Going self-employed in the UK transfers a great deal of responsibility onto your own shoulders. Where employees watch income tax and National Insurance quietly removed from their pay before it ever reaches the bank, the self-employed have to do that work themselves: work out what is owed, report it to HMRC through Self Assessment, and pay up on time. It can seem daunting at first, and the late-filing penalties are genuinely steep, but the system is anchored around a small number of dates and a couple of distinct taxes. Once you can see the shape of it, the process becomes far less intimidating. This guide walks through the mechanics: how Self Assessment works and when to file, how income tax is calculated on your profits, the two different kinds of National Insurance, the expenses you can knock off your bill, and how it all fits together in practice.
Self Assessment: Registration and Deadlines
Self Assessment is the route HMRC uses to collect income tax from people who are not taxed at source through the PAYE system. If you are self-employed, you register once and then file a return every year for as long as you trade.
The first thing to grasp is the unusual shape of the tax year, which runs from 6 April to 5 April. The 2025/26 tax year, for example, runs from 6 April 2025 through to 5 April 2026, and the filing and payment deadline for that year is 31 January 2027. Registration follows a slightly different rhythm: you need to register by 5 October after your first tax year of self-employment, so someone who starts trading during the 2025/26 year must be registered by 5 October 2026. Once you are in the system, two dates tend to rule your calendar — 31 January, when you file your online return and pay the tax for the previous year, and 31 July, when your second payment on account falls due if you are on the instalment scheme.
Missing that 31 January deadline triggers automatic penalties that climb quickly. A £100 penalty applies even if you are only one day late; after three months you start accruing £10 per day up to a maximum of 90 days; at six months you face £300 or 5% of the tax owed, whichever is larger; and another £300 or 5% lands at twelve months. Paying late also attracts interest on top of the amount you owe, so it is worth treating the filing date as immovable.
Income Tax Bands
Self-employed people pay income tax at exactly the same rates as employees; the only real difference is that you calculate and pay it yourself. Under the 2025/26 rates, your trading profits are stacked into the band structure below, with the Personal Allowance sitting at the bottom as the tax-free slice.
| Band | Taxable income | Rate |
|---|---|---|
| Personal Allowance | Up to £12,570 | 0% |
| Basic Rate | £12,571 – £50,270 | 20% |
| Higher Rate | £50,271 – £125,140 | 40% |
| Additional Rate | Over £125,140 | 45% |
The Personal Allowance is £12,570. For every £2 of income above £100,000 the allowance is reduced by £1, so it disappears entirely once income reaches £125,140 — a quirk that makes the effective marginal rate on that stretch of earnings much higher than the headline bands suggest. It is also worth being clear about the base your tax is calculated on: it is your trading profits, meaning self-employed income minus allowable business expenses, not your total turnover. That single distinction is why keeping on top of expenses is so valuable.
National Insurance for the Self-Employed
Alongside income tax, self-employed people pay two types of National Insurance, and the pair behave quite differently.
Class 2 is a flat weekly contribution of £3.45, about £179.40 a year, that applies once your profits exceed £12,570. It builds your entitlement to the State Pension and certain contributory benefits, so if your profits fall below the threshold you can still pay it voluntarily to keep your qualifying years ticking over — a decision worth making deliberately early in a business, when those years are cheapest.
Class 4 is closer to a straightforward tax. It is charged at 6% on profits between £12,570 and £50,270 and at 2% on anything above that line, with nothing due below £12,570. Unlike Class 2, it does not count towards your State Pension or contributory benefits. In effect you are paying a flat premium for pension cover and a percentage-based levy on top of that.
Allowable Business Expenses
You can deduct legitimate business expenses before any tax is calculated, and the definition of legitimate is reasonably generous. Common claims include office costs such as stationery, printing, postage and software subscriptions; travel covering fuel, public transport, parking and vehicle maintenance for business use; uniforms and protective clothing (though not everyday clothes); stock and raw materials; marketing from website hosting to social media promotion; professional fees such as accountancy and professional indemnity insurance; and the business-use portion of your phone and broadband bills.
Claiming for a Home Office
If you work from home, there are two routes. HMRC’s simplified expenses scheme provides a flat weekly amount based on how many hours you work at home each month:
| Hours worked from home per month | Flat rate per month |
|---|---|
| 25 – 50 | £10 |
| 51 – 100 | £18 |
| 101 or more | £26 |
That works out at roughly £6 a week on the lowest tier, and it is far easier to administer than the alternative — calculating your actual costs by reference to the proportion of your home used for business. If your real costs clearly exceed the flat rate, the actual-costs route may give a larger claim, at the price of more detailed record-keeping.
The Trading Allowance
For casual traders and side hustlers, the Trading Allowance offers an easy starting point: up to £1,000 of trading income a year is tax-free and does not even need to be declared. Once income passes £1,000, you can either deduct that allowance or deduct your actual business expenses, whichever gives the better result. For someone merely dipping a toe into self-employment — occasional freelance work or casual selling — it removes much of the paperwork entirely, though a genuine full-time business will almost always find proper expense tracking more rewarding.
A Worked Example: Freelance Designer
To see all of this running in practice, take Sarah, a freelance graphic designer. In the 2025/26 tax year she earns £45,000 in client fees and has £5,000 of allowable expenses, leaving her with £40,000 of trading profit. Her income tax follows the bands above: the £12,570 Personal Allowance is tax-free, and the remaining £27,430 is taxed at the basic rate of 20%, producing £5,486 of income tax.
On the National Insurance side, her profits sit comfortably above the £12,570 threshold, so she pays Class 2 at £3.45 a week — about £180 across the 52-week year against the £179.40 pure annual figure. Her Class 4 bill is 6% on the £27,430 between £12,570 and her £40,000 profit, which comes to £1,646. Adding the three together produces a total tax bill of £7,312: £5,486 of income tax plus £180 and £1,646 of National Insurance. That is an effective tax rate of roughly 18.3% measured against her £40,000 profit, or about 16.2% against her £45,000 of gross income.
Managing Self-Employed Tax in Practice
None of this requires exotic planning. The foundation is simple record-keeping from day one — invoices, receipts and bank statements — because HMRC expects you to hold onto your records for at least five years after the 31 January submission deadline. Accounting software such as FreeAgent, Xero or QuickBooks tracks income and expenses automatically and makes the annual return considerably less painful. A habit many freelancers adopt is transferring 25–30% of every payment into a separate savings account so the money exists when the bill lands, and remembering that if your tax bill exceeds £1,000, HMRC expects payments on account in January and July for the following year. Filing early rather than on the deadline leaves room to catch errors, and accountants’ fees are themselves an allowable expense — a point worth recalling as your affairs grow to include multiple income streams or property income. Reliefs such as pension contributions, Gift Aid donations and trading losses can all reduce the amount due, so it pays to look at those before you reach for the payment button. The system rewards the organised not the wealthy; keep good records, file on time, and the sums stay very manageable.