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How to fill in a self-assessment tax return

Commonly asked questions about filing your self-assessment tax return for the 2020-21 tax year.



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.


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 next blog 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.


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-employment, taxable 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:

  • Your National Insurance number.

  • Your UTR number.

  • Accounts, invoices, receipts or other records of income.

  • Records of any relevant expenses.

  • Contributions to pensions or charities

  • P60 and/or P45 forms.

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.


Richardson, D., 2021. How to fill in a self-assessment tax return. (online) Which? Money. Available at: <https://www.which.co.uk/money/tax/income-tax/self-assessment-tax-return/self-assessment-tax-al98c7k047tx> (Accessed 19 October 2021).









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