Avoid mistakes on the more complicated parts of your SA100 tax return

Let’s face it, unless you’re an accountant, you work for HMRC or you’ve been filling them out for donkey’s years, Self Assessment tax returns (AKA the “SA100”) can be difficult to understand. It’s a key reason why people make mistakes that can later cost them time and money. So, which parts of the SA100 can be more complicated and how can you avoid mistakes?

1. Unique Taxpayer Reference

Your UTR number is a unique 10-digit reference number that HMRC uses to identify you. HMRC issues your UTR number when you register for Self Assessment. You cannot file unless you include your UTR top left on the first page of your Self Assessment tax return, together with your National Insurance number and Employer Reference (if relevant). Don’t leave out any of these important details.

2. Supplementary pages

To report all of your taxable income/gains and claim related tax reliefs, you may need to complete and file one or more supplementary pages. They each have their own numbers and the most common are:

• the SA103S or SA103F (self-employment)
• the SA105 (renting out UK property or land)
• the SA108 (taxable capital gains from selling an asset)
• the SA104S or SA104F (taxable income from a business partnership)
• the SA102 (taxable income from full-time, part-time or casual employment or from being a company director)
• the SA106 (non-UK income or gains

On page 2 of your SA100 Self Assessment tax return, you must tick a box to confirm or deny earning taxable income from employment, self-employment, a partnership, UK property, overseas income/gains, trusts or capital gains. You must then fill out and file the relevant supplementary page(s) with your SA100.

Need to know! If you run more than one self-employed business, you must complete a separate SA103 for each. The same is true for ordinary business partnerships and employed roles. 

3. Additional information

Where relevant, you report some less common forms of income and tax reliefs via “additional information” pages. These can include Marriage Allowance, life insurance gains, “chargeable event gains”, Seafarer’s Earnings Deduction and “details of disclosed tax avoidance schemes”. Don’t make the mistake of leaving any of the above out or waste time trying to report via the main SA100 tax return.

4. Tax allowances and reliefs

Not claiming for all of your tax expenses is another common mistake to avoid, because it will mean your tax bill is higher than necessary. If you’re a sole trader or private landlord, you include summaries of your “allowable expenses” in your supplementary pages. You can claim for a proportion of your domestic expenses when running a business from your home. Tax reliefs for donations to charity and maintenance payments can also be claimed.

Need to know! If your expenses are less than £1,000, it’s more tax-efficient to claim the trading allowance (if you’re self-employed) or property allowance (if you’re a landlord). Both are worth £1,000 and you can claim both if you’re a sole trader with rental property. If the property is jointly owned, each owner can claim the £1,000 allowance.

Read our guides:

• 45 Allowable expenses you can claim when you’re a sole trader
• What expenses can landlords claim against tax?

5. Tax relief for working from home

If you’re an employee who must work from home (full time or part time) because you live too far away from your employer’s premises or they don’t have premises, you may be able to claim tax relief to cover business phone calls and additional gas and electricity use. You can either claim £6 a week without having to keep evidence of extra costs or the exact amount of extra costs with evidence (eg bills, receipts). If registered, you can claim via your Self Assessment tax return.

6. Marriage Allowance

This enables the transfer of £1,260 of Personal Allowance between spouses or civil partners (not people who live together) from lower earner to higher, which reduces the higher-earning spouse’s or civil partner’s tax by up to £252 a year. Normally the lower earning spouse or civil partner earns less than the Personal Allowance. You can claim Marriage Allowance through Self Assessment if registered.

Need to know! You can backdate your Marriage Allowance claim to include any tax year since 5 April 2018 during which you were eligible.

7. Registered pension schemes and overseas pension schemes

When completing your SA100, if you earn employed income, do not include payments you make to your employer’s pension scheme that are deducted from your pay before tax nor payments made by your employer. Higher and additional rate taxpayers employed with pension contributions coming out of pay after tax are entitled to further tax relief (include this in box 3 on SA100, TR4 – this is often missed out).

If your contributions and other pension inputs are more than the Annual Allowance (£60,000 in 2024/25), fill in boxes 10 to 12 on page Ai 4 of the ‘additional information‘ pages.

8. Interest payments

Within your SA100, you must report taxable interest you receive from bank, building society or other savings account. However, you do not have to include interest you receive from an ISA (Individual Savings Account), because this is tax-free.

9. UK dividends

The first £500 of dividend income you receive is tax-free (this is your dividend allowance). Above that threshold, you pay: 8.75% on dividend income if you’re within the basic rate band; 33.75% on dividend income if you’re within the higher rate band; and 39.35% on dividend income if you’re within the additional rate band. You should include all of your dividend income in your Self Assessment tax return, even if it’s less than £500, because it will count towards your basic or higher rate bands and may affect the rate of tax that you pay on dividends received over the £500 allowance.

Need to know! Make sure you sign and date your SA100 before filing, otherwise HMRC cannot accept it and it will send it back to you, which could mean you miss the filing deadline.

Using GoSimpleTax can ensure that you enter the necessary information in the right places within your SA100 and any additional or supplementary pages. It can also save you a lot of time and effort when filling in your Self Assessment tax return. Why not start your free trial today?

Blog content is for information purposes and over time may become outdated, although we do strive to keep it current. It's written to help you understand your Tax's and is not to be relied upon as professional accounting, tax and legal advice due to differences in everyone's circumstances. For additional help please contact our support team or HMRC.