UK Self Assessment Tax Return Guide for the Self-Employed
- Pick Wise Team
- May 10
- 6 min read
Completing your Self Assessment tax return can feel daunting, especially if it’s your first time. This comprehensive guide will walk you step-by-step through the process, highlight key deadlines, and outline practical tips to help you avoid common mistakes.
What Is Self Assessment?
Self Assessment is the UK’s system for collecting income tax from individuals and businesses whose tax is not automatically deducted from sources like wages, pensions, or savings. It’s a process that requires individuals to calculate and report their income to HMRC to ensure they pay the correct amount of tax.
If you’re self-employed, a freelancer, or a small business owner, you’ll generally need to register for Self Assessment and submit a tax return every year. This includes providing details of your income, expenses, and any tax reliefs or allowances you may be entitled to. Missing deadlines or filing incorrectly can result in penalties, so understanding the process is crucial for staying compliant.
Who Needs to Complete a Self Assessment Return?
You’ll need to file a Self Assessment Return if you meet any of the following criteria or fall into one of these specific categories listed below:
Self-employed (sole traders), with income over £1,000 (before deducting expenses)
Partner in a business partnership
You receive untaxed income such as rent, tips, foreign income, or dividends
You paid Capital Gains Tax when you sold something that increased in value
You had to pay the High Income Child Benefit Charge
For detailed eligibility, see the HMRC guide.
Key Self Assessment Deadlines
Meeting the deadlines for registering for Self Assessment and submitting your tax return is crucial to avoid penalties from HMRC. Missing these deadlines can result in fines, interest charges, and added stress, so it’s important to stay on top of your responsibilities.
Here are the key dates you need to remember and plan for:
In the UK, the tax year begins on 6 April and ends on 5 April of the following year.
If you need to file a tax return and haven’t done so before, you must notify HMRC by 5 October following the end of the tax year in question. You can tell HMRC by registering for Self Assessment.
If you’re doing a paper tax return, you need to submit it by midnight on 31 October.
If you’re doing an online tax return (recommended), you need to submit it by midnight on 31 January.
Any tax you owe must be paid by midnight on 31 January.
If you make advance payments on your bill (known as ‘payments on account’), a second payment deadline falls on 31 July.
An example timeline
John started a business and earned over £1000 from it in the 2024-2025 tax year. He registered for Self Assessment before the 31st of October 2025 deadline.
He decided to do an online return instead of a paper return. He submitted the online return in mid-January, before the deadline on the 31st of January 2026. He paid the tax he owed shortly after submitting his return and before the payment deadline of the 31st of January 2026.
Filing your Self Assessment can be time-consuming, so it’s wise to start early and leave yourself plenty of time before the deadline. If your situation is complicated, consider hiring an accountant to handle the process for you.
Missing the deadline could result in penalties and interest charges, so don’t delay.
Step-by-Step Guide to Filing Your Self Assessment Tax Return
1. Register for Self Assessment
If you’re registering for the first time, it’s important to sign up with HMRC online by 5 October following the end of the tax year in which you started trading. For example, if you began trading during the 2024/25 tax year, you must register by 5 October 2025.
Once you complete the registration process, HMRC will send your Unique Taxpayer Reference (UTR) by post. This 10-digit number is essential for managing your tax affairs, so be sure to keep it safe, as you’ll need it for submitting your Self Assessment tax return and any correspondence with HMRC.
2. Gather Your Information
The Self Assessment Return requires a significant amount of information about you and your business to ensure everything is reported accurately. It’s important to gather all the necessary details beforehand to make the process smoother and avoid delays.
Here are some of the key pieces of information you’ll need to have at hand before starting to fill in your return:
Your UTR, National Insurance number, and Government Gateway login
Business turnover and expenses
Records of income (invoices, receipts, bank statements)
P60/P45/P11D forms (if employed as well as self-employed)
Details of any other untaxed income (rental, dividends, etc.)
3. Log in and Start Your Return
Visit HMRC’s Self Assessment portal and log in using your credentials to manage your tax return online. If you’re submitting a paper return, make sure to download the correct form from the HMRC website, fill it out accurately, and post it well before the deadline to avoid any penalties.
4. Complete Each Section
The Self Assessment Return is divided into different sections to help ensure all relevant financial information is captured accurately. These sections include personal details such as your name, address, and contact information, self-employment details for information about your business income and expenses, and other income which covers earnings from sources like investments, rental properties, or pensions. Each section plays an important role in calculating your overall tax liability.
The personal information section is fairly self-explanatory. Just make sure that your name, address and contact details are all correct.
In the self-employment section, input all your business income, including earnings from freelance work or running your own company. Don’t forget to include details of your allowable expenses, such as office supplies, travel costs, or any other business-related expenses that can be claimed to reduce your taxable income.
In the final section, provide details of any additional income sources you may have, such as rental income from property, dividends from investments, capital gains from selling assets, or interest earned from savings accounts. Be sure to also include pension contributions you’ve made and any charitable donations, as these can sometimes qualify for tax relief or benefits, helping to reduce your overall tax burden.
5. Use Provisional Figures if Needed
If you don’t have your final profit figures ready by the deadline (e.g., your ‘accounting period’ doesn’t align with the tax year), you can use estimates, called ‘provisional figures’. Flag them on your return, then update HMRC once the correct figures are known (within 12 months of the filing deadline).
6. Check, Submit, and Save Proof
Double-check your tax return carefully to ensure there are no errors that could lead to costly mistakes, such as misreported income or missed deductions. Once you’ve reviewed everything, submit your return either online through the official tax portal or by mail, making sure to do so before the deadline to avoid penalties.
Don’t forget to save a copy for your records—it’s always helpful to have it on hand for future reference or in case of an audit.
7. Pay Your Tax Bill
Once you’ve submitted your tax return, HMRC calculates your tax liability. You must pay by 31 January following the end of the tax year.
You may need to make two ‘payments on account’ (advance payments for next year’s bill) in January and July.
Tips for a Smooth Submission
Here are some useful tips to help you easily and efficiently submit your tax return without stress:
Keep digital records throughout the year for easy filing.
Use accounting software to track income and expenses.
Double-check figures: Simple input errors can lead to penalties.
Set reminders for key dates and payment deadlines.
Speak to a professional accountant if your affairs are complex.
What If You Miss a Deadline?
HMRC may issue fines and penalties for missing Self Assessment deadlines, which can include an initial fixed penalty for late filing, daily penalties for prolonged delays, and even additional charges based on the tax owed. It’s important to submit your return on time to avoid unnecessary costs.
Here are the 2025 penalties stated on the HMRC website:
Filing your tax return late may result in the following penalties:
An initial fine of £100.
After 3 months, daily penalties of £10, capped at £900.
After 6 months, an additional charge of 5% of the tax owed or £300, whichever is higher.
Late tax payments incur a 5% penalty after 30 days, 6 months, and 12 months. You’ll also be charged interest on the amount owed.
You must pay within 30 days of the date on the penalty notice, or you may face further penalties.
Frequently Asked Questions
Can I amend my tax return after submitting it?
Yes, you have 12 months from 31 January to make corrections.
What expenses can I claim?
You can claim business-related expenses like office costs, travel, professional fees, phone plans, software subscriptions, and more. See allowable expenses.
Do I need to submit a return if I made a loss?
Yes, report your trading loss. You may be able to offset this against other income or carry it forward.
How do I pay?
Payment options include online banking, telephone banking, CHAPS, debit or corporate credit card online, and in-person at your bank.
Useful Resources
Final Thoughts
Filing Self Assessment doesn’t need to be stressful. Stay organised, keep up with deadlines, and don’t hesitate to seek expert advice for tricky situations. Timely and accurate submissions will help you avoid penalties and keep your focus on growing your business—not worrying about tax.
Good luck with your next tax return!
Some articles include affiliate links, which means we may earn a small commission if you make a purchase—at no extra cost to you. Rest assured, this does not influence our advice or recommendations in any way.
Comments