What SA103 is
The main Self Assessment return is the SA100. Extra 'supplementary pages' cover specific income types. SA103 covers self-employment as a sole trader. Online, you don't fill in separate forms — the questions appear when you say you were self-employed — but the structure is the same.
Short (SA103S) or full (SA103F)?
| Use SA103S (short) if… | Use SA103F (full) if… |
|---|---|
| Your turnover was below the VAT registration threshold (£90,000) | Turnover was above the VAT threshold |
| Your business is straightforward | You're changing accounting dates or have adjustments to make |
| You don't need to make adjustments for previous years | You're claiming certain reliefs or have more complex capital allowances |
Most sole traders and side-hustlers use the short pages.
What goes in each section
- Business details: description, postcode, start or end date if you started or stopped trading in the year.
- Income: your turnover (total sales before expenses), plus any other business income.
- Expenses: either total allowable expenses, or itemised by category (cost of goods, travel, premises, admin, staff and so on). Or the £1,000 trading allowance instead.
- Capital allowances: for equipment and vehicles, unless you use cash basis and claim them as expenses.
- Net profit or loss: worked out from the figures above.
- Tax deducted: for example CIS deductions if you're a subcontractor — see CIS refunds.
- Class 4 National Insurance: calculated on your profits; some people are exempt.
Box numbers change
HMRC renumbers boxes between tax years. Make sure you're using the form and notes for the correct year (2025/26 for returns due 31 January 2027).Figures to have ready
- Total sales / turnover for 6 April 2025 – 5 April 2026 (or your accounting period).
- Total expenses by category — see the allowable expenses checklist.
- Equipment or vehicles bought.
- Any CIS deductions (from your monthly CIS statements).
Doing it with TaxGo
You don't need to learn box numbers. Upload your figures and supporting documents — invoices, a spreadsheet, CIS statements — and TaxGo maps them to the right sections, asks only what's missing, and shows every figure with its source before you approve. See our self-employed page.
Common SA103 mistakes
- Entering profit as turnover. Turnover is all your sales before any costs; profit is what's left after expenses.
- Using the wrong period. Most sole traders now report on the tax year (6 April to 5 April) under the basis period rules that took full effect from 2024/25.
- Double-claiming. Claiming the £1,000 trading allowance and expenses, or simplified mileage and actual fuel costs.
- Forgetting CIS deductions. If you're a subcontractor, the tax already deducted reduces your bill — leave it off and you overpay.
- Missing capital allowances on tools, equipment or a van bought for the business.
If you've made a mistake on a return you've already sent, you can usually amend it online within 12 months of the 31 January deadline.