Sole trader Self Assessment: a complete beginner's guide
Self Assessment is HMRC's system for collecting tax from people whose income is not fully taxed before they receive it. For sole traders, it is the annual return that turns business records into a tax bill.
What Self Assessment means for a sole trader
As a sole trader, you are the business. You do not file company accounts or pay Corporation Tax. Instead, you report your self-employed profit on a personal Self Assessment tax return. HMRC then calculates Income Tax and National Insurance, along with any other parts of your return such as PAYE income, savings interest, student loans, or pension relief.
Who needs to file
You normally need Self Assessment if you trade as self-employed and need to report profit to HMRC. You may also need it if you have untaxed income from property, dividends, foreign income, or other sources. This guide focuses on the sole trader path.
If you have not filed before, read our HMRC sole trader registration guide first. You need to register before you can submit the return online.
What information you need
- Your Unique Taxpayer Reference, or UTR.
- Your Government Gateway login.
- Total business income for the tax year.
- Allowable business expenses, grouped clearly.
- Details of any PAYE income, pension contributions, student loans, or other income.
- Bank interest, dividends, property income, or other taxable amounts if they apply.
Turnover, expenses, and profit
The return starts with income and expenses, but tax is based on profit. Turnover is what clients paid you. Allowable expenses are business costs you can justify. Profit is what is left after those costs. If your records are messy, do not start with the HMRC form; start by cleaning the profit number.
How to submit online
Most sole traders file online through HMRC or through compatible accounting software. The online return walks through personal details, business income, expenses, tax adjustments, and final calculation. Before pressing submit, compare the result with an independent estimate so obvious mistakes stand out.
Use the self-employed tax calculator — add PAYE or other taxable income in the optional second field when you need combined banding.
When to file and pay
The online filing and payment deadline is normally 31 January after the tax year ends. For 2025/26, the tax year ends on 5 April 2026 and the online filing/payment deadline is 31 January 2027. If payments on account apply, a second instalment is due on 31 July.
What happens after submission
HMRC gives you a calculation and payment reference. Pay by the deadline using an approved method and keep confirmation. Save a copy of the submitted return, the calculation, and the records behind it. If you later discover an error, you can usually amend a return within HMRC time limits.
Beginner mistakes to avoid
- Entering turnover where the form expects profit or expenses.
- Forgetting PAYE income, which can push self-employed profit into a higher band.
- Ignoring payments on account until the January bill is larger than expected.
- Claiming expenses without evidence.
- Waiting until deadline week to find missing logins or bank records.
Next steps
Read the 2025/26 rates and thresholds guide to understand the calculation, then use the HMRC dates calendar to plan ahead.
Official source
HMRC's live filing guidance is on GOV.UK: Self Assessment tax returns.