Completing your first self-assessment tax return as a UK sole trader

Completing your first self-assessment tax return as a UK sole trader can feel daunting, but following a clear process makes it manageable. Register with HMRC as soon as you start earning from self-employment, then submit your return each year by the relevant deadline.

This guide explains the practical steps, what counts as allowable expenses, how to keep records, key deadlines and mistakes to avoid so you can file accurately and on time.

Understanding self-assessment as a sole trader

As a sole trader you are responsible for reporting your business income and expenses to HMRC through the self-assessment system. This applies whether you run a full-time business or a side hustle that has grown. You must register for self-assessment if your self-employed income exceeds the threshold that changes each year; always check the current rules on GOV.UK.

The process involves completing an online tax return that calculates your taxable profit after allowable expenses. Submitting on time avoids penalties, and good preparation makes the task straightforward even the first time.

Key deadlines you must meet

The tax year runs from 6 April to 5 April. Paper returns must reach HMRC by 31 October following the end of the tax year, while online submissions are due by 31 January. These dates can be affected by bank holidays and are subject to change, so always verify the exact deadline on GOV.UK before planning your submission.

If you miss the deadline you may face an automatic penalty. Late payment of any tax due also attracts interest and further charges. Set calendar reminders at least one month before the deadline and aim to complete your return in November or December to allow time for corrections.

Register for self-assessment as soon as you know you need to; HMRC usually sends you a Unique Taxpayer Reference within 10 working days of registration.

What records you need to keep

Accurate record keeping forms the foundation of a correct tax return. You must retain evidence of all income and expenses for at least five years from the 31 January submission deadline. Digital records are acceptable provided they are complete and accessible.

Sales invoices and receipts showing income received
Bank and credit card statements
Expense receipts and invoices, clearly marked with the business purpose
Mileage logs if claiming vehicle costs
Records of any private use of business items

Use accounting software or a simple spreadsheet to categorise transactions monthly. Review your records quarterly so that nothing is missed when you reach the filing deadline. This habit also helps you understand how your business is performing.

Allowable expenses: what you can claim

Allowable expenses are costs that are wholly and exclusively for your business. Common examples include office supplies, website hosting, marketing costs, accountancy fees and travel expenses directly linked to earning income. The exact list and limits change, so always check the latest guidance on GOV.UK or contact HMRC for clarification.

Some costs are partly private and partly business; you must apportion them reasonably. For example, if you use your home office for business, you can claim a proportion of heating, electricity and internet based on the space and time used for work. Keep detailed calculations to support your figures.

Do not claim personal expenses or costs that have a significant private element. Capital items such as equipment are usually treated differently through capital allowances rather than as immediate expenses; again, refer to current HMRC rules for the method that applies to your situation.

Step-by-step process to complete your return

Log in to your HMRC personal tax account or use compatible commercial software.
Enter your Unique Taxpayer Reference and the relevant tax year.
Complete the self-employment pages: report total turnover, then deduct allowable expenses to reach your profit.
Include any other income such as employment or savings interest in the appropriate sections.
Claim any available reliefs or allowances that apply to your circumstances.
Review the summary for accuracy, then submit before the deadline.
Keep a copy of the submitted return and the calculation.

Start early so you can gather missing documents. If your records are well organised the data entry takes far less time. Double-check figures against your bank statements and receipts rather than relying on memory.

Common mistakes to avoid

Many first-time sole traders make avoidable errors. Claiming non-business costs is one of the most frequent; always ask whether the expense would have been incurred if you were not trading. Mixing personal and business bank accounts often leads to confusion when separating transactions.

Failing to keep records for the required period can cause problems during any HMRC enquiry. Under-reporting income, even unintentionally, may result in penalties and interest. Over-claiming home office or vehicle expenses without proper apportionment is another common pitfall.

Leaving the return until the last few days increases stress and the chance of mistakes. Submitting without reconciling your figures to bank records often leads to errors that need correction later. If you realise you have made a mistake after submission, amend the return online as soon as possible within the allowed time frame.

Getting support and next steps

If your affairs are straightforward, completing the return yourself is realistic with the help of HMRC’s online guidance and tools. For more complex situations, consider speaking to an accountant who can explain your specific position without taking over the filing unless you choose.

Build good habits from day one: separate business finances, record every transaction promptly and review your records regularly. These practices reduce the workload at tax time and give you clearer insight into your business performance.

Organisations such as Loomworks in West Yorkshire can provide mentoring and practical support to early-stage founders navigating their first year of trading. Use free resources on GOV.UK and HMRC’s self-assessment helpline to fill any gaps in your knowledge.

Frequently asked questions

When do I need to register for self-assessment as a sole trader?

You should register with HMRC for self-assessment as soon as you start earning from self-employment and expect to exceed the current income threshold. Registration is required by 5 October following the end of the tax year in which you became self-employed. Always check the exact requirements on GOV.UK.

What is the deadline for my first self-assessment tax return?

For online filing the usual deadline is 31 January following the end of the tax year. Paper returns must be filed by 31 October. These dates can vary slightly with bank holidays, so confirm the precise deadline for your tax year on the HMRC website.

Can I claim my home office costs as a sole trader?

Yes, if you use part of your home exclusively for business you can claim a proportion of running costs such as heating, electricity and council tax. You must use a reasonable method to divide the costs and keep records showing how you calculated the business portion. Check current HMRC guidance for acceptable methods.

What happens if I miss the self-assessment deadline?

Missing the deadline usually triggers an automatic penalty. Further penalties and interest can apply if tax remains unpaid. You can appeal against a penalty in certain circumstances, but it is far better to file on time. Set reminders well in advance.

Do I need an accountant for my first tax return?

Not necessarily. If your income and expenses are simple you can complete the return yourself using HMRC’s online service. An accountant can still be useful for advice on allowable expenses and record keeping, especially if your business involves stock, capital equipment or international sales.