Completing your own Self Assessment tax return is entirely manageable. However, without the right preparation, it’s easy to make mistakes that can lead to penalties, missed tax savings, or unwanted attention from HMRC.
Whether you’re self-employed, a landlord, running a side hustle, or earning income outside PAYE, this guide highlights the most common Self Assessment mistakes for 2026, and how to avoid them.
Key deadline:
The filing and payment deadline for the 2025/26 tax year (6 April 2025 to 5 April 2026) is 31 January 2027.
1. Missing the Self Assessment Deadline
Missing the deadline triggers an automatic £100 penalty, even if no tax is due. Further penalties and interest can follow quickly.
How to avoid it:
Start early and set reminders. Leaving your tax return until January increases the risk of errors and delays.
2. Not Registering for Self Assessment on Time
If you’re new to Self Assessment, you’ll need a Unique Taxpayer Reference (UTR) before filing, which can take several weeks to arrive.
How to avoid it:
Register by 5 October following the end of the tax year in which you first earned untaxed income.
3. Forgetting to Declare All Income
One of the most common mistakes is failing to report all sources of income. HMRC expects full disclosure, including:
- Freelance or side hustle income (Etsy, Uber, tutoring, etc.)
- Rental income (including Airbnb)
- Interest, dividends, and investments
- Cryptocurrency income or disposals
- Capital gains
- Second or temporary jobs
How to avoid it:
Review bank statements and platform reports carefully. Never assume HMRC already has the full picture.
4. Claiming Incorrect Expenses (or Missing Them)
Some taxpayers under-claim and miss out on tax relief, while others over-claim and risk penalties.
Commonly missed allowable expenses include:
- Business use of your home
- Business mileage
- Pension contributions
- Gift Aid donations
- Marriage Allowance (if eligible)
- Trading or property allowance (£1,000 threshold)
How to avoid it:
Understand what qualifies as a legitimate business expense and keep clear records to support your claims.
5. Ignoring Capital Gains Tax (CGT)
With the CGT annual exemption reduced, more individuals now fall within reporting requirements.
How to avoid it:
If you’ve sold property, shares, or cryptoassets, ensure you keep records and assess whether a gain needs to be declared.
6. Being Caught Out by Payments on Account
Many first-time filers are surprised when HMRC requests payments on account, advance payments towards the next tax year.
How to avoid it:
Estimate your tax bill throughout the year and set aside funds regularly to avoid unexpected cash flow pressure.
7. Poor Record-Keeping Ahead of Making Tax Digital (MTD)
From April 2026, many self-employed individuals and landlords will need to comply with Making Tax Digital for Income Tax (MTD ITSA).
This includes:
- Keeping digital records
- Submitting quarterly updates
How to avoid it:
Start using digital tools or structured spreadsheets now to prepare for the transition.
8. Relying on Estimates Instead of Records
Guesswork leads to errors, missed claims, and inaccurate reporting.
How to avoid it:
Keep records throughout the year. Track income monthly and store receipts digitally wherever possible.
9. Entering Figures Incorrectly
Common errors include:
- Mixing up gross and net figures
- Entering tax deducted incorrectly
- Misclassifying employment income as self-employment
How to avoid it:
Take your time, double-check entries, and consider using accounting software with built-in validation.
10. Assuming PAYE Income Is Already Correct
If you’re employed and also complete a tax return, your PAYE tax code may already include adjustments.
How to avoid it:
Check your P60, P11D, and tax code carefully to avoid overpaying or underpaying tax.
11. Falling Victim to HMRC Scams
Scam emails and texts often increase around Self Assessment deadlines and can look convincing.
How to avoid it:
HMRC will never ask for payment or personal details via email or text. Always log in directly through GOV.UK or the official HMRC app.
12. Forgetting You Can Amend Your Tax Return
Mistakes can happen, but you have 12 months after the deadline to correct your return.
How to avoid it:
Review your submission and amend it promptly if anything changes or needs correcting.
Self-assessment doesn’t have to be stressful. The key is preparation, organisation, and accuracy.
Simple habits such as keeping digital records, starting early, and reviewing your return carefully can save time, money, and unnecessary worrys. If you would like further support on this, feel free to contact us at Greystone Advisory.
