The four common categories
Side income for UK employees usually falls into one of four categories, each with slightly different tax treatment.
Freelance / consulting work. You do paid work for clients on top of your job. Examples: web design, content writing, photography, tutoring, business consulting. Taxed as trading income — covered by the £1,000 trading allowance up to that level, then Self Assessment at your marginal rate above.
Marketplace selling. Selling products through eBay, Etsy, Vinted, Amazon, etc. The trading allowance covers up to £1,000 of revenue. Above that you're trading and need to register for Self Assessment. Note: occasional sales of personal belongings aren't "trading" — only systematic buying/making to resell counts.
Property rental. Renting out a room (Rent a Room scheme up to £7,500/year tax-free), short-let on Airbnb, or a buy-to-let. Different rules and a separate £1,000 property allowance.
Investment income. Savings interest above the Personal Savings Allowance (£1,000 for basic-rate, £500 for higher-rate, £0 for additional-rate taxpayers), dividends above £500. Reported on Self Assessment but not "side work" in the active sense.
This guide focuses on the first two — active side work that produces trading income.
The £1,000 trading allowance
For 2026/27, each UK individual gets a £1,000 trading allowance — gross trading income up to this level is tax-free and doesn't require declaration to HMRC.
The allowance is per individual, not per side hustle. If you sell £600 of crafts on Etsy AND earn £500 from weekend tutoring, that's £1,100 of total trading income — over the allowance, so Self Assessment becomes required.
Two ways to apply the allowance once you're over it:
- Use the full allowance: deduct £1,000 from your trading income. Pay tax on whatever's left.
- Use actual expenses: deduct your real costs (materials, equipment, software, business mileage, etc.) instead.
You choose whichever gives the better outcome. For low-overhead side work, the £1,000 allowance often wins. For activities with substantial costs (e.g., reselling, where you bought the items), actual expenses usually win.
How tax actually works on side income
Side income stacks on top of your employment salary in HMRC's tax calculation. Worked example:
- Employment salary: £40,000 (basic-rate band)
- Side income: £5,000 (your gross trading income)
- Trading allowance deducted: £1,000
- Taxable side income: £4,000
Your total income for tax purposes is £40,000 + £4,000 = £44,000. The £4,000 of taxable side income lands in the basic-rate band (because your total income is still below £50,270), so:
- Income Tax at 20%: £800
- Class 4 NI at 6%: £240
- Total tax on side income: £1,040
- Net side income after tax: £2,960 (from gross £5,000)
If the side income pushes your total income across £50,270 into the higher-rate band, the slice above £50,270 is taxed at 40% + 2% NI = 42% marginal rate.
For your specific marginal rate at any salary level, the Take-Home Pay Calculator shows it directly.
Registering with HMRC
If your trading income will exceed £1,000 in the tax year (6 April – 5 April), you must register for Self Assessment by 5 October following the end of that tax year. So trading income earned during 2026/27 must be registered by 5 October 2027.
The registration process:
- Go to gov.uk/set-up-sole-trader
- Sign in to your Government Gateway (or create an account)
- Provide your National Insurance number, address, business activity description, and start date
- HMRC issues you a Unique Taxpayer Reference (UTR) within ~10 working days
Once registered, you file Self Assessment annually by 31 January following the end of the tax year (online filing — paper deadline is 31 October).
Payment timing
Tax on side income is paid via Self Assessment, not through PAYE. Payments are due 31 January following the tax year end. So tax on 2026/27 side income is due by 31 January 2028.
If your Self Assessment bill exceeds £1,000, HMRC also requires payments on account:
- 50% of the next year's expected bill paid by 31 January
- Another 50% paid by 31 July
- Final balance reconciled the following 31 January
For someone starting side work that produces £5,000+ of profit, the second-year cashflow can be a surprise — you're effectively paying tax for both this year and a payment on account for next year in one go.
The practical setup
A weekend's worth of admin gets you operational:
Day 1: Register for Self Assessment at gov.uk. Open a separate bank account for side-work income (a free fintech account like Starling or Tide keeps the books clean even though it's not legally required for sole traders).
Day 1 evening: Set up a simple spreadsheet for invoices issued, expenses paid, and income received. Cloud accounting (FreeAgent, QuickBooks, Xero) is overkill for genuinely small side income but worth it if you expect to grow.
Day 2: Set up your first listing or contract. For freelance work, this might be a Fiverr / Upwork profile or a personal website. For marketplace selling, it's creating shop accounts and product listings.
Ongoing: log income and expenses as they happen, not in bulk at year-end. Reserve roughly 25-30% of net side income (gross minus deductible expenses) in a separate savings account for the eventual tax bill.
Common mistakes
- Forgetting to register: HMRC eventually catches up via marketplaces' data sharing. Late-registration penalties stack.
- Mixing personal and business expenses: pay business costs from the side-work account, not your personal current account.
- Not reserving for tax: spending all side income means scrambling for the January Self Assessment payment.
- Claiming personal expenses as business: HMRC's spot-check rate on sole-trader Self Assessment is rising; only claim costs that are wholly and exclusively for the business.
- Underestimating time: side work that pays £50/hr but takes 3 hours of admin per delivered hour pays £12.50/hr in practice. Track time honestly.
When to consider going further
Side income that grows beyond ~£15,000-£20,000 of annual profit starts to merit a more deliberate structure:
- At £15k-£40k profit: sole trader remains the right structure but more careful expense tracking matters
- At £40k+: consider whether to incorporate as a limited company (sole trader vs limited company)
- At any level: if the side work could replace your salary, how to replace your salary with freelance income covers the transition
For ideas on what side work pays and which to test first, see best side hustles UK. For freelancing specifically, how to start freelancing.
In short
Extra income alongside a full-time job is straightforward in tax terms: £1,000 trading allowance, then Self Assessment at your marginal rate. The setup is a weekend's work. The harder questions are which side work to pursue and how it interacts with your employment contract — neither has a single right answer.