Part Year Employment: Why Your Tax Code Underpays You
Started or left a job mid-tax-year? HMRC's part year employment tax code adjustment often leaves workers overpaying. Here's what actually happens.
You started a new job in October. By March, you've been overtaxed by hundreds of pounds. HMRC isn't going to text you about it.
Part year employment is one of the most reliably broken corners of the PAYE system. When you work for only part of a tax year, whether because you changed careers, took a career break, returned from leave, or were made redundant and then rehired, the maths that underpins your tax code stops working the way it should. The personal allowance, worth £12,570 in 2024/25, is assumed by HMRC to be spread evenly across 52 weeks or 12 months. If you only work for six of those months, the system frequently overcollects in the early weeks and is slow to correct itself. Sometimes it never does.
This is not your fault. It is a structural flaw in how PAYE annualises income, and it costs ordinary employees real money every year.
- When you work part of a tax year, HMRC's PAYE system often overtaxes you because it spreads your personal allowance assuming full-year employment.
- Common triggers include starting a job mid-year, leaving a job before April, returning from a career break, or being rehired after redundancy.
- You can claim a refund directly through HMRC or via your employer, but you usually have to ask. It rarely arrives automatically.
- Checking your tax code is the fastest way to spot whether an adjustment has been applied correctly or missed entirely.
- Most overpayments from part year employment are refundable up to four years after the tax year in question.
Why the Personal Allowance Maths Breaks Mid-Year
- Part Year Employment Tax Code Adjustment
- A change HMRC makes to an employee's tax code to reflect the fact they have worked for only part of the tax year, ensuring unused personal allowance from the months they were not employed is applied correctly to reduce their tax bill.
Every employee in the UK is entitled to a personal allowance: income you can earn before paying any tax. For 2024/25 that figure is £12,570. Under PAYE, your employer divides that allowance into equal weekly or monthly portions and applies it incrementally to each payslip. The assumption baked into this system is that you will be employed continuously from 6 April to 5 April.
That assumption fails the moment you start a job in, say, August. Your employer begins applying the personal allowance from August onwards, but HMRC's internal model initially treats the preceding April-to-July period as if you had been earning something during it. Depending on what information HMRC holds about your previous income, the tax code your new employer receives may not fully account for the allowance that was unused in those first four months.
The arithmetic is straightforward but the consequences are not trivial. Take someone starting a job on 1 August 2024 with a salary of £36,000. Over eight months (August to March) they earn £24,000. The correct tax calculation applies the full personal allowance of £12,570, leaving £11,430 taxable at 20 per cent: £2,286 in income tax. But if HMRC's system issues a tax code that treats the personal allowance as though it only started accumulating in August, the taxable income could be calculated as higher, triggering an overpayment that can easily reach £200 to £400 before any correction is applied.
The Three Scenarios That Trigger the Problem
Starting a New Job After a Gap
This is the most common trigger. If you spent time out of work, travelling, studying, or caring for a family member and then started a new job part-way through the tax year, the personal allowance for the months you were not employed is legitimately unused. A correctly applied part year employment tax code adjustment would carry that unused allowance forward, reducing your tax bill in the months you do work.
In practice, your new employer issues your first payslip using whatever emergency code HMRC sends, often 1257L on a Week 1/Month 1 basis, which explicitly ignores cumulative allowances and taxes each pay period in isolation. You can read more about what emergency codes cost in the short term in our post on Starting a New Job? Emergency Tax Codes Cost Real Money.
Leaving a Job Before the Tax Year Ends
If your employment ends in, say, January, you have effectively over-contributed through PAYE for a full year's worth of tax when you will only have earned perhaps nine months of salary. Your employer should issue a P45 showing tax paid to date. If your total earnings for the year are below the personal allowance, you may owe nothing and be owed everything back. If earnings exceed the allowance but the code was not adjusted, you may still have overpaid.
HMRC's end-of-year reconciliation process, the P800 notice, should in theory catch this. In practice, P800 letters typically arrive between June and October following the end of the tax year, meaning you could wait seven months or more for a correction that should have been automatic.
Returning After a Career Break Within the Same Tax Year
Perhaps the trickiest scenario. You leave a job in June, take three months off, then start a new role in September. Your P45 from the first job shows income and tax paid from April to June. Your new employer should use that P45 to set a cumulative tax code that gives you credit for what you have already earned and paid. If the P45 is lost, delayed, or incorrectly processed, the new employer defaults to an emergency basis code, wiping out the cumulative record and almost certainly overtaxing you through to April.
What Your Tax Code Letter Actually Means Here
The most important two-letter suffix on your tax code in a part year situation is the one that appears after the number: L, M, N, W1, or M1.
1257L on a cumulative basis is correct for most standard employees. It gives you one-twelfth of your annual personal allowance each month and carries forward any unused balance from earlier in the year.
1257L W1 or 1257L M1 (Week 1/Month 1) is the emergency basis. It taxes each pay period as if it were the first of the year, ignoring everything that came before. This is the code that silently overtaxes part-year workers. If you see W1 or M1 on your payslip and you are not a new starter in week one of the tax year, that is a red flag worth investigating immediately.
An 0T code means no personal allowance is being applied at all. If you see this and you have not already used your allowance elsewhere, you are almost certainly overpaying.
The fastest way to find out what code is currently applied to you, and whether HMRC has made the adjustment correctly, is to check your tax code at TapTax. You will not find out by waiting for a letter.
How HMRC Is Supposed to Fix This (And Why It Often Does Not)
HMRC's official position is that PAYE is self-correcting. The cumulative basis means that if you were overtaxed in April through June, your employer will collect less in July through September to compensate. By the end of the year, the total should be right.
That logic holds when you stay with the same employer all year. It collapses the moment you change employer, take a break, or start mid-year, because the cumulative record does not automatically transfer between payroll systems. The P45 process is meant to bridge that gap, but it depends on employees remembering to hand it over, employers processing it promptly, and HMRC's systems receiving and acting on the information before the next payroll run.
When those steps fail, which is more often than HMRC's communications suggest, the correction mechanism is a P800 tax calculation sent at the end of the year, or a Self Assessment return if you file one. Neither is fast, and neither is proactive. You have to know you are owed money before you can ask for it back.
For workers who have changed jobs recently, our post on Changing Jobs? Your Tax Code Update Can Go Wrong Fast covers the specific handoff failures that occur at employer transition in more detail.
How to Claim Back What You Are Owed
If You Are Still Employed
Contact HMRC directly via your Personal Tax Account or by calling 0300 200 3300. Explain that you started employment part-way through the tax year and believe your tax code does not reflect your unused personal allowance. HMRC can issue a revised code to your employer, who will then reduce future deductions until the overpayment is recovered. You do not receive a lump-sum refund in most cases; the adjustment is applied over remaining payslips.
If there are only a few months left in the tax year, the correction may not fully apply before April. In that case, the remainder is picked up via a P800 or you can submit a claim using form R40 (for those not in Self Assessment).
If You Have Left the Job
Your employer cannot adjust a code for a former employee. If you are not in Self Assessment and you are owed a refund, you claim directly from HMRC. The process depends on whether the tax year has ended:
- During the tax year: Use the HMRC online service or call to request an in-year repayment. You will need your P45.
- After the tax year: Wait for a P800 or contact HMRC proactively. You can claim back up to four tax years.
For a broader look at how these overpayments accumulate and how to calculate your likely refund, our Tax Code Overpayment Calculator: How Much Is HMRC Keeping? article walks through the arithmetic in plain terms.
A Concrete Example: What This Looks Like in Practice
Sarah is a project manager who left her job in May 2024 to travel. She returned to the UK and started a new role on 9 September 2024 on a salary of £45,000. Her new employer received an emergency code from HMRC: 1257L M1.
From September to March (seven months), Sarah earns £26,250. On a Month 1 basis, her employer applies one-twelfth of the personal allowance each month, which is £1,047.50. Over seven months that comes to £7,332.50 in personal allowance applied. Tax is charged on £26,250 minus £7,332.50, giving taxable income of £18,917.50 and a tax bill of £3,783.50.
The correct calculation, recognising that Sarah earned nothing between April and August, applies the full £12,570 personal allowance to her September-to-March earnings. Taxable income drops to £13,680, and her tax bill falls to £2,736. The M1 code has cost her £1,047.50 in excess deductions.
Sarah does not know this unless she checks her code. HMRC's P800 may arrive in September 2025, twelve months after she started the job, and only if HMRC's reconciliation process flags the discrepancy. That £1,047.50 has effectively been an interest-free loan to HMRC for the best part of a year.
Check your current tax code now to see whether you are in the same position.
The Cumulative vs. Non-Cumulative Code: A Practical Test
If you want to verify whether your employer is using a cumulative or non-cumulative code without waiting for HMRC correspondence, look at your most recent payslip. Find the "tax to date" and "pay to date" figures. Divide the "tax to date" figure by the "pay to date" figure to get your effective tax rate so far this year.
If your total pay to date is £20,000 and your tax to date is £2,800, your effective rate is 14 per cent. Given the personal allowance, that looks roughly correct for a standard employee with a cumulative code. If the same £20,000 pay to date shows tax of £3,500 or more, the non-cumulative code is costing you money.
For a more precise calculation based on your actual salary and tax code, the PAYE Calculator: Check Your Tax Code Is Correct tool lets you run the numbers against your payslip in under two minutes.
You can also cross-reference your tax code against different income scenarios using the TapTax salary tax calculator to see what your liability should look like across the year.
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The Broader Point About PAYE and Responsibility
The PAYE system is built on the assumption of continuous, single-employer employment. For the growing proportion of workers who take career breaks, switch jobs, work seasonally, or return from extended leave, that assumption fails silently and expensively. HMRC collects the overpayment, holds it, and waits. The onus is almost entirely on the individual to identify the problem, understand the mechanism, and initiate the claim.
That is not a neutral design choice. It is one that consistently benefits HMRC's cash flow at the expense of workers who may not know they are owed money at all. If you have experienced any of the triggers discussed here in the last four tax years, the possibility that you are owed a refund deserves more than a cursory glance at your last payslip.
Start by checking your tax code at TapTax. It takes less time than waiting for a letter that may never arrive.
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