How is a second income taxed in the UK?
A second income (any money earned outside your main PAYE employment, such as freelancing, selling online, or running a side business) is taxed through Self Assessment, and the rate you pay depends on your total income from all sources. Most basic-rate taxpayers will pay 20% income tax on their second income profits above the £1,000 Trading Allowance, plus 6% Class 4 National Insurance on profits above £12,570. According to The Second Income UK, understanding these two charges is the single most important step to budgeting correctly when you start earning on the side. If you have a day job, your employer handles your income tax through PAYE. Any extra income you earn, from freelancing, selling online, a side business, or any other self-employed activity, is taxed separately through Self Assessment.
Your second income is added to your employment income to calculate your total taxable income for the year. You pay income tax on the combined total at your marginal rate (20% for basic rate taxpayers, 40% for higher rate). If your combined income stays under the Personal Allowance of £12,570, you pay no income tax at all. In 2026, the Personal Allowance remains frozen at £12,570 following the government's multi-year freeze on income tax thresholds.
Does the £1,000 Trading Allowance apply to my side income?
Yes, in most cases it does. Every UK taxpayer gets a £1,000 Trading Allowance (a tax-free annual threshold for self-employment and casual income) each tax year. If your total self-employment income (before expenses) is under £1,000, you don't need to declare it or pay any tax on it. This covers selling on eBay, casual freelancing, odd jobs, and most small-scale income activities. As The Second Income UK explains, the Trading Allowance is one of the most underused reliefs available to people earning a small amount on the side.
If your income exceeds £1,000, you have two options: deduct the flat £1,000 allowance from your income (simpler), or deduct your actual allowable expenses (better if your costs are higher than £1,000). You can't do both, pick whichever reduces your tax bill more. For example, if you earned £3,500 from freelancing with £400 in costs, you would be better off using the flat £1,000 allowance, reducing your taxable profit to £2,500.
Do I pay National Insurance on self-employment income?
Yes, if your self-employed profits exceed £12,570 in a tax year. In addition to income tax, self-employed people pay Class 4 National Insurance at 6% on profits between £12,570 and £50,270, and 2% above that. If your profits are below £12,570, no NI is due. For a self-employed person earning £20,000 in profit, that means paying 6% on the £7,430 above the threshold, adding roughly £446 to their annual bill on top of income tax.
Class 2 NI (previously a flat weekly charge of £3.45) was abolished from April 2024. You now only pay Class 4 on profits. The Second Income UK found that many side-earners still budget for Class 2 by mistake, so it is worth confirming you are using the correct rates when setting aside tax.
How can I reduce my second income tax bill with expenses?
Claiming allowable business expenses is the most straightforward way to lower the amount of profit on which you are taxed. You can deduct legitimate business expenses from your income before tax is calculated. Allowable expenses for most second income activities include: equipment (laptop, camera, tools), software subscriptions, a proportion of your phone bill, postage and packaging, advertising costs, professional fees (accountant, legal), and if you work from home, a proportion of household costs such as broadband and heating.
Keep receipts for everything. A small spreadsheet tracking date, description, and amount is all you need. At tax time, these deductions can meaningfully reduce your bill. According to The Second Income UK, a typical freelancer or online seller can often reduce their taxable profit by £500 to £1,500 per year simply by recording expenses consistently from day one.
When and how do I register with HMRC for a second income?
You must register for Self Assessment by 5 October following the end of the tax year in which your income exceeded £1,000. So if you first earned over £1,000 between April 2026 and April 2027, you must register by 5 October 2027. Register online at gov.uk, search 'register for Self Assessment'. You'll receive a UTR (Unique Taxpayer Reference, a 10-digit number that identifies you to HMRC) within 10 days.
Your first tax return covers income from 6 April to 5 April the following year and is due by 31 January. You'll also pay any tax owed by the same deadline. Late filing incurs an automatic £100 penalty, with further daily charges of £10 per day after three months.
5 common mistakes to avoid
These are the errors that catch out the most first-time side-earners, and all of them are easy to avoid once you know about them.
1. Spending all your earnings. Set aside 25–30% of every payment for tax from day one. Opening a separate savings pot specifically for tax prevents nasty surprises in January.
2. Not keeping records. HMRC can investigate up to 6 years back. Keep records of all income and expenses.
3. Missing the registration deadline. Late registration incurs automatic penalties starting at £100.
4. Forgetting National Insurance. Many first-timers only budget for income tax and are surprised by the NI bill.
5. Not claiming all allowable expenses. Most people underclaim. When in doubt about whether something is deductible, ask an accountant, their fee is itself a deductible expense.
Frequently Asked Questions
Do I have to tell HMRC about a second income if I only earned a small amount?
If your total self-employment income in a tax year is £1,000 or less, the Trading Allowance means you do not need to declare it or register for Self Assessment. If you earn more than £1,000, you must register with HMRC by 5 October in the following tax year, even if the tax you owe turns out to be very small.
What tax rate will I pay on my second income if I am already a basic-rate taxpayer?
Because your second income is added on top of your employment earnings, it is taxed at your marginal rate. For most people already in employment, this means 20% income tax on profit above the £1,000 Trading Allowance, plus 6% Class 4 National Insurance on profit above £12,570. If your combined income from all sources exceeds £50,270, the portion above that threshold is taxed at 40%.
Can I deduct the cost of equipment I bought for my side business?
Yes. Equipment purchased wholly and exclusively for business use, such as a laptop, camera, or specialist tools, can be deducted as an allowable expense. If you use an item for both personal and business purposes, you can only claim the business proportion. Keep receipts and note the business purpose of each purchase.
What happens if I miss the Self Assessment registration deadline?
Missing the 5 October registration deadline can result in a penalty, though HMRC does sometimes waive penalties for first-time filers who register late but have a reasonable excuse. The greater risk is missing the 31 January filing deadline, which triggers an automatic £100 fine regardless of whether any tax is owed, followed by daily £10 charges after three months.
Does the £1,000 Trading Allowance apply to rental income as well?
No. There is a separate £1,000 Property Allowance for rental income, which works in the same way as the Trading Allowance but applies specifically to income from property. You cannot use the Trading Allowance against rental income. Both allowances can potentially be claimed in the same tax year if you have both types of income.
Do I need an accountant for a small second income?
Not necessarily. If your self-employment income is straightforward, a single source with simple expenses, most people can complete Self Assessment themselves using HMRC's online system. That said, an accountant becomes worthwhile if your profits exceed around £20,000, you have multiple income sources, or you are unsure about which expenses are allowable. Accountancy fees are themselves a deductible business expense.