The 12 Most Common Self Assessment Mistakes

These are the twelve most common Self Assessment mistakes — and how to avoid them.

1. Missing the deadline. File early. The penalty for being one day late is £100.

2. Using the wrong UTR. Double-check your 10-digit Unique Taxpayer Reference before submitting.

3. Forgetting income sources. Include all income — employment, self-employment, property, savings, dividends. HMRC has data from multiple sources and will spot discrepancies.

4. Not claiming all expenses. Review the full list of allowable expenses. Most sole traders underclaim by hundreds of pounds.

5. Confusing turnover with profit. Report your total turnover (before expenses), not your profit. Your expenses are deducted separately.

6. Mixing personal and business expenses. Only claim genuine business costs. Personal expenses on your business account must be excluded.

7. Forgetting payments on account. If your bill exceeds £1,000, you will owe advance payments for next year too.

8. Not claiming capital allowances. Equipment, vehicles, and other assets can be claimed through capital allowances.

9. Wrong accounting basis. Make sure you consistently use either cash basis or accruals basis, not a mix.

10. Not keeping records. If HMRC enquires and you cannot support your figures, penalties follow.

11. Ignoring student loan repayments. These are calculated on your Self Assessment and added to your bill.

12. Not seeking help when needed. If your affairs are complex, an accountant's fee is tax-deductible and could save you far more than it costs.

Accounted helps prevent most of these mistakes through automation, AI categorisation, and built-in checks.

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