Self Assessment for online sellers
If you sell as a sole trader and your sales went over £1,000 in a tax year (after refunds, before costs), you'll need a Self Assessment tax return. Here are the dates, what goes on it, and the mistakes that cost sellers money.
Checked against GOV.UK on 12 October 2026 · About a 7-minute read
1. The key dates
Not sure if you're trading at all? Start with our hobby or business guide.
| Date | What's due |
|---|---|
| 5 October | Register for Self Assessment, if this is your first year over £1,000 |
| 31 October | Paper returns (most people file online instead) |
| 31 January | Online return filed, tax owed paid, and first payment on account if you make them |
| 31 July | Second payment on account, if you make them |
For the tax year ending 5 April 2026, the online deadline is 31 January 2027.
2. What goes on the return
- Your total sales across all platforms, after refunds and before fees.
- Either your allowable expenses or the £1,000 trading allowance, but not both. Our hobby or business guide explains how to choose.
- Your other income, such as a salary, so the right rate applies.
3. Using your platform's annual report
The report your platform sends you (see our platform reporting guide) is a useful check, but don't copy its total straight onto your return. It covers January to December, not the tax year, and it's after the platform's fees. Use your own sales records for 6 April to 5 April, then compare them with the report.
4. Expenses online sellers often miss
- stock and materials
- postage and packaging
- platform, listing and payment fees
- advertising, such as Meta, Google or promoted listings
- website, app and software subscriptions
Keep receipts or exports for everything you claim.
5. National Insurance
If your profit is over £12,570, you pay Class 4 National Insurance: 6% up to £50,270 and 2% above. It's calculated on the return and paid with your tax.
The old flat Class 2 contribution is no longer compulsory. If your profit is over £12,570, your State Pension year is credited automatically. If it's under £7,105, you can choose to pay Class 2 voluntarily (£3.65 a week in 2026/27) so the year still counts towards your State Pension.
6. Payments on account
If your tax bill is over £1,000, HMRC usually asks for advance payments towards next year's bill, half in January and half in July. The exception is if you already pay most of your tax (80% or more) through PAYE.
Many sellers are caught out in their second year, when the January bill includes both the balance for last year and the first advance payment for this one.
Source: GOV.UK: Payments on account
7. Missing the deadline
For a normal Self Assessment return:
| How late | Penalty |
|---|---|
| 1 day | £100, even if you owe no tax |
| 3 months | £10 a day, up to £900 |
| 6 months | £300 or 5% of the tax due, whichever is higher |
| 12 months | Another £300 or 5%, whichever is higher |
| Paying late | Interest, plus 5% of the unpaid tax at 30 days, 6 months and 12 months |
If you're already in Making Tax Digital, different penalty rules apply. See our Making Tax Digital guide.
8. Getting close to Making Tax Digital?
Sole traders and landlords whose combined income from self-employment and property is over £50,000 joined Making Tax Digital from April 2026. The line drops to £30,000 from April 2027 and £20,000 from April 2028. Like the trading allowance, the test uses sales, not profit. Our Making Tax Digital guide explains what changes.
9. Getting help
If your affairs are simple, you can file yourself on GOV.UK. If you sell across several platforms, are near the VAT threshold or run a limited company, an accountant usually pays for themselves.
This guide explains published HMRC rules as they stood on 12 October 2026. It isn't personal tax advice. If your situation is unusual, speak to an accountant. Last checked: 12 October 2026.