The deadline for paper tax returns is rapidly approaching on 31 October, and it’s crucial that you allow plenty of time to complete your form correctly.
Waiting until the last minute could increase the risk of errors, which may result in you paying too much or too little tax.
So, if you’re self-employed, now is the perfect time to get your paperwork in order and start completing your tax return. Here are a few common mistakes to avoid.
1. Incorrect Unique Taxpayer Reference
Everyone who submits a self-assessment tax return must apply for a Unique Taxpayer Reference (UTR). This is a 10-digit number that will be posted to you within 15 days (if you live in the UK) of you registering for self-assessment.
The UTR is a crucial identifier, and if you fail to enter it on your form or write it incorrectly, this can cause complications and delays.
If you’re an individual or sole trader, you can find your UTR:
- In your personal tax account
- In the HMRC app
- On previous tax returns and other documents from HMRC.
Take your time and double-check that this number is correct on your form before submitting. If you can’t find your UTR, contact HMRC for help.
2. Not claiming allowable expenses
There are some allowable business expenses that can be deducted from your income to reduce your taxable profit.
These are costs that are incurred “wholly and exclusively” for business purposes, such as equipment, staff uniforms, and renting a premises. The government website has a complete list of allowable expenses.
Overlooking these allowable expenses could mean that you pay more tax than necessary. So, it’s worth speaking to a financial professional who can ensure that you’re claiming any tax relief you’re entitled to.
3. Missing deadlines
In 2025, government figures show that an estimated 1.1 million people missed the deadline for submitting their self-assessment tax return online.
If you file your return late, you’ll be charged an automatic £100 penalty, even if you don’t owe any tax. If you fail to submit your form within three months of the deadline, you’ll incur a further penalty of £10 a day, up to a maximum of £900.
So, it’s essential that you keep a note of key deadlines and allow plenty of time to meet them.
In 2025, the important dates to remember are:
- Register for self-assessment – by 5 October 2025
- Paper filing deadline – 31 October 2025
- Online filing deadline – 31 January 2026
- Payment deadline – 31 January 2026 (There is a second payment deadline of 31 July if you make payments towards your bill, known as “payments on account”).
4. Not reporting all taxable income
If you do this deliberately, you could be found guilty of tax evasion, which has serious consequences.
However, it’s possible to leave out taxable sources of income by mistake if you’re unclear on what should be included in your self-assessment form. In addition to your business income, this can include:
- State benefits
- Rental income
- Savings interest
- Pension payments
- Share dividend payments
- Tips, bonuses, and commissions
- Capital gains made from selling taxable assets
- All earnings from employment in the UK and overseas.
Keeping accurate records could reduce the risk of missing payments or expenses that should be included.
5. Overlooking tax-free allowances
Failing to claim the allowances you’re entitled to could result in a higher tax bill.
There are several types of tax relief for self-employed individuals, such as:
- The Trading Allowance – Up to £1,000 a year for eligible trading income.
- The Property Allowance – Up to £1,000 a year for individuals with income from land or property.
It may be worth speaking to a financial planner who can ensure that you’re making the most of any tax allowances available.
Correcting self-assessment errors
If you realise that you’ve made a mistake on your form after submitting it, you have 12 months after the deadline to amend your return. Your tax bill will be adjusted in line with your corrections.
However, starting your return early and allowing plenty of time to complete it could save you the stress of making errors or missing deadlines.
Get in touch
If you’re self-employed and need support reviewing and managing your finances tax-efficiently, we can help.
To learn more about working with us, please email hello@sovereign-ifa.co.uk or call us on 01454 416653.
Please note
This article is for general information only and does not constitute advice. The information is aimed at retail clients only.
All information is correct at the time of writing and is subject to change in the future.
Please do not act based on anything you might read in this article. All contents are based on our understanding of HMRC legislation, which is subject to change.
The Financial Conduct Authority does not regulate tax planning.
A pension is a long-term investment not normally accessible until 55 (57 from April 2028). The fund value may fluctuate and can go down, which would have an impact on the level of pension benefits available. Past performance is not a reliable indicator of future performance.
The tax implications of pension withdrawals will be based on your individual circumstances. Thresholds, percentage rates, and tax legislation may change in subsequent Finance Acts.
Approved by Best Practice IFA Group Ltd on 15/8/25