Being self-employed gives you freedom, but it also means managing your own taxes. HMRC data shows that over 750,000 people file their Self Assessment late every year, and expense errors cost self-employed taxpayers millions in lost deductions or penalties. Here are the 5 mistakes I see most often and exactly how to avoid them.
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Over 750,000 people miss the 31 January deadline every year. The first penalty is automatic and non-negotiable.
✅ How to avoid it: File by 31 January (online). Set a calendar reminder for 1 December. If you file by 30 December and owe less than £3,000, HMRC can collect it through your tax code. Consider filing in April or May — you'll know exactly what you owe and can budget for it.
How late
Penalty
1 day late
£100 fixed penalty
Up to 3 months
£100 + £10/day (max £900)
6 months
Above + 5% of tax due or £300
12 months
Above + 5% of tax due or £300
❌ Mistake 2: Claiming the Wrong Expenses
The most common error is claiming personal expenses as business costs. Daily lunch, commuting, and personal phone use top the list of rejected claims.
✅ How to avoid it: Only claim expenses that are "wholly and exclusively" for business. Claimable: tools, equipment, mileage (45p/mile first 10k), business phone, home office proportion, professional subscriptions, training, accountancy fees. NOT claimable: daily lunch, commuting to your usual workplace, personal clothing, fines. Keep every receipt for 5 years.
❌ Mistake 3: Not Registering for VAT on Time
You must register when your rolling 12-month turnover exceeds £85,000. Many self-employed people track this poorly and miss the threshold.
✅ How to avoid it: Check your rolling 12-month turnover monthly. Register within 30 days of exceeding £85,000. Late registration penalties: 5% if 9 months late, 10% if 18 months late, 15% beyond that. Consider voluntary registration below £85k if your customers are VAT-registered — you can reclaim VAT on business purchases.
❌ Mistake 4: Not Keeping Proper Records
HMRC requires you to keep records for 5 years. Many self-employed people lose receipts, mix personal and business spending, or rely on memory.
✅ How to avoid it: Open a dedicated business bank account. Use accounting software (FreeAgent, QuickBooks, Xero) or a simple spreadsheet. Photograph every receipt immediately. Log business mileage as you travel. Reconcile your accounts monthly, not once a year. HMRC can investigate 20 years back for deliberate evasion.
❌ Mistake 5: Ignoring Payments on Account
If your tax bill exceeds £1,000, HMRC requires two advance payments toward next year's bill — on 31 January and 31 July. Many first-time filers are shocked by this.
✅ How to avoid it: Plan for it. Your January payment = previous year's tax + first payment on account (50%). Your July payment = second payment on account (50%). If your income drops, you can reduce payments on account. Use our calculator to estimate your total January bill including payments on account.
Bonus: Choosing the Wrong Business Structure
Many self-employed people stick with sole trader status when a limited company would be more tax-efficient, or incorporate too early when sole trader is simpler.
When sole trader makes sense: Profits below £50,270, simple business, no plans to grow, want minimal admin.
When limited company makes sense: Profits above £50,270, want to retain earnings, plan to grow, want limited liability protection.
💡 Example: £60,000 Profit
Sole trader: Income Tax £9,432 + Class 4 NIC £1,286 + Class 2 NIC £179 = £10,897 total.
Limited company: Corporation Tax £9,450 + Dividend Tax £1,629 = £11,079 total.
At £60k, sole trader wins slightly. Above £70k, limited company typically becomes more efficient. Use our structure comparison calculator to find your crossover point.
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What happens if I miss the Self Assessment deadline?
Automatic £100 penalty from day one. After 3 months: £10/day up to £900. At 6 and 12 months: additional 5% of tax due or £300. Interest accrues from 1 February. File by 31 January to avoid all penalties.
Can I claim food and drink as a business expense?
Generally no unless travelling overnight for business. Daily lunch at your usual workplace is not claimable. Meals at client meetings may be claimable as entertainment. Keep receipts with business purpose noted.
Do I need to register for VAT as a sole trader?
Yes if your rolling 12-month turnover exceeds £85,000. Register within 30 days. Late penalties: 5% at 9 months, 10% at 18 months, 15% beyond. Consider voluntary registration below £85k if your customers are VAT-registered.
How long should I keep self-employed tax records?
At least 5 years after the 31 January submission deadline. For 2025/26 (filed January 2027), keep until January 2032. Keep invoices, receipts, bank statements, mileage logs. HMRC can investigate 20 years back for deliberate evasion.
What is payments on account and why do I owe extra tax?
Advance payments toward next year's bill if your tax exceeds £1,000. Two payments: 31 January and 31 July, each 50% of last year's tax. Your January bill = last year's tax + first payment on account. Can be a shock for first-time filers.