Self Assessment for Sole Traders: A Complete Guide

If you work for yourself in the UK, Self Assessment is how you report your profits to HMRC and pay your tax. For a first-time sole trader it can feel like a maze of forms, dates and unfamiliar terms. In practice it follows a clear sequence: register, keep records, work out your profit, file the return and pay the bill. This guide walks you through each stage. It is general information, not tax advice, and rates and thresholds change every tax year, so check the current figures on GOV.UK or with an accountant.

Employees have tax taken from their pay automatically through PAYE. Sole traders do not. Instead, after each tax year ends, you tell HMRC what you earned and what you spent on the business, HMRC calculates the tax and National Insurance due, and you pay it. That process is Self Assessment.

The same return also covers other income that is not taxed at source, such as some rental income or untaxed interest, but this guide focuses on the self-employment side.

Who needs to file a Self Assessment return

As a sole trader, you must register for Self Assessment if you earn more than £1,000 from self-employment in a tax year. That £1,000 test is about gross income, meaning what customers paid you before costs. If your gross trading income is £1,000 or less, the trading allowance gives you "full relief" and you may not need to tell HMRC, although there are exceptions.

You can start trading straight away, but you must register by 5 October after the end of the tax year in which you started. Registering late and not paying by 31 January can bring a "failure to notify" penalty. Once registered, HMRC expects a return every year until you tell them you have stopped trading.

The tax year and the Self Assessment cycle

The UK tax year runs from 6 April to 5 April. The current tax year is 6 April 2026 to 5 April 2027. Each return covers one complete tax year, and the key dates follow a yearly rhythm.

You do not have to wait until January. Filing in spring or summer gives you months to plan for the bill, and it means any mistakes can be fixed calmly.

Working out your profit

Tax is charged on profit: your business income minus allowable expenses. Getting this figure right is the heart of the return.

Business income

Include all income from your self-employment for the tax year: invoices paid, card and cash sales, platform payouts and any other business receipts. Cash basis is the default for sole traders, which means you count income when the money actually arrives and expenses when you actually pay them. You can opt for traditional accounting instead, for example if you hold a lot of stock or need accounts for finance.

Allowable expenses

You can deduct costs incurred wholly and exclusively for the business. Typical examples include office costs, business travel (not commuting to a regular place of work), stock, staff costs, financial costs, premises, marketing and training related to your current business. Clothing only counts if it is a uniform or protective wear, and entertaining clients is not allowable. For items with personal use, such as a phone, claim only the business share.

Shortcuts that can simplify things

• Trading allowance: instead of claiming actual expenses, you can deduct a flat £1,000. You cannot claim both. If your real expenses are higher than £1,000, claiming expenses is usually better.
• Simplified expenses: optional flat rates for working from home (if you work 25 hours or more a month there) and for vehicles (45p a mile for the first 10,000 business miles in a car or goods vehicle, 25p after that).
• Equipment: under the cash basis, most equipment purchases are simply claimed as expenses. Cars are an exception and use capital allowances.

How your tax is calculated

Once you enter your figures, HMRC's online service calculates your bill. It helps to understand what goes into it so the number does not come as a shock.

Income Tax

For 2026/27 in England, Wales and Northern Ireland, the Personal Allowance is £12,570. The basic rate of 20% applies on income from £12,571 to £50,270, the higher rate of 40% from £50,271 to £125,140 and the additional rate of 45% above that. The Personal Allowance falls by £1 for every £2 of adjusted net income over £100,000. Scotland has different bands, so Scottish taxpayers should check the Scottish rates.

National Insurance

Class 4 National Insurance is 6% on profits between £12,570 and £50,270 and 2% on profits above £50,270. If your profits are £7,105 or more, Class 2 is treated as paid, protecting your National Insurance record without a charge. With lower profits, you can pay voluntary Class 2 at £3.65 a week. Most self-employed people pay National Insurance through Self Assessment.

Other items

If you have a student loan, repayments on your self-employed income are calculated through your return and paid with the 31 January balancing payment. Pension contributions can also affect your bill: higher and additional rate taxpayers can claim extra relief on personal pension contributions through Self Assessment.

Payments on account: the first-year surprise

Payments on account are advance payments toward next year's tax bill. They catch many new sole traders out, because the first January bill can include both the full bill for last year and half of the next one.

• There are two instalments, each normally half of last year's bill (including Class 4 National Insurance), due 31 January and 31 July.
• They are not required if last year's bill was under £1,000, or if more than 80% of your tax was already paid at source, such as through PAYE.
• If you paid too little in advance, you settle the difference with a balancing payment on 31 January.
• If you expect lower profits, you can apply to reduce your payments on account.
• Student loan repayments and capital gains are paid with the balancing payment, not through payments on account.

How to file your return online

Most sole traders file online through their HMRC online account. The service guides you through the relevant pages and shows your calculation before you submit.

1. Gather your figures. Total income, expenses by category, any capital allowances and details of other income such as a salary, interest or rent.
2. Sign in to your HMRC online account. Use the sign-in details you set up when you registered, along with your Unique Taxpayer Reference.
3. Complete the self-employment section. Enter your income and expenses, or the trading allowance if you use it.
4. Add everything else. Include other taxable income, pension contributions and any reliefs that apply to you.
5. Check the calculation. Review the tax, National Insurance and any payments on account before you submit.
6. Submit and save your confirmation. Keep a copy of the return and the calculation; you may need them for a mortgage or loan application.
7. Pay by 31 January. Make sure the money reaches HMRC by the deadline, allowing time for your chosen payment method.

You can also use commercial software or an accountant. If you fall within Making Tax Digital for Income Tax, which applies from 6 April 2026 to sole traders and landlords with qualifying income over £50,000, you will need compatible software and will send quarterly updates during the year as well.

Penalties if you are late

Missing Self Assessment deadlines gets expensive quickly. For late filing, there is a £100 penalty straight away. After 3 months, £10 a day is added, up to £900. After 6 months, there is a further penalty of 5% of the tax due or £300, whichever is greater, and the same again after 12 months. Late payment brings penalties of 5% of the unpaid tax at 30 days, 6 months and 12 months, plus interest.

If you had a reasonable excuse, such as a serious illness, you can appeal. People who are within Making Tax Digital for Income Tax move to a points-based system for late submissions, which works differently.

Frequently asked questions

The bottom line

Self Assessment for sole traders comes down to a simple yearly routine: keep good records, work out your profit, file online well before 31 January and pay what you owe, including any payments on account. Start saving for tax from day one and the bill will never be a crisis.

Rates, bands and thresholds change every tax year, and Scotland sets its own Income Tax bands. Treat this as general information and check the latest guidance on GOV.UK or with an accountant if your situation is more complex.