More than 11.5 million people file a Self Assessment tax return in the UK each year, and anyone self-employed as a sole trader who earns more than £1,000 from their trade must be among them [1]. The return is how a self-employed person tells HM Revenue and Customs how much profit the business made, and how much Income Tax and National Insurance is due, because there is no employer to deduct tax at source through PAYE [2].
The process has a rhythm to it. A trader registers for Self Assessment, receives a Unique Taxpayer Reference, files the return after the tax year ends, and pays the bill by the deadline. Miss a step and the penalties mount quickly, starting at a fixed £100 even where no tax is owed [3]. From 6 April 2026, a further layer applies for higher-turnover traders, who must also keep digital records and send quarterly updates under Making Tax Digital for Income Tax [4].
This guide walks through the whole cycle: who has to file, how to register, which forms to complete, what goes on the return, the deadlines and payments on account, the penalty regime, and the shift to Making Tax Digital that is changing how sole traders report.
Key takeaways
- A sole trader must file a Self Assessment tax return if trading income exceeds £1,000 in the tax year [1].
- Registration is due by 5 October after the tax year ends, and the online return and payment are both due by 31 January [5].
- The self-employed pay Income Tax at 20%, 40% and 45% plus Class 4 National Insurance at 6% and 2%, all collected through the return [6].
- Late filing brings an immediate £100 penalty even when no tax is due, rising with daily charges after three months [3].
- Making Tax Digital for Income Tax applies from 6 April 2026 to those with qualifying income above £50,000, adding quarterly digital updates on top of the annual return [4].
Who has to file a self-employed tax return
A person must send a Self Assessment tax return if they were self-employed as a sole trader and earned more than £1,000 from that trade before deducting any expenses [1]. The £1,000 figure is the trading allowance, a tax-free threshold below which self-employment receipts do not need to be reported at all [7]. Anyone whose gross self-employment income sits at or below £1,000 is exempt from both the tax and the reporting requirement on that income [7].
The trading allowance can be used in two ways. A trader can either deduct actual business expenses from turnover in the usual way, or claim the flat £1,000 allowance instead, whichever leaves the lower taxable profit [7]. It cannot be both, and the allowance cannot create or increase a loss, so it is capped at the amount of income [7]. For a trader with £900 of expenses against £2,000 of turnover, claiming the £1,000 allowance leaves a lower profit than deducting the real costs, so the allowance wins [7].
Self-employment is not the only trigger for a return. A person may also need to file if they had other untaxed income, were a partner in a business partnership, or had to pay the High Income Child Benefit Charge, among other reasons [1]. Anyone unsure whether a return is required can use HMRC's online tool to check [1]. The difference between being taxed through the return and being taxed at source is explained in the Moonworkers guide to PAYE versus Self Assessment. Sole traders who also run a payroll for staff should keep the two duties separate, because employer PAYE obligations sit alongside, not inside, the personal return, and an HMRC-recognised payroll platform for sole traders handles the employment side while Self Assessment handles the profit.
Registering for Self Assessment
Before a return can be filed, a self-employed person has to register with HMRC. Registration is done online, and the deadline is 5 October following the end of the tax year in which the self-employment began [8]. A trader who started working for themselves in the 2026-27 tax year, which runs from 6 April 2026 to 5 April 2027, must register by 5 October 2027 [8].
Registration does two things at once. It sets up the Self Assessment record and it registers the person for Class 2 and Class 4 National Insurance, because the self-employed pay both through the same return [8]. After registering, HMRC issues a Unique Taxpayer Reference, a ten-digit number that identifies the taxpayer on every return and payment, followed by an activation code for the online account [9]. Because the code arrives by post and can take a week or more, registering well before the 5 October deadline avoids a last-minute scramble [9].
Registering late has a cost. A trader who misses the 5 October deadline and fails to pay the resulting tax bill on time can face a failure to notify penalty, calculated as a percentage of the tax owed [3]. Registering promptly, even before the first return is due, removes that risk entirely [8].
The forms: SA100 and SA103
A self-employed tax return is built from a core form plus supplementary pages. The main return is the SA100, which captures personal details and pulls together income from every source [10]. Self-employment income is reported on the SA103 supplementary pages, which come in two versions [10].
The table below sets out which self-employment form applies.
| Form | When to use it | What it covers |
|---|---|---|
| SA103S (short) | Turnover below the VAT threshold and simple accounts | Turnover, allowable expenses, net profit [[10]](https://www.gov.uk/self-assessment-tax-return-forms) |
| SA103F (full) | Higher turnover or more complex accounts | Detailed expense categories, capital allowances, adjustments [[10]](https://www.gov.uk/self-assessment-tax-return-forms) |
| SA100 (main return) | Every Self Assessment taxpayer | Personal details, all income, tax already paid [[10]](https://www.gov.uk/self-assessment-tax-return-forms) |
Most people file online rather than on paper, and the online service assembles the right pages automatically based on the answers given, so a trader does not have to choose between the short and full self-employment pages manually [11]. Filing online also extends the deadline from 31 October to 31 January and calculates the tax bill instantly [5].
What goes on the return
The self-employment pages ask for the business turnover, the allowable expenses, and the resulting net profit, which is the figure both Income Tax and National Insurance are charged on [11]. Allowable expenses are costs incurred wholly and exclusively for the business, such as stock, tools, business travel, a proportion of home running costs where the business is run from home, and professional fees [12]. Personal spending cannot be deducted, and getting the split right on mixed costs, such as a phone used for both business and personal calls, is a common area of error [12].
Good record-keeping underpins an accurate return. HMRC expects a self-employed person to keep records of all sales and income, all business expenses, and any personal income, and to retain them for at least five years after the 31 January filing deadline [11]. A trader who also employs staff will already keep detailed pay records, and running that side through payroll software for small businesses produces the wage figures that feed the expenses section of the return.
Income Tax and National Insurance on the return
The return calculates two taxes on the same profit figure: Income Tax and National Insurance, and the Moonworkers guide to how much tax a self-employed person pays works through the combined figures at several income levels. Income Tax follows the same bands as employment income, with the £12,570 Personal Allowance tax-free, 20% up to £50,270, 40% up to £125,140 and 45% above that, for the 2026-27 tax year [6]. The Personal Allowance itself tapers away above £100,000 of income, falling by £1 for every £2 over that figure [6].
National Insurance for the self-employed comes in two classes, both handled through the return. The figures for the 2026-27 tax year are set out below.
| Class | Applies to | Rate 2026-27 |
|---|---|---|
| Class 4 | Profits between £12,570 and £50,270 | 6% [[13]](https://www.gov.uk/self-employed-national-insurance-rates) |
| Class 4 | Profits above £50,270 | 2% [[13]](https://www.gov.uk/self-employed-national-insurance-rates) |
| Class 2 (voluntary) | Profits below the £7,105 Small Profits Threshold | £3.65 per week if paid voluntarily [[13]](https://www.gov.uk/self-employed-national-insurance-rates) |
Class 2 National Insurance changed structurally in recent years. A self-employed person with profits at or above the Small Profits Threshold of £7,105 is treated as having paid Class 2, and so builds up entitlement to the State Pension, without any charge falling due [13]. Someone whose profits fall below that threshold pays no Class 2 automatically, but can choose to pay it voluntarily at £3.65 a week to protect their contributions record [13]. Both Class 2, where paid, and Class 4 are collected through Self Assessment alongside the Income Tax [13].
Deadlines and payments on account
The Self Assessment calendar has several fixed dates, and missing any of them triggers a penalty. The key deadlines for a tax year that ends on 5 April are summarised below.
| Deadline | Date | What is due |
|---|---|---|
| Register for Self Assessment | 5 October | Tell HMRC a return is needed [[5]](https://www.gov.uk/self-assessment-tax-returns/deadlines) |
| Paper tax return | 31 October | File on paper if not filing online [[5]](https://www.gov.uk/self-assessment-tax-returns/deadlines) |
| Return through tax code | 30 December | File online to have tax collected via PAYE code [[5]](https://www.gov.uk/self-assessment-tax-returns/deadlines) |
| Online tax return | 31 January | File the return online [[5]](https://www.gov.uk/self-assessment-tax-returns/deadlines) |
| Pay the tax owed | 31 January | Settle the balancing payment and any first payment on account [[5]](https://www.gov.uk/self-assessment-tax-returns/deadlines) |
| Second payment on account | 31 July | Pay the second instalment towards next year's bill [[5]](https://www.gov.uk/self-assessment-tax-returns/deadlines) |
Payments on account catch many first-time filers by surprise. Where a tax bill is more than £1,000 and less than 80% of the tax was collected at source, HMRC requires two advance payments towards the following year's liability, each equal to half the current year's bill [14]. The first is due on 31 January alongside the balancing payment for the year just ended, and the second on 31 July [14]. This means a trader filing a first return can face a bill of one and a half times the tax actually due for the year, because the January payment covers the past year in full plus the first instalment of the next [14].
Anyone wanting to know the size of the bill before filing can use HMRC's estimate tool, which helps a trader budget for the January and July dates [15]. Setting money aside through the year, rather than finding it in January, is the single practical habit that keeps a self-employed person out of the penalty regime [14].
Penalties for filing or paying late
The penalty structure is deliberately steep, and it starts the moment the deadline passes. A return filed even a day late attracts an immediate £100 fixed penalty, and that penalty applies whether or not any tax is due, and even where the tax owed has already been paid [3]. The charges escalate the longer the return is outstanding.
The late filing penalties build up as follows.
| How late | Penalty |
|---|---|
| 1 day | £100 fixed penalty [[3]](https://www.gov.uk/self-assessment-tax-returns/penalties) |
| 3 months | £10 per day, up to a maximum of £900 [[3]](https://www.gov.uk/self-assessment-tax-returns/penalties) |
| 6 months | A further 5% of the tax due or £300, whichever is greater [[3]](https://www.gov.uk/self-assessment-tax-returns/penalties) |
| 12 months | Another 5% or £300, whichever is greater [[3]](https://www.gov.uk/self-assessment-tax-returns/penalties) |
Paying late is penalised separately from filing late. On top of interest charged on the outstanding amount, a trader who has not paid faces a 5% surcharge on the tax still unpaid at 30 days, again at 6 months, and again at 12 months [3]. A left-unfiled return can therefore rack up well over £1,000 in penalties before the tax itself is even counted [3]. HMRC does cancel penalties where a trader has a reasonable excuse, but the bar is high and the safest course is to file and pay on time [3].
Making Tax Digital for Income Tax
The biggest change to how self-employed people report their income is Making Tax Digital for Income Tax, which begins on 6 April 2026. From that date, sole traders and landlords with qualifying income above £50,000 must keep digital records and send quarterly updates to HMRC using compatible software [4]. Around 780,000 people are expected to join the service in its first year [16].
The rollout is staged by income level. Those with qualifying income above £50,000 are in from 6 April 2026, and those above £30,000 follow from 6 April 2027 [4]. The first quarterly update period runs from 6 April 2026 to 5 July 2026, with a submission deadline of 7 August 2026 for all affected customers [16]. Each update is a short summary of income and expenses sent through recognised software, and it takes minutes rather than replacing the full return [16].
The annual cycle does not disappear under Making Tax Digital. The tax return deadline remains 31 January, and the tax is still due by that date, so the quarterly updates sit on top of the existing obligations rather than removing them [16]. The direction of travel is unmistakable: reporting is moving towards frequent, software-driven submissions, the same shift that Real Time Information brought to payroll years ago. Sole traders who already file payroll in real time through an HMRC-recognised payroll engine will recognise the pattern, and the wider context sits in the Moonworkers guide to paying tax when self-employed.
Conclusion
Filing a self-employed tax return is a sequence of predictable steps rather than a single event: register by October, keep clean records through the year, file the SA100 and SA103 online by 31 January, and pay the bill, including any payments on account, on the same date. The penalty regime rewards early action and punishes drift, and the £100 that lands the moment a deadline slips is the easiest charge in the tax system to avoid.
What is changing is the frequency of reporting. Making Tax Digital for Income Tax pulls higher-turnover sole traders into quarterly updates from 6 April 2026, mirroring the real-time reporting that payroll adopted long ago. A trader who treats bookkeeping as a running task rather than a January panic will find the new regime far less daunting, and will keep more control over a tax bill that, handled well, holds no surprises.
FAQs
Do I have to file a tax return if I made a loss or earned very little?
A sole trader must file a Self Assessment return if trading income exceeded £1,000 in the tax year, even if the business made a loss overall [1]. Filing a loss can be worthwhile, because losses can often be set against other income or carried forward to reduce future tax [11]. Where gross self-employment income was £1,000 or less, the trading allowance means no return is needed for that income [7].
When do I need to register for Self Assessment?
Registration is due by 5 October following the end of the tax year in which the self-employment began [8]. A person who started trading in the 2026-27 tax year must register by 5 October 2027 [8]. Registering triggers the issue of a Unique Taxpayer Reference and an activation code by post, so allowing a week or more before the deadline is sensible [9].
How much National Insurance does a self-employed person pay?
For the 2026-27 tax year, the self-employed pay Class 4 National Insurance at 6% on profits between £12,570 and £50,270, and 2% on profits above £50,270 [13]. Class 2 is treated as paid automatically where profits reach the £7,105 Small Profits Threshold, with no charge due, while those below can pay it voluntarily at £3.65 a week to protect their State Pension record [13]. Both are collected through the annual return [13].
What is the penalty for filing my tax return late?
A return filed even one day after the deadline attracts a fixed £100 penalty, which applies regardless of whether any tax is owed [3]. After three months, daily penalties of £10 accrue up to a maximum of £900, and at six and twelve months a further 5% of the tax due or £300, whichever is greater, is added each time [3]. Paying the tax late brings separate 5% surcharges at 30 days, 6 months and 12 months, plus interest on the amount outstanding [3].
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