If you earn income from self-employment — whether as a sole trader, freelancer, contractor, or someone running a side business alongside a regular job — HMRC requires you to report that income through Self Assessment. Unlike employment, where your employer deducts tax at source via PAYE, self-employed income arrives untaxed. The Self Assessment tax return is how you account for it.

This self employed tax return guide is aimed at anyone new to filing, anyone who has been doing it themselves and wants to make sure they’re doing it right, and anyone who has been putting it off and needs a clear starting point. It covers registration, the filing process, what counts as an allowable expense, how payments work, and the mistakes that tend to cost people money.

UK tax rules change regularly. The figures and deadlines in this guide are current as of May 2026 and reflect the 2025-26 tax year. Where rules are particularly nuanced, we’ve flagged it — because “it depends” is sometimes the honest answer, and a good guide should tell you when that’s the case.

Not everyone who is self-employed automatically needs to file. HMRC uses a set of criteria, and you are required to complete a Self Assessment tax return if any of the following apply to you:

• Your self-employment income exceeded £1,000 in the tax year (the trading allowance threshold)
• You earned more than £100,000 from any source
• You received untaxed income of more than £2,500 from other sources, such as rental income or tips
• You are a company director
• You or your partner claimed Child Benefit and either of you earned over £60,000
• You have capital gains to report
• You made pension contributions over the annual allowance

The £1,000 trading allowance is worth noting. If your self-employment income was genuinely below £1,000 across the full tax year, you are not required to register or file — though you can still choose to, for example to claim a loss or build a National Insurance record.

What if your self-employment is a side income?

Many people who file Self Assessment are employed full-time and run a side business or take on freelance work on top. Both income streams need to be declared. HMRC cross-references PAYE records with Self Assessment returns, so omitting employment income — even if it was already taxed — can trigger queries.

Partnerships

If you are trading in partnership rather than as a sole trader, each partner files their own individual Self Assessment return covering their share of the profits. The partnership itself also files a Partnership Tax Return (SA800), which is separate from the individual returns.

Before you can file, you need to be registered. If this is your first time, the process is straightforward — but the deadline for registering matters.

The registration deadline

You must tell HMRC that you need to complete a Self Assessment return by 5 October following the end of the tax year in which you became self-employed. The UK tax year runs from 6 April to 5 April, so if you started self-employment during the 2025-26 tax year (6 April 2025 to 5 April 2026), you needed to register by 5 October 2026. Missing this date can result in a penalty, though HMRC does sometimes exercise discretion for first-time registrations.

How to register

You register online via HMRC’s website at gov.uk/register-for-self-assessment. For most sole traders, this means registering as self-employed, which also handles your Class 2 National Insurance contributions (though Class 2 NIC was abolished from April 2024, so only Class 4 applies for 2024-25 onwards).

Once registered, HMRC will issue you a Unique Taxpayer Reference (UTR) — a 10-digit number you’ll use on every return going forward. Allow up to 10 working days to receive it by post.

Already registered but haven’t filed in a while?

If you were registered in a previous year and didn’t file, your record may still be active. Check your Personal Tax Account on HMRC’s website before re-registering, as doing so twice can create duplicate records and unnecessary complications.

Missing a Self Assessment deadline costs money, and the penalties can stack up quickly. Here are the dates you need to have in your diary for the 2025-26 tax year (6 April 2025 to 5 April 2026).

What happens if you miss the January deadline?

An automatic £100 penalty applies the day after the deadline, even if you owe no tax at all. After three months, daily penalties of £10 per day (up to 90 days) begin to accumulate. At six months, a further 5% or £300 penalty (whichever is higher) applies. The longer you leave it, the more it costs — and HMRC does not tend to waive penalties without a genuinely reasonable excuse.

Payment on account explained

If your tax bill exceeds £1,000 and less than 80% of your income was taxed at source, HMRC requires you to make payments on account. These are advance payments towards next year’s bill, each equal to 50% of the current year’s liability. The first payment on account is due on 31 January alongside your main bill; the second falls on 31 July. First-year filers are often caught out by this — they budget for the tax they owe, only to find the January bill is effectively one-and-a-half times that amount.

Your Self Assessment return requires you to declare all income, not just self-employment earnings. This includes employment income, interest on savings, rental income, dividends, and any other untaxed receipts. HMRC’s systems increasingly cross-reference data from banks, letting agents, and employers, so omissions are more likely to be picked up than they used to be.

Calculating self-employment profit

Tax is charged on your profit, not your turnover. Profit is your total business income minus your allowable business expenses. Most sole traders use one of two accounting methods:

• Cash basis: You record income when it is received and expenses when they are paid. This is simpler and is the default for most sole traders and partnerships without corporate partners.
• Traditional (accruals) basis: Income and expenses are recorded when they are earned or incurred, regardless of when money changes hands. More complex, but sometimes necessary for larger businesses or those with significant stock.

Common allowable expenses for sole traders

Expenses must be incurred “wholly and exclusively” for business purposes. Common allowable categories include:

• Office costs — stationery, postage, software subscriptions
• Travel — fuel, train fares, parking (commuting to a permanent workplace does not qualify)
• Equipment and tools used for the business
• Marketing and advertising costs
• Professional fees — accountancy, legal advice
• A proportion of home costs if you work from home (calculated by area or using HMRC’s simplified flat rates)
• Business insurance

One important point: you do not send receipts or invoices to HMRC with your return. You simply enter the totals. But you are legally required to keep the underlying records for at least five years after the January filing deadline, in case HMRC opens an enquiry.

Once you know your taxable profit, the rates below determine how much you owe. These are the rates in force for the 2025-26 tax year.

Income tax

Note: the personal allowance tapers to zero for incomes above £100,000 at a rate of £1 reduction for every £2 earned over that threshold.

National Insurance for the self-employed

Class 2 NIC was abolished from April 2024. Self-employed individuals now pay only Class 4 NIC, calculated as follows for 2025-26:

• 6% on profits between £12,570 and £50,270
• 2% on profits above £50,270

Class 4 NIC is calculated automatically when you complete your Self Assessment return — it’s not a separate calculation you need to do yourself. However, if you want to maintain your state pension entitlement and your profits are below the lower profits limit, you may wish to pay voluntary Class 3 contributions. This is worth checking if your profits are consistently low.

The High Income Child Benefit Charge

If you or your partner received Child Benefit and either of you earned over £60,000 in 2025-26, you are required to declare this via Self Assessment and will face a clawback of some or all of the benefit received. The charge tapers in between £60,000 and £80,000.

Good record keeping is not just an HMRC requirement — it is the thing that makes your tax return accurate and makes the whole process significantly less painful. Many of the stresses people associate with filing a tax return trace back to poor records, not the filing process itself.

What records you need

For a self-employed sole trader, the minimum you should be keeping throughout the year includes:

• Sales records — invoices issued, income received, bank deposits
• Purchase records — receipts, invoices from suppliers, expenses paid
• Bank statements for any business accounts
• Mileage logs if you are claiming vehicle expenses
• Details of any assets purchased for the business (for capital allowances purposes)

How long to keep records

HMRC can open an enquiry into your return for up to 12 months after the filing deadline in straightforward cases — but this window extends to four or six years if they believe there has been a careless or deliberate error. The practical guidance is to keep records for at least five years after the 31 January filing deadline for that tax year. For the 2025-26 return (filed by January 2027), that means keeping records until at least January 2032.

Digital record keeping

You do not need to use software — a well-maintained spreadsheet is perfectly acceptable. That said, cloud accounting tools such as Xero make it substantially easier to reconcile income and expenses throughout the year rather than reconstructing them from memory in January. If you are VAT-registered, Making Tax Digital (MTD) already mandates compatible software; for those not yet caught by MTD, digital records are best practice regardless.

Filing online via HMRC’s website is the most common route, and for most sole traders it is straightforward once your records are in order. Here is the process from start to finish.

These are the errors that genuinely cost people money, and they come up regularly — not just in complicated situations.