Side hustle tax UK: getting your second income right

Side hustle tax UK is not a separate tax regime — it is ordinary Income Tax and National Insurance applied to a trade you run alongside employment. HMRC does not care whether trading is your main occupation or a sideline; the same registration and reporting rules apply once you are over the trading allowance, which is why HMRC side hustle enquiries and nudge letters treat a £3,000 resale shop the same as a full-time sole trader with the same turnover.

The starting point is the £1,000 trading allowance. It lets anyone earn up to £1,000 gross trading income in a tax year without needing to register for Self Assessment or pay tax on it. Gross means before costs — so £1,000 of sales, not £1,000 of profit — which catches out people who assume the allowance is measured on profit.

Once gross trading income passes £1,000, you must register for Self Assessment by 5 October following the end of the tax year in which trading started, and file a return by the following 31 January. Missing registration does not make the income exempt; it just risks penalties on top of the tax due. This is the single most common question we get asked: "do I pay tax on side income" if it was small — the answer turns on that £1,000 line, not on how the money feels.

For your tax return, you either deduct the £1,000 allowance from your income, or deduct your actual allowable expenses — never both. If your side hustle has genuine costs (stock, software, tools, a share of home running costs) above £1,000, claiming actual expenses will usually leave more profit sheltered than the flat allowance.

The profit that remains is added to your other income for the tax year, principally your employment income from your P60 or payslips, to work out your total taxable income. Tax is then calculated on that combined figure using the normal bands, so your side hustle profit is effectively taxed at your marginal rate, not a flat rate — which is the core mechanic behind second income tax questions.

This stacking effect surprises a lot of people. If your job already uses up your personal allowance and basic rate band, your side hustle profit could be taxed at 40% from the first pound of profit, even though the business itself is small. It is worth running a side hustle tax calculator style estimate before a big sales month, rather than finding out in January.

National Insurance follows a similar logic but is calculated separately. Class 1 NIC is deducted from your salary by your employer. Class 4 NIC applies to your self-employed profits above the lower profits limit, currently aligned with the personal allowance, and is charged at 6% up to the upper limit and 2% above it.

Voluntary Class 2 contributions can be worth paying even where profits are modest, because they protect your state pension record. We flag this on client returns rather than leaving it to be missed by default settings in filing software.

Losses matter too. A genuine trading loss in year one, common for anyone buying stock, tools or a website upfront, is not wasted — it can usually be carried forward to set against future profits from the same trade, which is one more reason to keep proper records from day one even while the numbers look unpromising.

Digital platform reporting rules mean marketplaces, content platforms and payment processors increasingly share seller data with HMRC automatically. This has closed a lot of the informal grey area that used to exist around small online selling, so treating the £1,000 threshold as a real line rather than a guideline is the safer approach — see does HMRC know about my online income for how that reporting actually works.

If your side hustle grows to the point where it produces meaningful, repeatable profit, the conversation moves on — should it become a limited company, do you need to think about VAT, should you be paying a spouse who genuinely helps. None of that is urgent at £2,000 of profit, but it is worth revisiting each year as the numbers change.

Record-keeping does not need to be elaborate at this scale. A simple spreadsheet or lightweight bookkeeping app tracking sales, costs and bank transfers is usually enough for a side hustle, provided it is kept up to date and the figures reconcile to your bank statements.

Source: Income Tax rates and allowances (gov.uk) · Self Assessment (gov.uk)