5 Common Tax Mistakes to Avoid in the UK

Navigating the UK tax system can feel overwhelming—whether you're a sole trader, small business owner, or an employee managing self-assessment for the first time. At Mytaxfiling, we provide trusted tax advice for individuals and businesses across the UK to help you avoid costly errors that can lead to penalties, interest, or missed savings.

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Here are five of the most common tax mistakes and how to avoid them.

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1. Missing the Self-Assessment Deadline

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Every year, thousands of taxpayers in the UK miss the self-assessment tax return deadline, which is 31st January for online filings and 31st October for paper returns. Missing the deadline results in an automatic £100 fine, which increases over time.

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Avoid it by:

• Marking the deadline on your calendar.
• Using HMRC’s email reminder service.
• Submitting your return early—even if you don't pay right away.

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At Mytaxfiling, we can help you prepare and submit your return accurately and on time.

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2. Not Registering for Self-Assessment When Required

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If you start earning additional income—through freelancing, rental income, crypto gains, or side businesses—you may need to register for self-assessment with HMRC. Many people assume that PAYE covers all tax liabilities, which is not always true.

Avoid it by:

• Registering as soon as you earn income outside PAYE.
• Checking HMRC’s eligibility criteria here.

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Mytaxfiling can help assess whether you need to register, and assist with the process.

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3. Forgetting to Claim Allowable Expenses

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Many small business owners, freelancers, and landlords forget to claim allowable expenses, reducing their taxable income unnecessarily. These can include:

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• Travel and mileage
• Home office costs
• Professional subscriptions
• Marketing and advertising

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Avoid it by:

• Keeping digital records of all expenses.
• Categorising them monthly to avoid end-of-year stress.
• Using tax software or hiring a tax adviser.

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Mytaxfiling can advise on what’s allowable based on your specific business activities.

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4. Incorrectly Reporting Income

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Reporting the wrong income figure—by double-counting or omitting certain sources—is a common issue. This is especially true for those with:

• Multiple income streams
• Foreign income
• Cryptocurrency or investment gains

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Avoid it by:

• Keeping accurate records of all sources of income.
• Using HMRC tools or a tax adviser to double-check figures.

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We can help consolidate your income sources and ensure your return is 100% accurate.

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5. Not Planning for Your Tax Bill

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One of the biggest mistakes is not setting aside money for your tax bill. Many self-employed individuals and new business owners are caught off-guard by their first payment, especially when it includes a Payment on Account for the following year.

Avoid it by:

• Putting aside 20–30% of your earnings.
• Using a separate savings account for taxes.
• Calculating estimated tax quarterly.

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Mytaxfiling offers forecasting tools and expert guidance to help you plan ahead with confidence.

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Final Thoughts

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Whether you're managing personal taxes or running a business, avoiding these common mistakes can save you stress, money, and time.

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At Mytaxfiling, our mission is to provide trusted tax advice for individuals and businesses across the UK. We’re here to simplify your tax journey—one return at a time.

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