Having an employer who runs PAYE does not take you out of Self Assessment. The £1,000 trading allowance test is the same whether the side work is your only income or your third income stream. Over £1,000 gross in the tax year and you register.
The two incomes are taxed together
When you file, you enter your employment income and the tax already deducted from it, then your self-employed profit. HMRC adds them up, works out the total tax due for the year, and subtracts what PAYE already collected. You pay the difference.
Usually there is a difference to pay, because your tax-free personal allowance and your basic-rate band were mostly used up by the job. That often means the side income is taxed from the first pound, at 20% if you are still within the basic-rate band, or 40% on any part that tips you over £50,270 of total income.
National Insurance works differently for each
Your job pays Class 1 National Insurance, taken from your wages. That does not carry over.
On the self-employed side you look at the profit on its own:
- Class 4 is 6% on profit between £12,570 and £50,270, then 2% above. The £12,570 floor applies to the self-employed profit by itself, so a small side profit often has no Class 4 to pay.
- Class 2 is £3.65 a week for 2026 to 2027. If your self-employed profit is £7,105 or more it is treated as paid automatically. Below that you can choose to pay it to keep your National Insurance record ticking over, though if the job already gives you a qualifying year you may not need to.
Where a side income bites
The main surprise is higher-rate tax. Your total income is what sets your tax band, so a job paying £45,000 plus £8,000 of self-employed profit puts roughly £2,730 above the £50,270 line, taxed at 40% with 2% Class 4 on top. The same logic applies to the High Income Child Benefit Charge, which starts once your combined income passes £60,000, and to student loan repayments, which take a percentage of everything above the plan threshold and pick up the self-employed slice through your return.
Then there are payments on account. If the tax you owe through Self Assessment, on top of anything already taken through PAYE, comes to more than £1,000, HMRC asks for two advance instalments towards the following year. The first year this happens can feel like paying one and a half times over, so it is worth knowing before the demand lands.
How the extra tax is collected
You pay it by 31 January after the tax year. If the amount you owe through Self Assessment is under £3,000 and you file online by 30 December, you can ask HMRC to collect it through your tax code over the next year instead of in one go. That spreads it across your payslips, though it does not reduce the total.
Keeping the two clean
Keep the side work in its own bank account if you can, or at least tag the transactions. Come January you want to be able to show, without digging, what came in and what you spent. A rough rule while you are earning: hold back 20% to 30% of every self-employed payment for tax, more if the job already puts you near the higher-rate band. How much tax to save has the fuller version.