Why has my tax code changed? Causes and fixes

GOV.UK lists 11 separate reasons why HMRC may change a tax code, from starting a new job to claiming Marriage Allowance or owing tax from an earlier year ^[1]. Once HMRC has the information it needs, it tells both the employee and the employer the new code within 15 working days ^[2].

A tax code change is not, in itself, a sign that something has gone wrong. The code is HMRC's instruction to the employer about how much tax-free pay to give in each period, and it moves whenever HMRC's picture of the employee's income or allowances moves ^[18]. A new company car, a State Pension, a second job or a Self Assessment debt all reshape that picture.

What makes a change confusing is that it usually arrives with little explanation. The employee sees a different number on the payslip, the take-home pay shifts, and the employer is the first person asked why. The employer, however, simply operates the notice HMRC sends and cannot alter the code on its own initiative ^[5].

This article explains how a tax code is built, the common reasons it changes, how in-year and annual changes differ, worked figures for the most frequent scenarios, how a new code reaches payroll, and what employees and employers should do when a change looks wrong.

Key takeaways

• HMRC changes a tax code when it expects the employee to pay a different amount of tax, for example after a new job, a new benefit in kind or a pension starts ^{[1] [18]}.
• HMRC notifies the new code to the employee and employer within 15 working days of having the right information ^[2].
• Monthly-paid employees usually see the change on the next or following payslip; weekly-paid employees by the third payslip ^[2].
• Underpaid tax, including Self Assessment bills under £3,000, can be collected by reducing the code ^{[7] [6]}.
• The employer must operate the code HMRC issues; the employee corrects errors directly with HMRC, most quickly through the online service or the HMRC app ^{[3] [5]}.

How a tax code is built

Every suffix code starts from the Personal Allowance, which is £12,570 for the 2026-27 tax year ^[14]. HMRC then subtracts amounts it wants to tax through the code, adds any extra allowances, divides the result by ten and attaches a letter ^{[4] [5]}.

The building blocks of the number

The table shows the typical items that move the number up or down. A change to any of them produces a new code.

When the deductions exceed the allowance, the code turns into a K code. Payroll multiplies the K number by ten and adds that amount to taxable pay, so K475 on a £27,000 salary produces taxable income of £31,750 ^{[5] [4]}.

The letters that can change

The letter is as significant as the number. A move from L to M or N signals a Marriage Allowance claim, a new S or C prefix signals a change of residence between Scotland, Wales and the rest of the UK, and a move to BR, D0 or 0T signals that the allowance is being used elsewhere ^{[4] [5]}. The guide to the 0T tax code explains the zero-allowance case in detail.

The common reasons a tax code changes

HMRC's own list of triggers is the best starting point. The table below sets out each reason GOV.UK gives and the usual direction of the change ^[1].

Job changes and second jobs

A new job is the most frequent trigger. Until the new employer receives a P45 or a completed starter checklist, it may operate a temporary code such as 1257L on a Week 1 or Month 1 basis, BR or 0T ^{[17] [4]}. Once HMRC links the employment to the employee's record, it issues a proper code and the temporary one disappears.

Where an employee holds two jobs at once, HMRC usually gives the full allowance to the main job and a BR code to the second ^{[5] [1]}. The guide to the BR tax code covers that arrangement and the allowance-splitting option. More on temporary codes sits in the article on emergency tax codes.

Benefits in kind and expenses

Most taxable benefits that are not payrolled are collected through the code, so a new company car or private medical cover reduces the allowance ^{[1] [6]}. When the benefit stops, the reduction is reversed and the code rises again. The article on how a P11D affects a tax code traces that link step by step.

That source of code changes is due to shrink. Mandatory payrolling of benefits begins with company cars, car fuel, vans, van fuel and medical benefits from 6 April 2027, with most other benefits following from 6 April 2028 ^[16]. Loans and accommodation remain voluntary ^[16]. The practical changes for employers are set out in the guide to mandatory payrolling of benefits.

Expenses work in the other direction. Approved job expenses that qualify for tax relief increase the code, so an employee who claims them sees a slightly higher take-home figure ^{[1] [4]}.

Pensions and the State Pension

The State Pension is normally paid without tax deducted, so HMRC collects the tax due on it by reducing the code on another job or pension ^[15]. In one illustration, a code of 205L reflects an allowance reduced by £10,520, which should match the taxable State Pension for 2026-27 ^[15].

Because the State Pension rises each April, HMRC lists a change in the weekly amount as a standard reason for a new code ^[1]. Starting a workplace or private pension also creates a new source of PAYE income that HMRC must code ^[11].

Marriage Allowance

Marriage Allowance lets one spouse or civil partner transfer £1,260 of Personal Allowance to the other, worth up to £252 for the 2026-27 tax year ^[8]. Neither partner may pay tax above the basic rate, or above the intermediate rate in Scotland ^[8].

Savings interest

Basic rate taxpayers have a Personal Savings Allowance of £1,000, higher rate taxpayers £500 and additional rate taxpayers nothing ^[10]. Where interest exceeds the allowance, HMRC can collect the tax by reducing the PAYE code, which is why GOV.UK lists savings interest as a reason for a change ^[1].

Tax owed, Self Assessment and the Child Benefit charge

HMRC can collect several kinds of debt through the code. A Self Assessment bill under £3,000 can be coded out where the taxpayer already pays tax through PAYE and filed by 31 October on paper or 30 December online ^[7]. Underpayments from an earlier year on the wrong code are collected in the same way ^[1].

The High Income Child Benefit Charge can also be paid through the code. The charge applies at 1% of Child Benefit for each £200 of adjusted net income above £60,000, and removes the benefit entirely at £80,000 ^[9]. Since October 2025, employees with PAYE income can ask HMRC to collect it by adjusting the code instead of filing a tax return ^[9].

Income above £100,000

The Personal Allowance falls by £1 for every £2 of adjusted net income above £100,000 and disappears at £125,140 ^[14]. A pay rise or bonus that pushes HMRC's estimate past £100,000 therefore produces a lower code, and one that pushes it past £125,140 produces 0T ^{[14] [4]}.

In-year changes and estimated income

Many changes arrive part-way through the tax year, prompted by Real Time Information from employers or by updates the employee makes online. HMRC recalculates the whole year when a change occurs ^[6].

How in-year adjustments work

Since 2 July 2017, HMRC has carried out "a full calculation for the year" when a code changes, and where extra tax is due the new code includes an in-year adjustment ^[6]. That adjustment is collected over the remaining weeks or months of the tax year, on a Week 1 or Month 1 basis ^[6].

Each new calculation replaces any earlier adjustment, so several code changes in one year do not stack up separate debts ^[6]. Where collecting the full amount would breach the 50% limit on deductions, the balance moves to the following year ^{[6] [5]}.

Why estimated income matters

HMRC's calculation depends on its estimate of annual income from each job and pension. The Check your Income Tax service shows that estimate and lets the employee update it, because out-of-date details can lead to paying too much or too little tax ^[3]. A one-off bonus or a temporary period of overtime can push the estimate up and reduce the code, even if annual pay ends up lower ^{[3] [6]}.

Annual changes at the start of the tax year

Not every change is a response to new circumstances. Each spring, HMRC issues codes for the new tax year, and employers apply them from 6 April ^[12].

The three types of coding notice

Employers view coding notices through PAYE Online or receive them through payroll software ^[13]. The table summarises the main types.

For 2026-27, the P9X tells employers to copy the authorised code from the 2025-26 record where no new code has arrived, and confirms that "the emergency code is 1257L for all employees" ^[12]. It also instructs employers not to carry over any Week 1 or Month 1 markings ^[12].

Why a basis change alone looks like a new code

An employee whose code moves from 1257L M1 to 1257L at the start of a tax year has, in effect, a new code even though the number is unchanged. Moving from Week 1 or Month 1 to cumulative means payroll looks back to 6 April, so any tax overpaid since then can be refunded on the next payslip ^{[4] [17]}.

Worked examples of a changed code

The figures below show how common triggers translate into a new code. They are illustrative: HMRC's own calculation also reflects any other adjustments on the employee's record.

A new company car mid-year

HMRC's PAYE Manual illustrates a £5,000 company car benefit added on 31 October. The full-year recalculation produces an in-year adjustment of £583.40, which HMRC's system converts into a code of K27 operated on a Month 1 basis ^[6]. The employee's take-home pay drops for the rest of the year, then the Month 1 marking falls away at the next 6 April ^[12].

A Self Assessment debt coded out

A basic rate taxpayer owes £1,500 from a Self Assessment return filed online before 30 December, and HMRC collects it through PAYE ^[7]. To collect £1,500 at 20%, HMRC needs to remove about £7,500 of allowance, so 1257L becomes roughly 507L for the following tax year ^{[14] [4]}. Spread across 12 months, that is £125 a month of extra tax.

A pay rise above £100,000

An employee whose estimated adjusted net income rises to £110,000 loses £5,000 of allowance, leaving £7,570 and a code of 757L ^[14]. If the rise happens mid-year, HMRC's in-year adjustment collects the shortfall for the months already paid over the rest of the year ^[6].

How a new code reaches the payslip

HMRC sends the employee a coding notice and sends the employer a P6 or P9 ^{[13] [18]}. Timing then depends on pay frequency and on when the notice lands relative to the payroll cut-off.

Timing by pay frequency

GOV.UK sets out when the new code should appear once HMRC has notified it ^[2].

If the new code does not appear in that window, the employee should check that the employer has received the notice ^[2]. Employers can check notices in PAYE Online, in payroll software or through the PAYE Desktop Viewer ^[13].

Refunds and underpayments after the change

On a cumulative code, payroll recalculates tax from 6 April and either refunds overpaid tax or collects the shortfall gradually, subject to the 50% limit ^{[5] [4]}. Anything left after the tax year closes is reconciled through a P800 tax calculation, sent between June and March of the following year ^[11]. Debts above £3,000 that cannot be collected through the code are handled by Simple Assessment ^[11].

What to do when a new code looks wrong

A changed code is only a problem when HMRC's information is wrong. GOV.UK places the responsibility for checking on the taxpayer, stating that it is the individual's responsibility to make sure the right amount of tax is paid ^[1].

Steps for the employee

The quickest route is the Check your Income Tax online service or the HMRC app, which shows the current code, the Personal Allowance, estimated income and whether the code has changed ^[3]. The service also lets the employee update income, employer and pension details ^[3].

Employees who have just started a new job should wait 35 days before contacting HMRC, since the starter information may still be on its way ^[2]. Once corrected, the new code reaches the employer within 15 working days ^[2].

Steps for the employer

The employer's role is narrow but strict: operate exactly the code and basis HMRC notifies, from the next payday, and keep the most recent notice on file ^{[12] [13]}. Payroll cannot reverse a code because an employee disputes it; the employee must resolve the dispute with HMRC ^{[2] [5]}.

Accurate Full Payment Submissions reduce unnecessary code changes, because HMRC's estimates are built on the pay data employers report ^{[6] [3]}. HMRC-recognised SME payroll software imports P6 and P9 notices automatically and applies the right basis without manual re-keying.

Bureaux and embedded payroll

Accountants handling many client schemes receive coding notices in volume, especially around 6 April. A payroll bureau platform that matches each notice to the correct employee and flags codes still on a Week 1 or Month 1 basis keeps those changes under control ^{[12] [13]}.

Platforms that embed UK payroll need the same capability through code. An HMRC-recognised payroll API should accept code updates at any point in the year, store the basis separately from the code, and recalculate cumulatively as soon as a new notice applies ^{[5] [6]}.

Decode a new tax code

Before raising a query, an employee or employer can decode the old and new codes side by side with the Moonworkers tax code checker, which applies the 2026-27 allowance and bands to any code.

Conclusion

A changed tax code is PAYE doing what it was designed to do: keeping deductions in step with an individual's real circumstances during the year, so fewer people face a bill or a refund after 5 April. Most changes trace back to a short list of causes, chiefly new jobs, benefits, pensions, allowance transfers and tax owed, and each leaves a recognisable signature in the number and letters of the code.

The next shift will come from the payroll side. As mandatory payrolling of benefits takes effect from April 2027, a large category of code adjustments will move into the payroll calculation itself, and codes should become simpler and more stable. Employers who report accurately in real time and operate notices promptly will see the benefit first.

Frequently asked questions

Why has a tax code changed without any letter from HMRC?

HMRC may have notified the change through the online account or the HMRC app rather than by post, and the employer receives its own notice separately ^{[3] [18]}. The Check your Income Tax service shows the current code and whether it has changed. Common triggers include a new benefit, a pension or tax owed from an earlier year ^[1].

Can an employer change an employee's tax code?

No. The employer must operate the code HMRC notifies or, for a new starter without a code, the code the starter procedure requires ^{[5] [17]}. If the code seems wrong, the employee contacts HMRC, which issues a corrected code to both parties within 15 working days ^[2].

Why has a tax code gone down after a pay rise?

HMRC estimates annual income from the pay employers report, so a rise or bonus can lift the estimate ^[3]. Above £100,000 of adjusted net income the Personal Allowance is tapered, and an in-year adjustment may also collect tax underpaid earlier in the year ^{[14] [6]}. Updating the estimate online corrects a figure inflated by a one-off payment.

Will a tax code change produce a refund?

It can. If the new code is higher and operated on a cumulative basis, payroll recalculates from 6 April and refunds the excess on the next payslip ^[4]. Any overpayment still outstanding at the end of the tax year is reconciled through a P800 tax calculation sent between June and March ^[11].