMRC website.

    Alternatively, you can use the Which? tax calculator to send direct to HMRC for a small fee. 

    You can also submit a paper tax return in the post. You can find HMRC's address on your forms and any other correspondence.


    How do I pay my tax return?


    You can pay via online or telephone banking, CHAPS, debit card online, at your bank or building society, Bacs, Direct Debit or cheque through the post. 

    You can no longer pay HMRC at the Post Office or via credit card.



    How can I check if my tax payment has been received?


    You can view your HMRC online account to check if your payment has been received – it should show as ‘paid’ three to six working days after you made the payment. 

    If you paid by post, you can include a letter with your payment asking for a receipt from HMRC. They should send this back to you by post.


    What if I can’t pay my tax bill?


    If you’re struggling to pay your tax bill, contact HMRC and make a ‘payment proposal’. This is an alternative way of paying your bill, either through monthly or quarterly payments. 

    HMRC will consider this proposal, and may ask for more information about other assets you have, such as savings and investments, before accepting the offer.



    What if the estimate for my next year’s income is incorrect?


    Those with business or property income must make payments in advance via payment on account. Your tax bill is then settled the following January (ie January 2022 for the 2020-21 tax year). 

    If you have overpaid, you’ll get a refund plus interest. If you’ve underpaid, you’ll have to pay extra. You can also ask HMRC to adjust your payments on account if you believe you will have a smaller tax bill this year.

    We explain how this works in our guide to paying tax when self-employed.


    What if I spot mistakes on my tax return?


    If you've overpaid and the error is down to HMRC, you can generally claim a full rebate - but you'll need to do this within four years of the end of the tax year you're claiming for. 

    So, if you paid too much tax for the 2019-20 tax year, you'll have to make a claim before 5 April 2024. 

    If you've paid too much tax because of your own mistake – such as not claiming an allowance you're entitled to – you can correct your tax return within the first year of filing it.

    Errors may also mean that you underpay tax. In this case, you should contact HMRC as soon as possible to correct your return. Failing to correct an error may lead to a steep fine.

    You can find out more in our guide to late tax returns and penalties for mistakes.


    What if my tax return is late?


    There is an automatic £100 fine if you submit your tax return after the 31 January deadline, and the penalties go up over time. You may also face late fines on any tax payment you owe that is overdue.

    If you miss the deadline, file your tax return as soon as possible to avoid fines building up.

    You can find out more in our guide to late tax returns and penalties for mistakes