Tax Code With Week 1 Basis: Why You're Overpaying

If your payslip shows a tax code followed by "W1", "M1", or the words "non-cumulative", HMRC is essentially taxing you as though every pay period is a fresh start with no memory of what you've already paid. That sounds harmless. It isn't.

The Week 1 basis tax code is one of the most misunderstood notations in the UK payroll system, and for a significant chunk of employees, it quietly creates an overpayment that HMRC will not automatically correct. You have to act. Here is what is actually happening, why it happens, and what it costs you if you do nothing.

What "Week 1 Basis" Actually Means

Under the normal, cumulative basis, your employer's payroll software adds up everything you have earned and paid in tax since the start of the tax year, then works out whether you have paid the right amount so far. If you had a month of lower earnings, the following month's payroll automatically adjusts downwards to compensate. The system self-corrects continuously.

The Week 1 basis does none of that. Each pay period, your employer simply takes one week's (or one month's) worth of personal allowance, taxes the rest at the appropriate rate, and moves on. No lookback. No correction. No memory.

For most people, this produces roughly the right result, but only if your earnings are perfectly consistent every period. The moment you have a pay rise, a bonus, a period of unpaid leave, or a gap in employment, the cumulative system would normally smooth things out. The Week 1 basis cannot. It keeps taxing you on the assumption that this week looks exactly like every other week, right up to 5 April.

Why HMRC Issues a Week 1 Code

HMRC does not issue non-cumulative codes out of malice. It does so because it lacks the information needed to issue a proper cumulative code confidently. Specifically, a W1/M1 suffix typically appears in these situations:

You Have Started a New Job

If you start a job without a P45 from your previous employer, and HMRC does not yet have your full earnings history for the year, your new employer will often receive a code on a non-cumulative basis. This prevents them from overcorrecting in either direction while the picture is unclear.

Your Tax Code Was Changed Mid-Year

When HMRC issues an emergency or revised tax code partway through the year, it will sometimes apply the W1/M1 suffix to prevent your employer from attempting a large, potentially disruptive adjustment to catch up. This is especially common after a coding notice is triggered by a change of circumstances, a benefit-in-kind adjustment, or a query on your income.

You Returned to Work After a Long Absence

Returning from extended sick leave, maternity or paternity leave, or a career break can disrupt your cumulative record sufficiently that HMRC defaults to the non-cumulative basis while it reconciles your position.

An Emergency Code Was Applied

The emergency tax code (currently 1257L) is often applied on a Week 1 basis. If your employer cannot confirm your tax code before your first payslip, HMRC instructs them to use the emergency code non-cumulatively. This is supposed to be temporary. It frequently is not.

The Real Cost of a Non-Cumulative Code

Let us make this concrete. Suppose you earn £55,000 a year and were on a career break for four months, returning to work in August. By the time you return, five months of the tax year have passed. Under a cumulative code, your employer would know you have only earned income for one of those five months and would adjust your tax accordingly, spreading your personal allowance and rate bands correctly across the year.

Under a Week 1 basis, none of that catches up. Your employer simply taxes each month as if it were a standard month. You receive five-twelfths of your personal allowance during the break (from a previous employer or not at all) and continue receiving one-twelfth per month on your return. If the earlier months were untaxed or undertaxed, that could work in your favour. But if you had tax deducted on income that did not reflect your true annual picture, the non-cumulative code will not correct it.

For higher earners, the exposure is sharper. If a bonus is paid while you are on a W1 code, it is taxed entirely in isolation. There is no scope for the payroll system to recognise that you have already used part of your basic rate band. The result can be an effective overtaxation on that single payment, with no automatic remedy until you either get the code corrected or HMRC issues a P800 at year end.

Even then, HMRC's P800 process, which we have covered in detail in our post on Annual Tax Calculation P800: What HMRC's Letter Really Means, is not guaranteed. HMRC issues P800s where it believes a reconciliation is owed, but the data feeding that process depends on accurate reports from employers. If your payroll records are consistent with the W1 code, HMRC may see no discrepancy to correct.

How to Spot a Week 1 Code on Your Payslip

Your payslip or your PAYE coding notice (form P2) will usually show the tax code. The non-cumulative basis is indicated by:

• The letters W1 after the code (for weekly pay): e.g. 1257L W1
• The letters M1 after the code (for monthly pay): e.g. 1257L M1
• The word X in some older or simplified payroll outputs
• A note on your coding notice stating "non-cumulative" or "week 1/month 1 basis applies"

If your payslip simply shows a code like 1257L with no suffix, you are almost certainly on the cumulative basis, which is correct for the vast majority of employees.

Not sure what your current code is or whether it is correct? Check your tax code free at /check-my-tax-code and find out in minutes.

Why the Code Does Not Fix Itself

This is the frustrating part. HMRC's systems are largely automated, but the trigger to remove the W1/M1 suffix requires either an employer action (submitting accurate year-to-date figures via RTI, Real Time Information) or an explicit update from HMRC issuing a fresh cumulative coding notice.

Employers do not always initiate this. Many payroll operators simply process what the coding notice says without querying whether the non-cumulative flag should be lifted. HMRC, for its part, will not always reissue a cumulative code unless prompted by new information or by you contacting them directly.

The net effect is that a W1/M1 code can persist for an entire tax year. And because the non-cumulative basis does not produce a running total of under or overpayments within your payroll, the discrepancy only becomes visible at year end, if at all.

This is distinct from, say, an incorrect code number (like a T code reducing your allowance) where the error is immediately visible in every payslip as a higher-than-expected deduction. With W1/M1, the monthly deduction can look plausible, particularly if your earnings are relatively stable, while a structural undercorrection accumulates silently.

For more on how HMRC coding notices work and what the letters in your code actually mean, Tax Code Suffixes UK: What the Letter After the Number Does is worth reading alongside this post.

What to Do If You Are on a Week 1 Basis

Step 1: Confirm the Basis

Check your most recent payslip and your PAYE coding notice. Log in to your HMRC Personal Tax Account at gov.uk (or use the HMRC app) and look at the coding notices section. Your current code and its basis should be listed there.

Step 2: Work Out Whether It Is Causing an Overpayment

If you started the year with a gap in employment, variable earnings, or changed jobs without a P45, there is a reasonable chance the non-cumulative basis has left you overtaxed. Compare your actual year-to-date earnings with what a standard 1257L cumulative calculation would produce.

You can use our salary tax calculator to model what your expected cumulative tax position should be, then compare it against what your payslips show has actually been deducted.

Step 3: Contact HMRC to Request a Cumulative Code

Call HMRC's income tax helpline (0300 200 3300) or use the online messaging service in your Personal Tax Account. Explain that you are on a W1/M1 basis and provide your year-to-date earnings and tax paid. HMRC can issue a revised cumulative coding notice to your employer, which should result in an automatic adjustment in your next payslip.

Do not wait until 5 April. The sooner this is corrected, the sooner your payroll starts adjusting. Waiting until year end means a longer wait for any refund via P800 or Self Assessment.

Step 4: Follow Up With Your Employer

Once HMRC issues a new cumulative code, your employer's payroll department needs to action it. This sometimes requires a nudge. Ask your payroll team to confirm they have received the updated coding notice and applied it cumulatively.

If You Have Multiple Income Sources

The W1/M1 complication is even more pronounced if you have more than one source of PAYE income, for example a second job or a pension alongside employment. HMRC allocates personal allowance and rate bands across income sources via the coding system. If one source is on a non-cumulative basis, the interaction between the two becomes especially difficult for payroll to handle correctly.

Our multiple income tax calculator can help you model your expected combined tax position if you are in this situation.

If you also receive Child Benefit and earn above £60,000 (or between £60,000 and £80,000 under the High Income Child Benefit rules as updated from April 2024), an incorrect W1 code on your primary employment could interact badly with the High Income Child Benefit Charge. Our Child Benefit tax calculator lets you check your exposure.

The Accountability Question

It is worth asking why this situation arises in the first place. HMRC's RTI system, introduced in 2013, was specifically designed so that employers report pay and tax data to HMRC in real time, giving HMRC a live picture of every employee's position. In theory, HMRC should never need to resort to a non-cumulative code for long, because it has the data to issue an accurate cumulative one almost immediately.

In practice, the system relies on employers submitting accurate, timely data and HMRC's systems processing it without delay. Both conditions fail often enough that W1/M1 codes persist well beyond their intended temporary status. According to HMRC's own annual reports, coding errors affect millions of taxpayers each year, with a significant proportion receiving P800 underpayment or overpayment notices.

The burden of correction falls almost entirely on the employee. HMRC's automation will not always catch it. Your employer's payroll team may not flag it. The only person consistently motivated to resolve it is you, because only you lose money if it goes uncorrected.

If you have recently had a tax code change you did not expect, a W1/M1 suffix appearing at the same time is a strong signal that HMRC issued an interim code while waiting for more information. That interim status may already have expired. Check your tax code now at /check-my-tax-code and do not assume the system has caught up.

Conclusion

A tax code with a Week 1 basis is not an obscure technicality. It is a live mechanism that can silently shift money from your pocket to HMRC's, with no automatic trigger to return it. The W1 or M1 suffix on your payslip is HMRC effectively saying: we do not have the full picture, so we are taxing you in the simplest way possible. The problem is that simple is not always correct.

If you started a new job without a P45, returned from a leave period, or had your code changed mid-year, there is a real chance you are sitting on an overpayment right now. Do not wait for HMRC to find it. Check your tax code free at /check-my-tax-code and find out today.