Loads of people run a business alongside a job — a weekend trade, freelance evenings, a growing side hustle. It's completely normal, and the tax isn't as scary as it looks once you see how the two sides fit together. Here's the map.

Two income sources, one tax return

Your employer handles the job through PAYE — tax and National Insurance come off your payslip automatically. Your self-employed profits go through Self Assessment. The clever bit: your Self Assessment return pulls in both — your employment income (from your P60) and your self-employed profit — and works out the total tax due, giving you credit for the PAYE tax you've already paid. So you're not taxed twice on the same money; the return just settles the difference.

You only get one personal allowance

The £12,570 tax-free personal allowance is yours, not one per source. Usually your job uses it up (through your tax code), which means your self-employed profit is often taxed from the first pound — commonly at 20%, or 40% if your combined income pushes you into the higher-rate band. This is the number-one surprise for side-hustlers: your side income is taxed on top of your job, at your highest rate, not afresh with its own allowance.

National Insurance: you might pay two types

  • Class 1 — deducted from your salary through PAYE (the employee's NI).
  • Class 4 — on your self-employed profits, via Self Assessment (see our NI guide).

You can pay both — but there's an annual maximum so you don't overpay across the two. If you earn well in both, this can mean a refund or adjustment; it's the kind of thing worth having an accountant check.

The £1,000 trading allowance

If your self-employed income is small, remember the £1,000 trading allowance: earn under £1,000 gross from your side activity and you generally don't need to declare it at all. Over £1,000 and you register for Self Assessment — but you can still deduct the flat £1,000 instead of actual expenses if that's better for you.

The set-aside still applies Because your side income is taxed at your marginal rate (often 20% or 40%) with no personal allowance left, set aside a healthy chunk of every payment — 20–30% depending on your total income — so the January bill is covered. Your job's payslip won't cover your side-hustle tax; that's on you.

How we make it simple

We handle exactly this every day: reconciling PAYE with self-employed profit, using the trading allowance when it helps, checking the NI maximum so you don't overpay, and making sure you declare what you need to and nothing you don't. From £19 + VAT a month. Get started.