Tax Guides

How to fill in a self-assessment tax return form online

How to fill in a self-assessment tax return form online

How to fill in a self-assessment tax return form online

December 8, 2020

In order to be able to complete a self-assessment tax return online, you must first create a Government Gateway account. Once registered, you will be sent a 10 digital Unique Taxpayer Reference (UTR) number with an activation code in the post. You will need these details to sign into your Government Gateway portal online for the first time. Bear in mind that this letter can take up to ten working days to arrive by post or twenty one if you live abroad, so make sure you factor this in for the self-assessment tax return deadline of 31st January.

To file your self-assessment tax return easily, it is best if you prepare and collect all relevant information you may need beforehand such as:

  • Your 10 digital UTR number
  • Your National Insurance number
  • Details of your untaxed income including amounts and dates of when these were received
  • Records of expenses relating to self-employment
  • Any contributions to charity or pensions which are eligible for tax relief
  • P60 or other records showing how much income you have received which you have already paid tax on

The self-assessment tax return form is then split into two parts – the main section which is referred to as SA100 and the supplementary pages. Before attempting to complete either, we would strongly recommend you read the helpsheets supplied by HMRC which provide comprehensive guidance on how to complete each section. The main SA100 section covers taxed and untaxed income, pension contributions, charitable donations and any taxable benefits. The SA100 is then broken down further into smaller parts:

  • Starting your tax return
    Enter your personal details such as date of birth, contact details and National Insurance number
  • What makes up your tax return
    Complete all the Yes/No tick boxes which apply to yourself. This tells HMRC why you are completing a self-assessment tax return and indicates whether or not supplementary pages need to be completed
  • Income
    This covers income arising from interest and dividends from UK banks and building societies, UK pensions, annuities and other state benefits received, and other UK income which is not covered on supplementary pages. Only fill in the amounts if any of these sections are relevant to you – if not, leave blank.
  • Tax reliefs
    Enter amounts for any payments into registered pension schemes (including overseas pension schemes but excluding your employer’s pension scheme if you opt into a workplace pension) and charitable donations. Also confirm if you receive Blind Person’s Allowance, amounts repaid for student loan or postgraduate loan, details of any child benefits received by you or your partner (only if your personal income is over £50,000), and/or enter details if you wish to transfer your Personal Allowance to your spouse or civil partner.
  • Finishing your tax return
    If you owe tax to HMRC you can select for HMRC to collect this through a tax code adjustment applied to your salary direct from your employers. If you are owed a refund, you should enter your bank details for HMRC to pay this either to yourself or a nominee. Finally, make sure to sign and date your tax return so that the entire form is complete.

How to complete supplementary pages of a self-assessment tax return


Only the relevant supplementary pages should be completed where you have additional untaxed income that has not been reported in the SA100. The most common sources of additional income often arise from:

  • Self-employment income – you will need to complete SA103
  • Rental property income – you will need to complete SA105
  • Declaring capital gains – you will need to complete SA108

Self-Employment income (SA103)

If you are completing a self-assessment tax return to declare self-employment income, make sure you have completed all the necessary steps required to have registered as self-employed with HMRC first. The entire SA103 form is made up of the following parts:

  • Your name and UTR number
  • Business details
    Enter basic business details such as a short description as well as dates of when you started trading
  • Business income
    Enter total figure of your annual turnover
  • Allowable business expenses
    Only complete this section if your annual turnover is over £85,000. You will be required to provide figures for each category of business expenditure you have incurred.
  • Net profit or loss
    Be sure to enter the figure in the correct designated space (do not complete both spaces)
  • Tax allowance for vehicles and equipment (capital allowances)
    You may wish to speak to a chartered accountant for expert advice on what constitutes capital allowances as accounting for these will help lower your tax bill.
  • Calculating your taxable profits
    Note that your taxable profits may not be the same as your net profit as you may have accounted for non-allowable expenses. Make sure you understand how to make adjustments.
  • Total taxable profits or net business loss
    Only enter a figure in either box 31 or 32, not both.
  • Losses, Class 2 and Class 4 National Insurance contributions (NICs) and CIS deductions
    You will only need figures for boxes 33 – 35 if you entered a loss for box 32.

Rental Property income (SA105)

Landlords needs to complete the SA105 form which consists of two parts – one for furnished holiday lets and the second for all other rental properties. You can complete this supplementary page by filling in details for:

  • Your name and UTR number
  • UK property details
    Fill in basic UK property details including number of properties let out, if any of these are jointly let and if you will be claiming Rent a Room relief
  • Furnished holiday lettings (FHL) in the UK or European Economic Area (EEA)
    Enter figures for the total income as well as any relevant allowable deductibles as listed in this section (boxes 5.1 to 19)
  • Property income
    This income should be from property which excludes FHL, Real Estate Investment Trust or Property Authorised investment Funds dividends/distributions.
  • Property expenses
    HMRC sets out certain expenses which you can claim that will help to reduce your tax bill. Put in total figures for each category listed that is applicable (boxes 24 to 29).
  • Calculating your taxable profit or loss
    This section allows you to claim for other allowances to reduce your tax bill as well as bringing forward previous years’ losses or to carry forward this year’s losses.

Declare Capital Gains Tax (CGT) (SA108)

When you dispose of certain assets (whether these have been sold for a gain or gifted) you may incur capital gains tax charges. This needs to be declared through a self-assessment tax return so that tax can be paid on the gains. Even where you have gifted an asset for free, for tax purposes, if the asset has increased in value over time since being in your possession, it will be treated as you having gifted that gain to the recipient. You will need to fill in the following sections which apply to your circumstances:

  • Your Name and UTR
  • Residential property (and carried interest)
    Note that since 6 April 2020, if you disposed of any residential property you must report and pay for CGT within 30 days of disposal (this means that the 31 January deadline does not apply if you only need to complete a personal tax return for this reason).
  • Other property, assets and gains
    You can include disposals of other types of assets here that is not property, shares or securities (shares and securities continue as a separate section after). This may include items of value (usually worth £6,000 or more) such as antiques, collectibles, artwork etc.
  • Listed shares and securities
  • Unlisted shares and securities
  • Loss and adjustments
    To reduce your CGT bill, complete this section for available losses brought forward to the year you are reporting, or you can reduce a future CGT bill by indicating that you would prefer the loss in this financial year to be carried forwards.
  • Non-residential Capital Gains Tax (NRCGT) on UK property or land and indirect disposals
    This is to be completed if you are non-UK resident but have UK property that has been disposed of.

Other supplementary pages


If you do not need to complete a self-assessment tax return for any of the above reasons, then you may need to complete other supplementary pages:

  • Additional information (SA101) – use this for less common types of income, deductions and tax reliefs such as SEIS or EIS, as well as Married Couple’s Allowance.
  • Employment (SA102) – this needs to be completed if you are an employee with income that is taxed through PAYE
  • Partnership (SA103S or SA103F) – there is a short form and a full form for partnerships. See HMRC’s notes to determine which you need to complete.
  • Foreign income (SA106) – applicable for those with foreign income, including but not limited to interest from offshore savings, dividends from foreign companies and income from oversea pensions or property.
  • Trusts (SA107) – for those who are a beneficiary or those who are a settlor putting money or assets into a trust or settlement
  • Minister of Religion (SA102M) – these pages can be completed by anyone who is a minister or employee acting as a minister of any religion or faith
  • Lloyd’s underwriters (SA103L) – only suitable for those who have been trading as an underwriting member of Lloyd’s and receive a form CTA1 and CTA 2 from Lloyd’s.

For any queries or help required to get your self-assessment tax return completed, please call us on 01865 24 55 11.


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