Tax on a second job explained

Why a second job gets a different tax code

Your tax-free Personal Allowance can only be given to one employer at a time, since PAYE needs a single place to apply it. In practice, that's almost always your main job, leaving a second job with a BR code (everything taxed at 20%, no allowance) or sometimes D0/D1 if you're already a higher or additional-rate taxpayer. This isn't a penalty for having two jobs — it's just HMRC avoiding giving you the same tax-free allowance twice.

A worked example: £30,000 main job plus £10,000 second job

Main job on the standard 1257L code, £30,000 a year: the £12,570 allowance leaves £17,430 taxable, all within the 20% basic-rate band — £3,486 of Income Tax. Second job on a BR code, £10,000 a year: there's no allowance to apply, so the whole £10,000 is taxed at 20% — £2,000. Add the two together: £5,486 total Income Tax across both jobs.

Compare that with the same £40,000 coming from a single employer instead: the £12,570 allowance leaves £27,430 taxable, all still within the basic-rate band, giving exactly £5,486 of Income Tax — identical to the two-job total. As long as your combined income from both jobs stays within the basic-rate band, splitting it across two employers with a BR code on the second one gives exactly the same total Income Tax as if one employer paid it all.

National Insurance works differently — and can work in your favour

Unlike Income Tax, employee National Insurance is calculated separately for each job against its own threshold, rather than on your combined income. In the example above, the main job's £30,000 generates NI of (£30,000 − £12,570) × 8% = £1,394.40. The second job's £10,000 a year sits entirely below the £12,570 NI threshold on its own, so it generates no NI at all, even though combined income across both jobs is £40,000.

Compare that with the same £40,000 from a single employer, which would generate (£40,000 − £12,570) × 8% = £2,194.40 of NI — £800 more than the two-job total in this example. Splitting modest income across multiple small jobs, each under the NI threshold on its own, can genuinely mean paying less total NI than earning the same amount from one employer.

When it works the other way

The flip side applies if both jobs individually pay well above the NI threshold: because the 8% rate applies up to £50,270 per job rather than per person, someone with two separate jobs each paying £40,000 could end up paying more combined NI than someone earning the same £80,000 from a single employer, since neither job on its own reaches the point where the rate drops to 2%. Worked through: two jobs at £40,000 each generate £2,194.40 of NI apiece — £4,388.80 combined — compared with £3,610.60 from a single £80,000 employer, a difference of £778.20 purely from how NI is split across two separate employments.

HMRC allows people in this situation to apply for NI deferment on one job, which caps how much is deducted at source and avoids genuinely overpaying across the two — it's not automatic, so it has to be actively requested.

What to check if a second job becomes your main income

Job roles can change — a 'second job' that grows to pay more than your original main job doesn't automatically swap tax codes between employers. If this happens, it's worth telling HMRC directly so the Personal Allowance can be moved to whichever job genuinely is your main income now, rather than leaving the smaller job with the allowance and the larger one taxed entirely at BR.

Common mistakes

• Assuming a BR code on a second job is a mistake or a penalty — it's usually correct, simply reflecting that your Personal Allowance is already allocated to your main job.
• Forgetting self-employed second income needs separate reporting — a side business alongside a PAYE main job generally still needs declaring through Self Assessment, regardless of how the main job is taxed.
• Not checking if a second job pushes combined income into a higher tax band — Income Tax looks at everything together via Self Assessment or an adjusted code, even though each job is taxed individually through payroll in the meantime.
• Assuming NI deferment applies automatically — it has to be applied for if both jobs individually cross the NI threshold and you're at risk of overpaying across the two.
• Overlooking that multiple small jobs under the NI threshold can mean paying less total NI than one larger job — useful to know, but not something to rely on without checking your specific combined position.