Are You Making These 7 Common Self Assessment Tax Return Mistakes?

Clear records, accurate figures and reliable support can make your Self Assessment easier to manage.

The 31 January 2027 Deadline

If you are self-employed, a sole trader, a landlord, or have other untaxed income, you may need to file a Self Assessment tax return.

For the 2025/26 tax year, the online filing deadline is 31 January 2027. Any balancing payment due for 2025/26 must also be paid by this date. If payments on account apply to you, your first payment towards the 2026/27 tax year may be due at the same time.

The paper filing deadline is earlier, on 31 October 2026. You can check the latest dates on the official GOV.UK Self Assessment deadlines page.

The deadline can feel a long way away, but gathering records and checking your figures takes time. Here are seven common mistakes to avoid.

1. Leaving Your Tax Return Until The Last Minute

One of the most common mistakes is waiting until January to begin.

Last-minute filing can create avoidable problems. You may struggle to find missing invoices, bank statements, receipts or details of additional income. You may also discover that you need to register for Self Assessment or recover access to your HMRC online account.

Technical issues and payment delays can add further pressure. A return submitted after the deadline can lead to an automatic £100 late filing penalty, even if you do not owe any tax.

The practical solution: start gathering your information early. Set aside time to review your records, identify missing documents and prepare your return before the final weeks of January.

2. Forgetting To Declare All Your Income

Your tax return must include all relevant taxable income, not only the income from your main business.

This may include:

• Sole trader or freelance income.
• Income from a second job or side business.
• Property or rental income.
• Dividends and savings interest.
• Foreign income.
• Capital gains where relevant.
• Benefits or other taxable income.

It is easy to overlook occasional freelance work, a small online business or income received through a separate bank account. However, HMRC expects your return to provide a complete picture of your taxable income.

Review your bank statements, invoices, payment platforms and other financial records. Compare these figures with your bookkeeping records so that omissions and duplicate entries are less likely.

The practical solution: maintain accurate records throughout the year. Up-to-date bookkeeping gives you a clearer view of your income and makes it easier to prepare an accurate return.

3. Using Figures From The Wrong Tax Year

The UK tax year runs from 6 April to 5 April the following year. This is different from the calendar year and may not match your business accounting period.

For the 2025/26 return, you generally need to report income and expenses relating to the period from 6 April 2025 to 5 April 2026.

Using figures from the wrong period can affect your taxable profit and lead to an inaccurate return. This is particularly easy to do when you are working from bank statements, invoices or accounting software without checking the dates carefully.

The practical solution: label your records clearly by tax year. Check the dates of invoices, payments and expenses before entering figures into your return. If you use accounting software, review the reporting period before exporting information.

A careful review at this stage can prevent confusion later and give you greater confidence in the figures submitted.

4. Claiming The Wrong Expenses, Or Missing Allowable Costs

Expenses can reduce your taxable profit, but they must meet HMRC’s rules.

Business expenses generally need to be incurred wholly and exclusively for the purpose of your trade. Personal expenses are not normally allowable. Common mistakes include claiming private travel, ordinary clothing, personal entertainment or the full cost of an item that is partly used personally.

The opposite mistake is also common. Some self-employed people fail to claim legitimate business costs because they are unsure what qualifies. This can result in paying more tax than necessary.

Depending on your circumstances, relevant business costs may include professional fees, business insurance, office costs, certain travel expenses, software subscriptions and a reasonable business proportion of some home-working costs.

Keep receipts, invoices and clear explanations for your claims. Where an expense has both business and personal use, calculate and claim only the appropriate business proportion.

The practical solution: review your expenses carefully and keep supporting evidence. If you are unsure whether a cost is allowable, obtain professional guidance before including it.

5. Forgetting About Payments On Account

Your January bill may be more than the tax owed for the year just ended.

If your Self Assessment tax bill is above the relevant threshold and most of your tax has not already been collected at source, HMRC may require you to make payments on account. These are advance payments towards your following year’s tax bill.

Usually, the first payment is due on 31 January and the second payment is due on 31 July. Each payment is normally based on part of your previous tax liability.

This can create a cash-flow challenge because your January bill may include:

• The balancing payment for the previous tax year.
• The first payment on account for the next tax year.

The practical solution: check your previous tax calculation and budget for the full amount due. Do not assume that your January payment will only cover the tax already calculated for the year.

If your income has reduced significantly, it may be possible to apply to reduce your payments on account. This should be considered carefully, because reducing them without a reasonable basis may leave you with an unexpected balance later.

6. Submitting The Return But Forgetting To Pay

Filing your tax return and paying your tax are separate responsibilities.

You may submit your return on time but still face interest and late payment penalties if the tax is not paid by the due date. This is a common issue when someone files early but does not make a note of the payment deadline.

Check your HMRC online account and allow enough time for your payment to reach HMRC. Bank transfers, payment methods and processing times can vary.

The GOV.UK guidance on Self Assessment penalties explains the possible consequences of late filing and late payment.

The practical solution: record both deadlines in your calendar. Keep confirmation of your payment and review your HMRC account afterwards to ensure the balance has been received correctly.

7. Not Checking The Return Before Submission

Small data-entry errors can change your tax calculation.

Before submitting, check your:

• Name, address and National Insurance details.
• Unique Taxpayer Reference.
• Income figures.
• Expense totals.
• Pension contributions and Gift Aid payments.
• Student or postgraduate loan details.
• PAYE income and tax already paid.
• Property, dividend or foreign income sections where relevant.
• Payments on account already made.

Compare the return with your bookkeeping records, bank statements, P60, P45, P11D and other relevant documents. Do not rely only on a quick visual check. Review each section and confirm that the figures relate to the correct tax year.

If you later discover a genuine error, HMRC usually allows you to amend your return within 12 months of the Self Assessment deadline. For the 2025/26 return, the normal amendment deadline is 31 January 2028. You can find more information on the GOV.UK Self Assessment corrections page.

The practical solution: complete a structured final review before submitting. Accurate records and a second pair of eyes can provide valuable reassurance.

A Practical Self Assessment Checklist

Before 31 January 2027, make sure you have:

• Confirmed whether you need to file a Self Assessment return.
• Gathered all income records for the 2025/26 tax year.
• Reviewed business and personal bank statements.
• Checked that expenses are allowable and supported by evidence.
• Confirmed whether payments on account apply.
• Reviewed any tax already paid through PAYE or previous payments.
• Checked your personal and financial details.
• Filed the return and paid the amount due.
• Saved a copy of the submitted return and payment confirmation.

Taking these steps early can simplify the process and reduce the risk of missing important information.

Get Reliable Self Assessment Support

Are you concerned that your records are incomplete, your expenses are unclear or your tax bill may be difficult to manage?

S&G Advisory provides a dependable Self Assessment tax filing service for sole traders, self-employed professionals and individuals with additional income. We can help organise your information, review your figures and prepare your return accurately and efficiently.

Our support is tailored to your needs. You receive clear guidance without unnecessary complexity, helping you remain compliant and make more confident financial decisions.

For personalised support, contact S&G Advisory or email info@sgadvisory.co.uk. We are here to help you manage your Self Assessment with greater clarity and peace of mind.