Tax Code New Job UK: What Actually Happens on Day One
Starting a new job in the UK? Your tax code on day one shapes every payslip you get. Here's what HMRC does, what can go wrong, and how to fix it fast.
Starting a new job next Monday? HMRC has already made a decision about how much tax you'll pay before you've even met your new manager. Whether that decision is correct is a different matter entirely.
Your tax code at a new job in the UK is not a formality. It is a live calculation that determines how much of your salary you actually take home from the first payslip onwards. Get it wrong and you could be overpaying tax for months without realising it. Get it right and you start your new role with exactly what you're owed in your pocket.
This post covers the specific sequence of events that governs your tax code when you change jobs, what the most common errors look like in practice, and what you can do today to make sure you are not silently subsidising HMRC's cash flow.
- Your new employer needs a starter checklist or P45 from your old job to set your tax code correctly from day one.
- If neither document is available, HMRC defaults you to an emergency tax code, often 1257L W1/M1, which can cause significant overpayment.
- A wrong tax code at a new job can persist for an entire tax year unless you actively intervene.
- HMRC will not automatically alert you if your code is wrong; the responsibility to check sits with you.
- Checking your tax code takes five minutes at /check-my-tax-code and could recover hundreds of pounds.
What HMRC Actually Does When You Start a New Job
When you leave your previous employer, they should issue you a P45. This document is not a bureaucratic souvenir. It tells your new employer three critical things: your tax code at the point you left, how much you have earned in the current tax year, and how much income tax has already been deducted.
Armed with that P45, your new employer's payroll software passes the information to HMRC, which then issues a tax code notice (an P6 or P9 form, which you will not see directly) instructing the employer on what code to use going forward. The system works on a cumulative basis, meaning HMRC and your employer both keep a running total of your pay and tax across the whole tax year. If you overpaid at your old job, the cumulative calculation at your new one should, in theory, correct it automatically.
That is the theory. Reality, as anyone who has changed jobs mid-year will recognise, is messier.
- P45
- A form issued by an employer when an employee leaves. It records the employee's tax code, total pay to date, and total tax deducted in the current tax year. Giving this to a new employer allows payroll to apply the correct tax code from the start.
The Emergency Tax Code Problem
Not everyone has a clean P45 to hand when they start a new role. Your old employer might issue it late. You might have had a gap between jobs. You might be starting your very first job. In each of these cases, your new employer cannot apply your proper cumulative tax code because they have no information to work from.
So what happens? Your employer asks you to complete a starter checklist (formerly called a P46), which asks a deceptively simple question: is this your only job, your main job, or a second job?
If you tick the wrong box, the consequences land directly in your bank account. Tick Statement C (second job or pension) when this is actually your main job, and your employer will apply the BR code, taxing every penny of your new salary at 20% with no personal allowance at all. On a £55,000 salary, that is a tax code error worth over £2,500 per year.
Even if you tick the correct box, if your employer does not have your P45 they will almost certainly apply an emergency tax code: 1257L on a Week 1/Month 1 (W1/M1) basis. This is the non-cumulative version of the standard code, meaning each pay period is taxed in isolation. Your allowances reset every month as if you had earned nothing before. This sounds harmless until you remember that it ignores any tax you already paid in the months before joining. The result is almost always overpayment.
We have covered the mechanics of this in detail in Cumulative vs Non-Cumulative Tax Code: The Hidden Cost and Tax Code With Week 1 Basis: Why You're Overpaying.
The Three Job-Change Scenarios and What Each One Means for Your Tax Code
Scenario One: You Have a P45 and Hand It Over Promptly
This is the cleanest outcome. Your previous employer issues the P45 quickly (legally, they must do so on or before your leaving date, or by the next payday if that is sooner). You give it to your new employer's HR or payroll team on or before your first payday.
Payroll enters the figures, HMRC is notified, and your cumulative code carries forward. If you overpaid at your old job because your leaving date fell mid-month, that credit rolls forward automatically. Your first payslip at the new employer reflects your real year-to-date position.
Still check the payslip. Errors in data entry happen, and a miskeyed earnings figure can produce a wrong tax calculation even with a clean P45 in the system.
Scenario Two: You Have a Gap Between Jobs
If you were unemployed for any period before starting the new role, you might have received Jobseeker's Allowance or Employment and Support Allowance. Both are taxable. Your P45 from your old employer will not include these, because they are paid by the DWP, not your former employer.
In this case, HMRC should update your record automatically through real-time information (RTI) data shared by DWP. In practice, this update sometimes lags. If you start your new job and payroll has no clear picture of your year-to-date income, you are likely to land on an emergency code again.
The fix is straightforward: contact HMRC directly on 0300 200 3300 or update your details through your Personal Tax Account. HMRC can issue your new employer an updated code within a few weeks, though that delay still costs you money in the interim.
Scenario Three: This Is Your First Ever Job in the UK
No P45 exists. Your employer gives you a starter checklist. Statement A applies: this is your first job since the start of the tax year and you have not been receiving taxable benefits, a student loan, or a state pension.
HMRC will allocate code 1257L and, once your employer starts submitting payroll data via RTI, the cumulative record begins from your first payday. As long as you tick Statement A correctly and your employer submits accurately, this scenario is actually the least likely to result in a long-term error, though your first payslip may still show an emergency basis code until HMRC confirms the allocation.
Why the Wrong Code Persists Longer Than It Should
HMRC operates on real-time information submitted by employers through RTI. In an ideal world, every error would self-correct within a pay period or two. In the real world, two things conspire against that:
First, RTI data quality is patchy. If your employer's payroll system carries over an incorrect year-to-date earnings figure, HMRC's own system cannot tell the difference between a genuine figure and a data entry error. It simply processes what it receives.
Second, HMRC issues tax code change notices (P6 forms) to employers, not employees. You are the last person in the chain to find out that your code has changed, and you may never find out at all unless you actively look at your payslip and compare the code against what it should be.
The practical consequence is that a wrong tax code applied in, say, April, can run all the way through to March without a single alert reaching the employee. HMRC will eventually reconcile through the P800 annual tax calculation (explained in Annual Tax Calculation P800: What HMRC's Letter Really Means), but that letter arrives months after the tax year ends. You wait 12-18 months to recover money that was never HMRC's to take.
What Your First Payslip Should Show
When your first payslip arrives, check three things before you check anything else.
The tax code itself. For most UK employees in 2024/25, the standard code is 1257L. If you see BR, D0, D1, or any code followed by W1 or M1, flag it immediately. BR means no personal allowance is being applied. D0 means everything is taxed at 40%. These are not normal codes for a new starter with no other income.
The year-to-date figures. If your P45 was processed correctly, the year-to-date pay and tax figures should reflect your earnings at your old job plus your first month at the new one. If year-to-date shows only the current month's earnings, your employer is running you on a non-cumulative basis, which may mean you are overpaying.
The tax deducted this month. Do a rough sense check. On a salary of £60,000 (£5,000 per month), your monthly tax on a correct 1257L code should be approximately £920. If your payslip shows £1,200 or more, something is off.
If anything looks wrong, check your tax code now at /check-my-tax-code before raising it with payroll. Having the correct code confirmed first makes the conversation with HR significantly easier.
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Second Jobs and Overlapping Income
If you are starting a new job while still receiving pay from the old one during a handover period, or if the new role is a second concurrent job rather than a replacement, the tax code picture changes entirely.
Your main job should carry your personal allowance (code 1257L or similar). Any secondary employment gets code BR at minimum, taxing all earnings from that source at 20% with no allowance. If your combined income is likely to exceed £50,270, some of that secondary income should attract 40% tax, and HMRC may apply code D0.
The complication arises if HMRC gets the allocation wrong: applying the personal allowance to the wrong employer, or applying BR to what is actually your only job. Both situations cost you money. The check your tax code tool at /check-my-tax-code will flag if your allowance is allocated incorrectly across multiple employment records.
If your household income sits above £60,000 and you or your partner receive Child Benefit, a new job that pushes you across that threshold can also trigger the High Income Child Benefit Charge. Worth running through the numbers at our salary tax calculator before the first payslip arrives.
What to Do If Your Code Is Wrong
Do not wait for HMRC to spot it. The onus is on you.
Step one: Check your current tax code. Your payslip shows it, and so does your Personal Tax Account at gov.uk. Alternatively, check your tax code at /check-my-tax-code in under five minutes.
Step two: If the code is wrong, contact HMRC directly. Do not ask your new employer to fix it without first getting the correct code from HMRC, because payroll teams can only apply what they are instructed to apply.
Step three: Once HMRC issues a corrected code, your employer will apply it from the next pay period. If you have overpaid, the cumulative system will refund the difference through your payslip, typically by deducting less tax in subsequent months rather than issuing a cash refund. If you are approaching April and the underpayment cannot be corrected in time, HMRC will issue a P800.
If you believe the overpayment is substantial and the tax year is nearly over, it may be worth reading Tax Refund for High Rate Taxpayers With a Wrong Code to understand how the refund process works in practice.
The One Thing Most New Starters Never Do
Here is what the whole system assumes: that you will check. HMRC does not send you a welcome letter when you start a new job confirming your tax code. Your employer does not alert you if the code on file looks unusual. The payroll software processes whatever code it has been given.
The single most effective thing you can do when you start a new role is look at your first payslip and verify that the tax code is what it should be. Not after three months. Not when a colleague mentions they got a P800. On payday, before you spend anything.
We started with the observation that HMRC has already made a decision about your tax before you met your new manager. That decision deserves five minutes of scrutiny. Check your tax code at /check-my-tax-code today, and make sure the decision is the right one.
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