How Do I Complete HMRC Self Assessment as a UK Sole Trader in 2026?
Filing your tax return might not be as creative as designing a brilliant mug or crafting the perfect slogan for a t-shirt, but it's just as important for keeping your business running smoothly. At ByDesignUK, we've helped thousands of UK customers navigate the entrepreneurial journey, and we know that managing your HMRC self assessment properly is crucial when you're running your own show as a sole trader in 2026.
Whether you're selling custom gifts, printed apparel, or running any other sole trader business, understanding how to complete your self assessment correctly saves you stress, money, and potential penalties. Let's walk through exactly what you need to know for the 2025/26 tax year.
What exactly is HMRC self assessment for sole traders?
HMRC self assessment is the system you use to report your income and pay tax as a sole trader. Unlike employees who have tax deducted automatically through PAYE, you're responsible for calculating and declaring your own earnings, expenses, and tax liability each year. The self assessment covers income from self-employment, untaxed interest, rental income, and certain other sources that don't have tax deducted at source.
When do I need to register and file my return in 2026?
You must register for self assessment by 5 October 2026 if you became a sole trader during the 2025/26 tax year (which ran from 6 April 2025 to 5 April 2026). Your deadline to file online is 31 January 2027 for the 2025/26 tax year, with the same date for paying any tax owed. Paper returns have an earlier deadline of 31 October 2026, though online filing gives you three extra months and is generally simpler.
What income and expenses should I include?
You need to declare all income from your sole trader activities, including sales, fees, and any other business revenue. For expenses, HMRC allows you to deduct legitimate business costs such as stock or materials, premises costs, marketing, professional fees, business travel, and equipment. Keep detailed records and receipts for everything. You can also claim simplified expenses for vehicles, working from home, and living on your business premises if applicable.
How do I actually complete the online self assessment form?
Log into your Government Gateway account and access the self assessment section. You'll complete the SA100 main form plus the SA103S or SA103F pages for self-employment. Enter your business income in the self-employment section, then list your allowable expenses by category. The system calculates your profit automatically, applies your personal allowance, and works out your tax bill including Class 2 and Class 4 National Insurance contributions. Double-check everything before submitting, as errors can trigger enquiries.
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What happens if I miss the deadline or make mistakes?
Late filing triggers automatic penalties starting at £100, with additional charges if you're more than three months late. Late payment incurs interest and further penalties. If you make an honest mistake, you can amend your return within 12 months of the filing deadline. Deliberate errors carry much heavier penalties, so accuracy matters. If you're genuinely unable to file on time due to exceptional circumstances, contact HMRC immediately to request an extension.
Frequently Asked Questions
No, your self assessment calculates both income tax and National Insurance contributions together. Class 2 NI applies if your profits exceed £12,570, and Class 4 NI is a percentage of profits between £12,570 and £50,270. You pay everything in one lump sum by 31 January.
Yes, you can use simplified expenses (a flat rate based on hours worked from home) or claim a proportion of actual costs like rent, utilities, and internet. The simplified method is £10-26 per month depending on hours. Keep records of your working pattern to justify your claim.
You still need to file a self assessment return if you're registered as self-employed and earned over £1,000 from self-employment. However, you won't owe income tax if your total income is below £12,570 (the 2026 personal allowance), though you may still owe Class 2 National Insurance.
If your tax bill exceeds £1,000, HMRC requires payments on account (advance payments toward the following year's tax). You'll pay 50% on 31 January and 50% on 31 July. This spreads the burden but requires careful budgeting throughout the year.
