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UK Tax Codes Explained

When a UK employee’s net pay suddenly drops (or they receive a bill for “underpaid tax”), the culprit is often a change to their tax code. If you manage UK employees from overseas, this page gives you the essentials: what a tax code is, why it changes, what you must do as an employer, and how to keep employees calm and compliant.
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What is a UK tax code?

A tax code is HMRC’s instruction to the payroll on how much of an employee’s earnings should be tax-free in the current tax year. Payroll uses that code to spread the tax-free amount evenly over the year so the right amount of PAYE income tax is deducted each pay period. HMRC issues the code directly to the employer/payroll agent and to the individual.

Example: Code 1257L means a standard personal allowance of £12,570 for the year (numbers reflect £12,570 with the last digit dropped; “L” is the common suffix for the basic allowance).

Why employees get upset about tax codes

  • Net pay can change mid-year. HMRC can revise a code part way through the year (for example, after benefits are reported or new information is received), which immediately affects take-home pay.
  • Under/over-deductions build up silently. An incorrect code can mean not enough (or too much) tax is collected during the year, leading to unexpected bills or refunds after year-end.
  • Employers don’t control the code math. Payroll only receives the final code, not HMRC’s workings, so you can’t “fix” the amount in-house. Only the employee (or their tax agent) can ask HMRC to amend it.

Who sets the code — and what’s in it?

HMRC sets the code and sends it to you (or your payroll provider) electronically. You must operate exactly the code HMRC issued — there’s no discretion to override it, even if everyone suspects it’s wrong.

A tax code starts with the personal allowance and is then adjusted for items HMRC expects to tax or relieve through PAYE, such as:

  • Benefits in kind (e.g., private medical insurance, company car/fuel) unless fully payrolled,
  • Prior-year underpaid tax to be collected in-year,
  • Job expenses/allowances HMRC has agreed to give “through the code.”


Practical impact: when you submit P11D benefits after year-end, HMRC may use that data to change the next year’s code, affecting net pay.

Common letters you’ll see (quick decoder)

  • L – standard personal allowance
  • M / N – Marriage Allowance transferred in/out
  • S / C – Scottish / Welsh taxpayer (different income tax bands apply)
  • BR / D0 – no allowance; tax everything at 20% / at 40% (often temporary)
  • 0T – no allowance available (tax at progressive rates)
  • W1 / M1 – “week 1 / month 1” emergency basis (doesn’t look back)
  • K – negative allowances (deductions exceed allowances)
  • T – HMRC needs to review elements annually


(Your employees will see these on their coding notice; you’ll only see the final code value to operate.)

How to prevent noise and payroll surprises (HR playbook)

  1. Proactive comms: At the start of each UK tax year (April), send a short explainer: “You’ll receive a coding notice from HMRC; check it and tell HMRC if it’s wrong.”
  2. Point to the right channel: Share HMRC’s Personal Tax Account link so employees can view and update their code online fast (changes feed straight to payroll).
  3. Benefits timing: Warn staff that submitting benefits (P11D) or switching to payrolled benefits (mandatory from April 2027) can alter codes and net pay.
  4. Offer help: Let employees forward coding notices to us for a quick review; if they want, we can act as their personal tax agent to correct HMRC directly.
  5. Escalation path: If an employee faces hardship due to an in-year underpayment coded in, they can ask HMRC to spread or remove the in-year recovery (it may then be collected next year).

How we help your UK workforce (and you)

  • Tax code triage: We review employees’ coding notices and flag likely errors.
  • Agent support (optional): With the employee’s consent, we contact HMRC to correct the code on their behalf and monitor for updates.
  • Payroll alignment: We check for new codes daily and update payroll immediately, minimising back-pay shocks.
  • Comms toolkit for HR: Ready-to-use email templates and FAQs you can forward to staff when codes change (examples mirrored from our client guidance).

FAQ for HR

If HMRC sent that code, you must use it. The employee should check their coding notice and either adjust it in their Personal Tax Account or authorise us to liaise with HMRC.
No. You must apply the HMRC code immediately. We can help the employee ask HMRC about phasing any in-year underpayment recovery.
HMRC often reduces next year’s allowances to collect tax on benefits. That change arrives as a new code.
No — only the individual gets that breakdown. We can review their notice with them and outline the correction for HMRC.

Sample message you can send to employees

About your tax code

HMRC sets your tax code and sends it to us. We must use the code exactly as issued. If your net pay changes unexpectedly, please log in to your Personal Tax Account to review your code and request any corrections. If you’d like help, our advisers can review your coding notice and (with your consent) contact HMRC to update it.

Need a calm pair of hands?

If tax codes are creating noise in your UK team, we’ll set up a tax-code support lane for your employees and give HR a simple communications toolkit. That keeps your people focused — and your payroll compliant.

FAQs

A tax code is a number and letter combination issued by HMRC that tells the employer how much tax-free income an employee can earn before tax is deducted. It ensures the correct amount of tax is collected through PAYE.
Tax codes can change when an employee’s circumstances change – for example if they start receiving a benefit-in-kind (like private medical insurance or a company car), if they owe tax from a previous year, or if HMRC updates its estimates of their income.
Not usually. Tax codes are set by HMRC, not the employer. The employer simply applies the tax code provided by HMRC through payroll. If the code is incorrect, the employee must contact HMRC to have it corrected.
The numbers (e.g. 1257) show how much tax-free income the employee has. The letters (e.g. L, M, N, K, BR, D0, T, S, C) indicate adjustments or special rules, such as Scottish or Welsh tax, marriage allowance transfers, or benefits.
Employees should check their payslips, coding notices, or HMRC’s online tax account. If their net pay suddenly decreases or they receive an unexpected tax bill, it may indicate their code needs reviewing.

No. Only HMRC can change a tax code. Employers must use the code issued by HMRC. However, HR managers can support employees by explaining what tax codes mean and encouraging them to contact HMRC directly.

Employers can:
Provide employees with payslips showing the code used
Share HMRC’s contact details for tax code queries
Engage UK tax advisors (like us) to explain codes, review cases, and help resolve disputes with HMRC
If an incorrect code is applied, too much or too little tax may be deducted. HMRC will usually correct this by adjusting the tax code later in the year. Employees may also need to file a UK tax return to reconcile underpaid or overpaid tax.
Because the impact is seen in their payslip. When take-home pay changes, employees often assume the employer made an error. In reality, tax code changes are HMRC-driven. HR managers should be ready to explain this clearly to reduce employee frustration.
Yes. If an employee has more than one job or pension, HMRC will split the personal allowance across employments or issue special codes (such as BR or D0) to ensure the right tax is collected.

Contact Our Experts

Get in touch to confirm your UK PE position and meet your UK tax obligations correctly. Whether you’re just getting started or need to fix past compliance, we’re here to help.

Sarah English

Senior Manager

Head of Direct Taxes

sarah.english@tuerner.com
+44 (0) 1636 526 075

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