Filing your Self Assessment Tax Return for 2025 may seem straightforward at first glance, but the process can become complicated if not handled with care. Thousands of UK taxpayers face penalties each year not because they fail to file altogether, but due to avoidable errors that lead to misreporting or missed deadlines.
Whether you're self-employed, a landlord, a freelancer, or someone with multiple income sources, it's crucial to understand the pitfalls of Self Assessment filing. In this guide, Quick Tax Returns outlines the top 10 common mistakes and how to avoid them—ensuring accuracy, compliance, and peace of mind.
1. Missing the Filing Deadline
The deadline for submitting your online Self Assessment Tax Return for the 2024/2025 tax year is 31 January 2026. If you're filing a paper return, the deadline is 31 October 2025.
Why it matters:
Missing these dates results in automatic penalties—even if you don’t owe any tax. The longer you delay, the higher the penalties.
Solution:
Set digital reminders in advance, and aim to file at least two weeks before the deadline to allow for any last-minute corrections or technical issues.
2. Failing to Register for Self Assessment
If you are new to Self Assessment (for example, you started a side business or became a landlord), you must first register with HMRC. Failure to do so could prevent you from receiving your Unique Taxpayer Reference (UTR), which is essential for filing.
Solution:
Register by 5 October 2025 following the end of the tax year in which you earned untaxed income. Keep your UTR and Government Gateway ID secure.
3. Incorrect Personal or Business Details
Even a simple error in your National Insurance number, address, or bank details can delay your refund or lead to processing issues.
Solution:
Double-check every entry before submitting. If your circumstances change (e.g., marital status or business structure), update HMRC promptly.
4. Forgetting to Declare All Sources of Income
One of the most common mistakes is underreporting income. This includes forgetting to declare:
- Rental income
- Dividends and investments
- Foreign income
- Freelance or gig economy earnings
- Side businesses
Solution:
Maintain proper records throughout the year and consult with a tax advisor if you’re unsure what qualifies as income.
5. Overlooking Allowable Expenses
Many taxpayers pay more tax than they need to because they don’t claim deductible business expenses, such as:
- Office supplies
- Business travel
- Software and subscriptions
- Marketing costs
- Home office costs (if applicable)
Solution:
Keep digital records and receipts, and use accounting software or expert support to calculate legitimate deductions accurately.
6. Misreporting VAT or Class 2/4 National Insurance Contributions
If you're self-employed and VAT-registered, or if your income exceeds certain thresholds, you’re responsible for correctly calculating and reporting VAT and NICs.
Solution:
Use reliable software or a tax professional to manage these complex figures. Ensure you stay updated on changing thresholds and rules from HMRC.
7. Failing to Keep Proper Records
HMRC requires you to keep records for at least 5 years after the 31 January submission deadline. Poor or missing documentation can lead to errors and penalties during audits.
Solution:
Store records digitally in organised folders or cloud systems. Save:
- Invoices and receipts
- Bank statements
- Employment contracts
- Rental agreements
8. Guessing or Estimating Figures
Some taxpayers input rough estimates, hoping they'll be close enough. This is risky and can lead to discrepancies during HMRC reviews.
Solution:
Use accurate figures. If you're waiting on documents (like P60s or dividend statements), wait until you receive them before filing or flag them clearly to HMRC.
9. Not Using the Correct Tax Code
Using an outdated or incorrect tax code can result in underpaid or overpaid tax, potentially affecting your return.
Solution:
Check your current tax code through your HMRC online account or on your payslips. Contact HMRC for clarification if anything seems off.
10. Neglecting Professional Help When Needed
DIY filing can be fine if your tax situation is simple, but if you're managing complex income streams, overseas earnings, or business deductions, professional help is invaluable.
Solution:
Work with a qualified tax advisor or use expert Self Assessment services like Quick Tax Returns to ensure everything is accurate, optimised, and fully compliant.
Pro Tip: File Early, Not Just on Time
Submitting your return well before the deadline gives you time to:
- Budget for your tax bill
- Fix errors
- Seek professional guidance
- Avoid last-minute stress
Remember, filing early doesn't mean paying early—the payment deadline remains 31 January 2026.
Why Avoiding These Mistakes Matters
Avoiding these common errors can help you:
- Save money by claiming eligible deductions and avoiding fines
- Gain peace of mind by staying compliant with HMRC regulations
- Protect your reputation as a responsible taxpayer
- Improve your financial planning by knowing your exact tax liability
By taking the time to understand and sidestep these mistakes, you ensure a smoother, more confident tax return process in 2025.
FAQs: Self Assessment Tax Return 2025
1. What is the deadline for filing the 2025 Self Assessment Tax Return?
The deadline for online filing is 31 January 2026. For paper returns, it's 31 October 2025.
2. Do I need to file a Self Assessment if I'm employed full-time?
Only if you have additional untaxed income—such as rental income, freelance work, or dividends. If you’re unsure, check with HMRC or a tax advisor.
3. Can I amend my Self Assessment after submission?
Yes, you can amend your return within 12 months of the original deadline (i.e., until 31 January 2027 for the 2024/25 return).
4. What happens if I make a mistake in my Self Assessment?
Minor mistakes can be corrected. However, serious errors or omissions could result in penalties and interest. HMRC may investigate if discrepancies are found.
5. Are there penalties for filing late, even if I owe no tax?
Yes. HMRC imposes a £100 penalty for missing the deadline, even if your tax bill is £0. Additional fines apply for longer delays.
6. What expenses can I claim as self-employed?
You can claim for business-related costs such as travel, office expenses, tools, and a portion of household bills if you work from home.
Final Thoughts
Filing your Self Assessment Tax Return for 2025 doesn’t have to be a stressful or costly process. By avoiding the common mistakes outlined above, you not only meet your legal obligations but also take control of your financial wellbeing.
At Quick Tax Returns, we’re dedicated to helping UK taxpayers navigate Self Assessment with clarity, accuracy, and support. If you're unsure about any part of the process, it’s always worth getting expert advice rather than risking avoidable errors.
Stay informed. Stay compliant. And make tax season stress-free.