Filing your Self Assessment tax return can be a daunting task, especially if you’re self-employed or running a small business. With tight deadlines and strict rules from HMRC, even a small mistake can lead to penalties, unexpected tax bills, or delays in refunds.
To help you stay on track and stress-free, here are five of the most common tax return mistakes — and how to avoid them.
1. Missing the Deadline
Mistake: One of the most frequent errors is simply missing the deadline. The Self Assessment deadline is 31 January for online returns (covering the previous tax year ending 5 April).
Consequences: Even if you owe no tax, a late submission results in an automatic £100 penalty. Additional fines apply the longer you delay.
How to avoid it:
- Add the deadline to your calendar
- Start your return early
- Use HMRC’s email reminders or an accountant to keep you on track
2. Incorrect or Missing Information
Mistake: Entering incorrect figures or leaving out essential details like your Unique Taxpayer Reference (UTR), income from other sources, or claiming the wrong expenses.
Consequences: Mistakes can trigger HMRC enquiries, result in underpaid tax, or delay your tax refund.
How to avoid it:
- Gather all your records (P60s, invoices, bank statements) before you begin
- Double-check all entries
- Consider using accounting software or working with a professional
3. Forgetting to Declare All Income
Mistake: Omitting additional income such as rental earnings, dividends, bank interest, freelance side gigs, or foreign income.
Consequences: HMRC uses data matching tools to detect undeclared income, which can result in penalties and interest.
How to avoid it:
- Keep a list of all income streams
- Review bank statements and financial documents
- Be honest—if you earn it, declare it
4. Claiming Disallowed Expenses
Mistake: Claiming personal expenses (like clothing, travel to a permanent workplace, or daily meals) that aren’t “wholly and exclusively” for business purposes.
Consequences: You could face penalties and interest if HMRC deems your claims inaccurate or excessive.
How to avoid it:
- Learn what qualifies as an allowable expense
- Keep receipts and accurate records
- When in doubt, ask your accountant or check HMRC’s guidelines
5. Not Keeping Proper Records
Mistake: Failing to keep records of income, expenses, or receipts for at least 5 years after the 31 January submission deadline.
Consequences: If HMRC investigates and you can’t provide proof, they may estimate your tax bill or impose penalties.
How to avoid it:
- Store records digitally using accounting software
- Organise documents by category and date
- Regularly update your records throughout the year
Final Thoughts
Filing your tax return doesn’t need to be stressful — but it does require attention to detail. Avoiding these common mistakes can save you time, money, and hassle. If you’re ever unsure, it’s worth seeking help from a qualified accountant or tax advisor to make sure you’re staying compliant and tax-efficient.
