Filing a Self Assessment tax return can seem simple, but small mistakes can sometimes lead to an incorrect tax bill, unnecessary stress or even penalties.

Whether you’re self-employed, a landlord, a company director or have additional income, knowing the common mistakes can help you submit your return accurately and on time.

Here are some of the most common Self Assessment mistakes in the UK and how to avoid them.

1. Missing the Self Assessment Deadline

One of the most common mistakes is leaving your tax return until the last minute.

For most taxpayers filing online, the Self Assessment deadline is 31 January following the end of the tax year. For the 2025/26 tax year, the online filing deadline is 31 January 2027.

Preparing your return early gives you time to find missing records, check your figures and plan for the tax payment.

2. Forgetting to Declare Additional Income

Your main employment or business may not be your only taxable income.

You may also have income from:

• Freelance or side-business work
• Property rental
• Dividends
• Savings and investments
• Overseas income

However, not every small amount of additional income automatically requires a Self Assessment return. For certain trading or miscellaneous income, the £1,000 trading allowance may apply, depending on your circumstances.

Always check the rules for your particular income.

3. Claiming the Wrong Expenses

If you’re self-employed, certain allowable business expenses can reduce your taxable profit.

Common examples include business insurance, professional fees, advertising, office costs and certain travel expenses.

However, personal spending cannot normally be claimed simply because it was paid from a business bank account.

Tip: Keep receipts and records throughout the year so you can identify genuine allowable expenses.

4. Forgetting Legitimate Expenses

Some people make the opposite mistake — they don’t claim expenses they are entitled to claim.

Forgetting allowable business costs can increase your taxable profit and potentially increase your tax bill.

Good bookkeeping makes it easier to track expenses and avoid missing legitimate claims.

5. Confusing Turnover With Profit

If you’re self-employed, remember that turnover and profit are not the same thing.

For example:

Turnover: £40,000
Allowable expenses: £8,000
Profit: £32,000

Using turnover instead of the correct taxable profit can result in an inaccurate tax calculation.

6. Forgetting About Payments on Account

Your Self Assessment bill may include not only tax owed for the previous tax year but also payments on account towards your next tax bill, if they apply to you.

These advance payments are generally due on 31 January and 31 July.

This is why a first Self Assessment bill can sometimes be higher than expected.

7. Using Figures From the Wrong Tax Year

The UK tax year runs from 6 April to 5 April the following year.

It’s easy to accidentally include income or expenses from the wrong period, especially when you have several accounts or irregular income.

Always check that your figures relate to the correct tax year before submitting your return.

8. Forgetting Capital Gains or Foreign Income

Certain asset sales may create a Capital Gains Tax reporting requirement. Similarly, depending on your circumstances, foreign income such as overseas rental income, dividends or interest may need to be reported.

Don’t assume that these amounts can simply be left out of your Self Assessment return.

9. Not Checking Your Return Before Submission

A quick review before submitting can help catch simple errors.

Check your:

• Income
• Expenses
• Pension information
• Capital gains
• Foreign income
• Tax already paid
• Payments on account

If something doesn’t look right, investigate it before submitting the return.

10. Ignoring a Mistake After Submission

If you discover that you’ve made an error after submitting your tax return, don’t ignore it.

In many cases, you can amend your return within the permitted amendment period. For most online returns, amendments can generally be made within 12 months of the statutory filing date.

If you’re unsure how to correct an error, getting professional tax advice can help.

How to Avoid Self Assessment Mistakes

A few simple habits can make your tax return much easier:

• Keep receipts and financial records throughout the year.
• Track all sources of income.
• Keep business and personal transactions separate where possible.
• Review your expenses carefully.
• Start your tax return well before the deadline.
• Keep money aside for your tax bill and payments on account.
• Check your return carefully before submitting it.

Frequently Asked Questions (FAQs)

1. Can I correct a Self Assessment mistakes after submitting my return?

Yes. In many cases, you can amend your tax return within the permitted amendment period. The correction may change the amount of tax you need to pay or receive.

2. What happens if I forget to declare income?

If you realise you’ve missed taxable income, deal with it as soon as possible. Depending on the circumstances, you may need to amend your return or use another HMRC correction process.

3. Can I claim all my business expenses?

No. Only expenses that meet the relevant tax rules can generally be claimed. Personal expenses cannot normally be claimed as business expenses.

4. What are payments on account?

Payments on account are advance payments towards your future Self Assessment tax bill. Where they apply, they are generally paid twice a year on 31 January and 31 July.

5. Can an accountant help with Self Assessment?

Yes. An accountant can help you organise your records, identify allowable expenses, prepare your tax return and reduce the risk of common mistakes.

Final Thoughts

Self Assessment mistakes don’t have to be stressful.

Most mistakes can be avoided by keeping good records, understanding what income and expenses need to be reported, and giving yourself enough time to check your return.

If you are unsure about your tax position, getting professional advice before submitting your return can help you avoid costly mistakes later.

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