A Self Assessment tax return is HMRC’s system for self-employed individuals to declare how much tax they owe based on their earnings. Whether or not you’re new to tax, the process can be overwhelming if your finances are complex, and many people have trouble completing it correctly.
According to the latest statistics from HMRC, an estimated 1.1 million Self Assessment customers missed the deadline of 31 January 2024 for filing their 2022/23 tax returns. So, if you’ve made a mistake before, missed a deadline, or are feeling anxious about completing your first tax return, you’re not alone.
This guide explores the top six most common (and costly) Self Assessment tax return mistakes and what you can do to avoid them in the future.
1. Missing/incorrect UTR or NI number
One of the most common tax return mistakes is entering the wrong Unique Taxpayer Reference (UTR) or National Insurance (NI) number. In some cases, these are not included at all.
A UTR is issued by HMRC when you register for Self Assessment or set up a limited company (Corporation Tax UTR). Without this 10-digit number, HMRC won’t know who you are when you submit your tax return. You should use your personal UTR for your Self Assessment return, as opposed to any Corporation Tax UTRs you may have.
If you’ve misplaced or forgotten your UTR, you can find it in your Personal Tax Account, in the HMRC app, on previous tax returns (if applicable), or in other HMRC documents you have received.
Alternatively, you can contact the Self Assessment helpline if you’re self-employed, or request a copy if you’re a limited company owner. Your UTR should be posted to you within 10 working days, but it could be longer, so if the Self Assessment deadline is looming, be sure to give yourself plenty of time and avoid a late submission charge.
Finally, your NI number ensures that your National Insurance contributions (NICs) and taxes are properly recorded under your name. If you don’t know your NI number, you can find it in your Personal Tax Account, the HMRC app, or on any employment document such as a payslip or P60. If you don’t have these, contact HMRC.
How to avoid
To avoid this common mistake, it’s useful to have your UTR and NI numbers on hand somewhere that’s memorable and easily accessible to you, especially during tax season. Sensitive information shouldn’t be written down on a piece of paper. Instead, try noting these numbers down in a secure encrypted notes app.
2. Over or under-claiming business expenses
Another common tax return mistake is claiming the wrong expenses or an incorrect amount. Depending on the nature of your work, there are certain running costs that you can get back in your annual tax return if they are “wholly and exclusively for trade” purposes.
However, some people claim expenses that they’re not entitled to, or try to claim personal rather than business expenditures. There could be several reasons for this, such as genuine oversight, lack of knowledge, or an attempt to reduce tax liability. Either way, providing incorrect information in your tax return could result in a considerable penalty.
There are also people who under-claim. Perhaps they’re simply unaware of the expenses they are entitled to, but omitting them in a tax return could lead to an unnecessarily high tax bill.
How to avoid
The best thing to do is to familiarise yourself with the list of allowable expenses. When completing your tax return, you don’t need to provide proof of expense, but you should keep a clear record of all your business receipts to ensure that you claim the correct amount.
3. Failure to declare all income sources
It’s vital to report all your earnings to HMRC to make sure that you’re paying the correct amount of income tax. This includes:
- Your salary including any benefits or bonuses
- Income from a holiday/rental property
- Interest accrued from savings
- Investment income (e.g. dividends)
- Pensions and income from abroad
- Government benefits (e.g. maternity pay)
However, leaving some income sources out is a common tax return error. If done intentionally for tax evasion purposes, this is illegal and can have serious consequences, but it can also be a genuine mistake.
How to avoid
It is your responsibility to keep a clear and complete record of all your income. If there are documents or details missing from your personal files, this will result in an error when completing your tax return. You could face a penalty for providing inaccurate information or incur a higher tax bill than needed
To avoid this common mistake, you should use a reliable and efficient tracking system for your earnings, and declare them all correctly in your tax return.
4. Supplementary pages missing
People often forget or don’t realise that they need to provide extra information about their income. If HMRC needs more detail to assess your tax return, you’ll need to provide supplementary pages along with your SA100 (the main Self Assessment tax return form).
The supplementary pages you’ll need to complete depend on your role and income source, such as:
- SA102 – employees and company directors
- SA103S – self-employed and sole traders
- SA104S – business partnership
- SA105 – UK property income
- SA106 – non-UK income
- SA108 – capital gains
- SA109 – Non-UK or dual residents
How to avoid
The best way to make sure that you provide the correct supporting documents in your tax return is to check the full list of supplementary pages and their requirements.
5. Failure to declare Notice of Coding
If your employment circumstances change, HMRC may amend your PAYE tax code. You’ll receive a letter to explain why your tax code has changed. This is called a Notice of Coding.
When submitting your tax return, you will need to include these details in the Tax Adjustment section for the tax year in question. However, this is a step that many people miss.
It’s important to check your PAYE tax code and declare a Notice of Coding, as it could mean that you’ve over or underpaid tax.
Failure to include this in your Self Assessment tax return could cause complications later. For example, you may be charged twice – once through your PAYE tax deductions and another through your tax return, making it more difficult to resolve with HMRC further down the line.
How to avoid
If you’ve received a Notice of Coding in the post, be sure to keep this for your records.
Ahead of tax season, you might find it useful to make a checklist of all the documents you’ll need to file your tax return correctly, including the Notice of Coding. You can tick sections off as you work your way through the SA100 form, making sure that nothing has been missed.
6. Forgetting about benefits/tax-free allowances
There are different types of benefits that you might be eligible for as a business owner. However, forgetting to take advantage of them and declare them in your tax return is a huge error that could cause you to pay too much tax.
Some of the tax-free allowances you could claim are:
- Childcare benefits – Up to £4,000 per child
- Maternity pay – Up to £184.03 a week
- Marriage allowance – Transfer up to £1,260 of your personal allowance to your spouse or partner
- Rent a Room scheme – Up to £7,500 per tax year for letting out furnished accommodation in your home.
How to avoid
HMRC offers several benefits that you could be entitled to as a business owner. It’s important to do your research and find out which allowances you’re eligible for.
Remember that tax-free allowances count as income and must be declared in your tax return. Doing so could significantly lower your tax bill.
Other mistakes to avoid in your Self Assessment tax return
Other common errors to watch out in the tax return process are:
- Missing the Self Assessment registration deadline (5th October)
- Missing the Self Assessment tax return deadline (31st January)
- Ticking the wrong boxes on the form
- Not budgeting for the tax bill
- Failure to declare future business income (sole traders, unincorporated landlords, and business partnerships with an annual turnover of under £150,000, who use cash-basis accounting, must declare business income that they have yet to receive at the point of submitting their tax return)
- Failure to declare interest received on bank accounts
How to amend your tax return
If you’ve filed your tax return and made a mistake, you can make changes within 12 months of the Self Assessment deadline. For example, you have until 31st January 2025 to correct any errors in your tax return for 2022/23. Depending on the mistake, you may have to pay more tax or claim a refund if you’ve overpaid.
Conclusion
The first time you file a tax return can be complicated and stressful, and it’s easy to make a mistake. However, late submissions, late payments, and providing inaccurate information can extend the process and result in penalties, so it’s important to do your homework well in advance. It might also be worth getting professional help if you’re unsure how to file your tax return correctly.
We hope you have found this guide informative and helpful in understanding some of the most common Self Assessment tax return mistakes. If you have any Self Assessment tips, please share them in the comments below, or get in touch with us if you have any questions.
