A Step-by-Step Guide to Filing Self Assessment as a Sole Trader (From a Tax Expert)
Filing a Self Assessment tax return can feel daunting the first time, especially if you’re running your own business. However, once you understand the structure, it becomes a straightforward annual process rather than a stressful event.
As a tax professional would advise clients: the key is preparation, not panic.
Below is a clear, practical walkthrough of how sole traders in the UK should approach their Self Assessment tax return.
Table of contents: tap to view sections
- 1. Understand whether you are required to file
- 2. Register with HMRC (if you are new)
- 3. Keep accurate and organised records
- 4. Calculate your taxable profit
- 5. Complete your Self Assessment tax return online
- 6. Review your tax calculation carefully
- 7. Submit your return before the deadline
- 8. Pay any tax due
- 9. Retain your records for compliance
- Final thoughts from a tax perspective
1. Understand whether you are required to file
If you are trading as a sole trader and your self-employment income exceeds £1,000 in a tax year, you are generally required to register for Self Assessment and file a return.
The UK tax year runs from 6 April to 5 April, and your filing obligation is assessed based on that period.
Even if tax has been paid elsewhere, you may still need to report income if it is untaxed.
2. Register with HMRC (if you are new)
Before you can file, you must register as self-employed. This is done through HMRC, after which you will receive a Unique Taxpayer Reference (UTR).
You will also set up your Government Gateway account, which becomes your portal for all future submissions.
A common mistake here is delaying registration—this can result in penalties if deadlines are missed later.
3. Keep accurate and organised records
Good record-keeping is the foundation of a stress-free tax return. At minimum, you should maintain:
- All business income records (invoices, sales logs, bank statements)
- Receipts for allowable business expenses
- Records of mileage and travel (if applicable)
- Details of any other taxable income
- Pension contributions or relevant adjustments
From a compliance perspective, HMRC requires records to be kept for at least five years after the 31 January submission deadline.
4. Calculate your taxable profit
Your tax liability is based on profit, not turnover:
Total income – Allowable business expenses = Taxable profit
Allowable expenses must be incurred “wholly and exclusively” for business purposes. This includes costs such as:
- Office supplies and software
- Business phone usage
- Professional services (e.g., accountants, legal advice)
- Travel costs incurred for work
A disciplined approach to expense categorisation can significantly reduce errors and overpayments.
5. Complete your Self Assessment tax return online
This filing walkthrough should be read alongside our guide to preparing your own tax return, which focuses on organising records and deciding what help you need. For tax years within MTD, see our complete MTD guide: the annual return is completed through compatible software.
You will file using the SA100 main return, alongside the self-employment supplementary pages (SA103).
Within the return, you will be asked to provide:
- Personal details
- Business income and expenses
- Adjustments (if any)
- Additional income sources
The online system will guide you through each section and automatically calculate your liability.
6. Review your tax calculation carefully
Once completed, HMRC will generate an automatic calculation of:
- Income Tax owed
- National Insurance contributions (Class 2 and Class 4 where applicable)
This is a critical stage where errors in input will directly affect your tax bill, so review figures carefully before submission.
7. Submit your return before the deadline
The key deadlines are:
- 31 October – paper filing (rarely used now)
- 31 January – online filing and payment deadline
Late submissions trigger automatic penalties, starting at £100, even if no tax is owed.
8. Pay any tax due
Your tax bill is typically due by 31 January, and in some cases, you may also need to make payments on account for the following year.
These advance payments often surprise new sole traders, so it is important to budget for them early.
9. Retain your records for compliance
Once submitted, do not discard your documentation. HMRC may enquire into your return, and you must be able to evidence:
- Income received
- Expenses claimed
- Supporting invoices and receipts
A well-organised digital folder system is usually sufficient for most sole traders.
Final thoughts from a tax perspective
Self Assessment is less about complexity and more about consistency. Sole traders who maintain clean records throughout the year rarely struggle when January arrives.
The most successful approach is simple:
- Record everything as you go
- Separate business and personal finances
- Treat tax as a monthly consideration, not an annual shock
With the right habits in place, Self Assessment becomes a routine administrative task rather than a source of stress.