Filing your self-assessment tax return can feel like a job that is easy to put off, but leaving it too late often leads to unnecessary stress and avoidable penalties. With the 31 January deadline approaching each year, it pays to be organised and clear on what HMRC expects.
Samantha Newport, director at Modus Accountants, shares practical tips to help small business owners and individuals file accurately and on time.
1. Register with HMRC as early as possible
If you are new to self-assessment, registering early is essential. HMRC will send your Unique Taxpayer Reference (UTR) and activation code by post, which can take over a week to arrive.
You typically need to register by 5 October following the end of the tax year. Leaving it too late can mean you are unable to file on time, even if you are otherwise prepared.
This is a common issue for new sole traders, company directors filing for the first time and individuals with new income streams.
2. Include all sources of income
One of the most frequent mistakes is forgetting to declare all income. HMRC receives data from multiple sources, so missing something can raise red flags.
You should include:
- Employment income from your P60
- Benefits and expenses shown on a P11D
- Self-employment income
- Dividend income from shares
- Bank interest (outside ISAs)
- Rental income
- Foreign income where applicable
Even small amounts, such as interest from savings accounts, should not be overlooked.
If your financial situation has changed during the year, take extra care to ensure everything is captured.
3. Understand what you can claim as expenses
Claiming the right expenses can significantly reduce your tax bill, but it is important to stay within HMRC rules.
Allowable expenses must be “wholly and exclusively” for business purposes. Common examples include:
- Office costs such as stationery and software
- Travel costs, including mileage
- Professional fees and subscriptions
- A portion of home costs if you work from home
Where many people go wrong is either underclaiming or claiming incorrectly.
If you are unsure, it is worth reviewing guidance or speaking to a professional. You can also explore our guide on 5 business expenses you might not know you could claim for more detail on lesser-known allowances.
4. Keep accurate and up-to-date records
Good record-keeping makes self-assessment far easier and reduces the risk of errors.
You should keep:
- Invoices and receipts
- Bank statements
- Payroll records if applicable
- Details of any other income
HMRC expects records to be kept for at least five years after the 31 January submission deadline.
Using bookkeeping software or professional bookkeeping services can save time and give you confidence that your figures are accurate.
5. Be aware of payments on account
Many taxpayers are caught off guard by payments on account. If your tax bill exceeds £1,000, you may be required to make advance payments towards the next year’s tax.
These are due:
- 31 January
- 31 July
Each payment is typically 50 percent of your previous year’s tax bill.
Failing to plan for this can create cash flow pressure, even if your business is performing well. Our article on cash flow vs profit and why profitable businesses can still struggle explains why this happens and how to manage it.
6. Double-check everything before submitting
Errors on your tax return can lead to delays, enquiries, or penalties.
Before submitting, check:
- Figures match your records
- All sections are completed
- Personal details are correct
- You have included all income sources
It is much easier to fix mistakes before submission than after. While HMRC does allow amendments, these can take time and may trigger further checks.
7. Do not leave it until the last minute
January is always a busy time, and HMRC’s online system often experiences heavy traffic close to the deadline.
Leaving your return until the final days increases the risk of:
- Missing information
- Technical issues
- Rushed errors
Starting early gives you time to gather documents, ask questions, and ensure everything is accurate.
If you need support, working with self assessment service professionals can remove the pressure and ensure everything is handled properly.
8. Understand the penalties for late filing and payment
HMRC penalties can escalate quickly, so it is important to take deadlines seriously.
If you miss the 31 January deadline:
- An immediate £100 fixed penalty applies
- After 3 months, daily penalties of £10 can be charged (up to £900)
- After 6 months, further penalties apply based on the tax owed
- Interest is charged on late payments
Even if you cannot pay your tax bill in full, it is still better to submit your return on time and then speak to HMRC about payment options.
9. Get professional advice if your situation is complex
Tax rules can become more complicated if you:
- Run a limited company
- Have multiple income streams
- Employ staff
- Are VAT registered
In these cases, working with experienced accountants in Oxfordshire can help ensure compliance and identify opportunities to improve your tax position.
Support with related areas such as payroll services or VAT services can also help keep your finances organised throughout the year, not just at filing time.
Conclusion
Filing your self-assessment tax return does not need to be stressful if you stay organised and give yourself enough time. Understanding what HMRC expects, keeping accurate records, and being aware of deadlines can make a significant difference to both your experience and your final tax position.
Small mistakes or missed details can lead to penalties or missed opportunities to claim legitimate expenses. Taking a proactive approach, and seeking advice where needed, helps ensure everything is handled correctly the first time.
If you would prefer peace of mind, working with experienced professionals can take the pressure off and allow you to focus on running your business with confidence.
FAQs
Who needs to complete a self-assessment tax return?
You may need to file if you are self-employed, a company director, receive rental income, earn significant dividends, or have untaxed income.
What happens if I miss the deadline?
You will receive an automatic £100 penalty, even if you have no tax to pay. Further penalties and interest can apply if the delay continues.
Can I amend my tax return after submitting it?
Yes, you can usually amend your return within 12 months of the filing deadline. However, it is best to get it right first time where possible.
What expenses can I claim?
You can claim expenses that are wholly and exclusively for business use. This includes costs like travel, office supplies, and professional fees.
Do I need an accountant for self-assessment?
Not everyone does, but if your finances are more complex or you want peace of mind, professional support can help you avoid mistakes and stay compliant.