Filing a Self Assessment tax return is the annual obligation that most UK sole traders dread, even though the mechanics are straightforward once you know what HMRC wants. This guide takes you from registration through final declaration, with the dates, the supplementary pages, and the planning points that catch first-time filers out.
Who must file
You must register and file a Self Assessment return if any of the following apply:
- You earned more than £1,000 from self-employment in the tax year
- You are a partner in a partnership
- You receive untaxed property income above the property allowance
- You are a company director with untaxed income
- You owe Capital Gains Tax outside the Real Time CGT service
The tax year runs 6 April to 5 April. You must register by 5 October following the end of the tax year you started trading.
Key deadlines
| Event | Deadline |
|---|---|
| Tax year ends | 5 April |
| Register for Self Assessment (first year) | 5 October |
| Paper return | 31 October |
| Online return and balancing payment | 31 January |
| First payment on account | 31 January |
| Second payment on account | 31 July |
Miss the 31 January and HMRC issues an automatic £100 penalty. Daily £10 penalties kick in after three months, and tax-geared penalties apply after six and twelve months.
What you actually file
Your main return is the SA100. As a sole trader you also complete supplementary pages:
- SA103S if your circumstances meet the current short-page conditions
- SA103F when the full self-employment pages are required, including where turnover is at or above the VAT registration threshold
- SA105 for UK property income
- SA108 for capital gains
- SA106 for foreign income
The self-employment pages report turnover, allowable expenses, capital allowances, and net profit. HMRC then combines this with any other income to calculate income tax and Class 4 National Insurance, plus any voluntary Class 2 contributions.
Payments on account
If your bill exceeds £1,000 (and less than 80% of tax was collected at source), HMRC asks for payments on account: 50% of the previous year’s bill paid each January and July, with a balancing figure the following 31 January.
| Year | January payment | July payment |
|---|---|---|
| Year 1 | Full bill (no prior year) | Nothing |
| Year 2 | Year 1 balancing + 50% POA | 50% POA |
| Year 3 | Year 2 balancing + 50% POA | 50% POA |
The big shock is the 31 January of Year 2, when you pay last year’s full bill plus the first half of this year’s estimate. Budget for it.
Common mistakes
- Forgetting to register by 5 October and incurring failure-to-notify penalties
- Confusing the cash basis with the accruals basis (most small sole traders use cash)
- Missing the trading allowance (£1,000 tax-free) for very small side income
- Not claiming legitimate home office, mileage and software expenses
- Treating drawings as a deduction (they are not)
- Submitting the wrong number of self-employment pages when running multiple trades
Filing through MTD
Since 6 April 2026, sole traders and landlords with qualifying income over £50,000 have replaced the annual return with Making Tax Digital quarterly updates and a final declaration. The threshold falls to £30,000 from 6 April 2027 and £20,000 from 6 April 2028, so move into compatible software before your income crosses the line to avoid a rushed migration. See our preparing for MTD and Self Assessment pages, plus the official HMRC Self Assessment guidance. See pricing for sole-trader bookkeeping with receipts, invoices and reports in one place.