Changing Jobs? Your Tax Code Update Can Go Wrong Fast
When you change your main job, HMRC's tax code update process is slower and messier than they admit. Here's what to check before it costs you.
When did you last change employer? If the answer is "recently" and you haven't checked your tax code since, there is a reasonable chance HMRC is collecting more than it should, quietly, every single pay day.
Changing your main job employer triggers one of the most error-prone moments in the UK tax system. The process depends on a chain of communication between you, your old employer, your new employer, and HMRC, and at any point in that chain something can go wrong. The result is almost always the same: you pay too much tax and wait months to get it back.
- When you change main employer, HMRC should update your tax code automatically, but delays of 6-12 weeks are common.
- Without a P45, your new employer is required to put you on an emergency tax code, which can cost you hundreds of pounds.
- HMRC may continue sending your old tax code to your new employer if the records are not reconciled correctly.
- You can speed up the process by contacting HMRC directly or using your Personal Tax Account, rather than waiting for automation to kick in.
- Check your tax code on your first payslip from your new employer; do not assume it is correct.
The Handover Nobody Tells You About
When you leave a job, your employer is legally required to issue you a P45. This four-part form records your pay and tax to date for the current tax year and tells your new employer which tax code to use. In theory, your new employer submits the relevant section to HMRC via PAYE Real Time Information (RTI), HMRC updates their records, and you are taxed correctly from day one.
In practice, the handover breaks down with remarkable regularity.
Your old employer might be slow issuing the P45. You might start your new job before it arrives. Your new employer's payroll department might process it after your first pay run has already closed. Or, in the case of many small businesses running monthly payroll, the submission might not reach HMRC until weeks after you started.
During any gap in that process, your new employer has no choice but to apply an emergency tax code.
- Emergency Tax Code
- A temporary tax code (typically 1257L W1/M1 or BR for the 2024/25 tax year) applied by an employer when they have not received tax code information from HMRC or a P45 from the employee. It taxes each pay period in isolation, ignoring your annual personal allowance, and can result in significant overpayment.
The emergency code 1257L W1/M1 is less punishing than the BR code (which applies a flat 20% to all earnings with no personal allowance at all), but it still ignores any tax you have already paid in the current year and any allowances beyond the basic. If you left your old job mid-year having used some of your personal allowance there, the W1/M1 code will not carry that forward. You will be taxed as if the year started fresh.
For someone earning £55,000 a year, moving jobs in month six of the tax year, that error can easily produce an overpayment of £300 to £600 before HMRC corrects it.
Why the Automatic System Is Not as Automatic as HMRC Suggests
HMRC's RTI system, introduced in 2013, was supposed to end this problem. Employers submit payroll data every time they pay staff, giving HMRC near-real-time visibility of who is working where and on what salary. The theory is that HMRC can spot when you have moved employer and issue a revised tax code without you doing anything.
The reality is that RTI tells HMRC what your new employer is paying you. It does not automatically tell them that the old employer relationship has ended cleanly, particularly if your old employer was slow issuing a P45 or made errors in their final submission. HMRC's systems then have to reconcile two sets of employment records, and until they do, your new employer may be sitting on an old or incorrect code.
This is compounded by a structural irony: HMRC issues tax code notices (form P9 or P6, depending on whether it is a bulk annual update or an in-year change) to employers, not to employees. You may be the last person in the chain to know your code has changed, and the only way to find out proactively is to check yourself.
If you want to understand what your current code actually means in pounds and pence, our Free Tax Code Calculator UK: What the Number Actually Tells You post breaks down the number in plain English.
What to Do the Moment You Start a New Job
Do not wait for your first payslip to assume everything is fine. Here is the sequence that actually protects you.
Step 1: Hand over your P45 immediately
Give your P45 to your new employer's payroll team on your first day, not your first week. If your old employer has not issued it yet, chase them in writing. They are legally obliged to provide it without delay.
Step 2: Check your Personal Tax Account
Log into your HMRC Personal Tax Account at gov.uk and navigate to the PAYE section. You can see the tax code currently assigned to your new employment. If your new employer has not yet been linked to your record, or if the code shown is an emergency code, you will know before your first pay run rather than after.
Step 3: Contact HMRC directly if there is a problem
You do not have to wait for the system to self-correct. Call HMRC's income tax helpline on 0300 200 3300 or use the secure messaging function in your Personal Tax Account. Explain that you have changed employer and provide both employers' PAYE references if you have them. HMRC can issue a corrected P6 notice to your new employer within days. The automated system may take weeks to do the same thing unprompted.
Step 4: Check your first payslip carefully
When your first payslip arrives, look at the tax code shown. It should be 1257L (for the 2024/25 tax year, assuming no adjustments) without the W1 or M1 suffix. If it shows W1/M1, BR, or 0T, you are on an emergency or non-cumulative code and you need to act.
You can check your tax code free at /check-my-tax-code to see whether what your employer is using matches what HMRC has on record.
The Specific Problem of Multiple Jobs and a Change of Main Employer
The situation becomes meaningfully more complicated if you had a second job or a side income alongside your main employment. Your tax code is not just about one employer; it distributes your personal allowance across income sources, and HMRC's system is optimised around a single, stable employment.
When your main employer changes, HMRC should reassign your personal allowance to the new main employment and maintain any secondary codes for other income. In practice, what sometimes happens is that the old main employer continues to appear as the primary record temporarily, your new employer is assigned a secondary code (BR or D0) by mistake, and you end up paying basic or higher rate tax on your entire salary at the new job with no personal allowance applied at all.
This is not a hypothetical edge case. It is a predictable consequence of how HMRC's systems sequence employer changes, and it can persist for a full pay cycle or longer before anyone notices.
Our post on Multiple Employment Tax Code: Why HMRC Gets It Wrong covers the mechanics of this in detail. If you also have a side hustle or freelance income on top of your employment, Side Hustle Tax Code: Why HMRC's Maths Is Off is worth reading alongside this one.
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The Overpayment That Sits There Quietly
Here is the scenario nobody warns you about clearly enough. You start a new job in, say, October. Your new employer uses an emergency code for the first month because your P45 was delayed. By November, your code is corrected to 1257L. You assume everything is fine.
But November's correction is not retroactive. The month you were on an emergency code still has an overpayment sitting in it. Under a cumulative tax code like 1257L (without W1/M1), your payroll software will recalculate your total tax liability for the year-to-date each month and adjust accordingly. So in theory, the November payslip should pull back the excess tax from October automatically.
In practice, this only works cleanly if the code was corrected before November's payroll was processed. If the correction came in after the payroll closed, you are waiting until December for the adjustment, or potentially until your end-of-year tax reconciliation. HMRC's P800 process, which is meant to identify and repay overpayments after the end of the tax year, is reliable but slow. Refunds under P800 can take until October or November following the April year end.
For a higher earner, that is a meaningful float HMRC is holding. As we noted in our Tax Code Overpayment Calculator: How Much Is HMRC Keeping? post, the aggregate overpayment across UK employees runs into hundreds of millions of pounds in any given year.
If you suspect you have overpaid, you do not have to wait for P800. You can claim a repayment directly through your Personal Tax Account or by writing to HMRC.
One More Trap: Employer Changes and Benefits in Kind
If your old employer provided a taxable benefit (a company car, private medical insurance, or a season ticket loan, for example), that benefit will have been factored into your tax code as a deduction. When you leave, your old employer is supposed to notify HMRC that the benefit has ended. If they are slow to do so, HMRC may carry the benefit deduction forward into your new tax code, reducing your effective personal allowance at a job where you receive no such benefit.
The result is that you pay more tax on your new salary to cover a benefit you no longer receive. It is not common, but it is not rare either, particularly in cases where the old employer's HR and payroll teams do not communicate cleanly at the point of departure.
If your new tax code is lower than you expected (a number lower than 1257, or with a letter suffix like L with unexpected deductions), run through our Verify Tax Code Accuracy UK: A Forensic Checklist to identify what is pulling it down.
The Check You Can Do Right Now
Changing main job is the single most common trigger for a tax code error in PAYE, and the good news is that it is also one of the most straightforward to catch. You do not need an accountant. You need your payslip and five minutes.
Look at your current payslip. Find the tax code. If it is not 1257L (or a code that reflects a legitimate adjustment you are aware of), something may be wrong. Check your tax code free at /check-my-tax-code and see exactly what HMRC has on record for you.
The chain of employer-to-HMRC communication has already let you down once if your code is wrong. Don't wait for it to fix itself.
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