If you’ve noticed your tax-free allowance has gone down, HMRC has adjusted your tax code because something in your tax position now needs to be collected through PAYE. The standard personal allowance is £12,570, frozen at that level since 2021 and set to stay there until at least April 2031.1GOV.UK. Income Tax – Maintaining the Personal Allowance and the Basic Rate Limit When HMRC reduces the tax-free portion of your code, more of your salary becomes taxable and your take-home pay falls straight away. The cause is almost always one of a handful of things: high earnings, employer benefits, past underpayments, other untaxed income, an emergency code, or a change to Marriage Allowance.
You’re Earning Over £100,000
This is the single biggest reason people see their allowance shrink sharply. Once your adjusted net income crosses £100,000, your personal allowance drops by £1 for every £2 you earn above that threshold. By the time your income reaches £125,140, the allowance is gone entirely and every pound is taxable.2GOV.UK. Income Tax Rates and Personal Allowances Your tax code will often switch to “0T,” meaning no personal allowance is being applied.3GOV.UK. Understanding Your Employees Tax Codes – What the Letters Mean
The effect in that band is severe. Income between £100,000 and £125,140 is effectively taxed at 60%: you pay 40% on the earnings and lose £1 of tax-free allowance for every £2 earned, and that lost allowance was shielding income from 40% tax.
“Adjusted net income” is not simply your gross salary. HMRC takes your total taxable income and subtracts certain deductions, including pension contributions paid gross and Gift Aid donations.4GOV.UK. Personal Allowances – Adjusted Net Income That gives you a lever. Increasing pension contributions can push your adjusted net income back below £100,000 and restore the allowance. Someone earning £105,000 who makes £5,000 in additional pension contributions recovers the full personal allowance, saving around £2,500 in tax on top of the normal tax relief on the contribution itself.
HMRC uses real-time payroll data to estimate your annual earnings, so if your pay is variable, your code may change more than once in a year. Timing lags can mean you overpay or underpay for months before the position settles after the tax year ends.
You Get Benefits in Kind From Your Employer
Non-cash perks such as a company car, private medical insurance, or a low-interest loan are taxable. HMRC usually collects the tax by reducing your personal allowance so it comes out of your salary gradually. If your company car has a taxable value of £5,000, your tax-free allowance drops from £12,570 to £7,570, and you pay tax on the extra £5,000 through payroll.
Most benefits have historically been reported on a P11D after the tax year ended, and HMRC then adjusted the following year’s code on the assumption you’d receive similar benefits.5GOV.UK. Expenses and Benefits for Employers – Reporting and Paying That is where surprises happen. If you switched to a more expensive car or gained a new benefit, your code drops once HMRC processes the updated P11D, and the change feels sudden because last year’s data is catching up with this year.
From April 2027, employers will have to report most benefits in kind through payroll in real time, with voluntary adoption available from April 2026.6GOV.UK. Getting Ready for Mandatory Payrolling of Benefits in Kind If your employer has already moved to payrolling, tax on your benefits comes off each payslip directly rather than through a code reduction, which can also change how your allowance looks on paper.
You Underpaid Tax in a Previous Year
When you owe tax from a prior year, HMRC prefers to collect it by lowering your current tax-free allowance rather than asking for a lump sum. This often happens when you change jobs and both employers briefly apply the full personal allowance, or when HMRC’s earlier estimate of your untaxed income was too low.
There are limits. If the underpayment is under £3,000, HMRC will normally code it out. For underpayments above £3,000, they will only do so if your income exceeds £30,000. The deduction taken from your pay in any period cannot exceed half your pre-tax wages.3GOV.UK. Understanding Your Employees Tax Codes – What the Letters Mean HMRC sends a P2 Notice of Coding setting out the arithmetic: your personal allowance, the deduction for the prior-year debt, and the resulting code.7HM Revenue and Customs. PAYE Manual – Coding – P2 Notice of Coding
Check the figures on the P2 carefully. HMRC sometimes bases the adjustment on outdated information, and you can challenge it through your personal tax account or by phone.
You Have Other Income That Isn’t Taxed at Source
Income that arrives without tax already deducted gets swept into your PAYE code so HMRC can collect what’s owed. The most common example is the State Pension: it’s taxable, but the Department for Work and Pensions pays it gross. HMRC accounts for this by reducing the personal allowance on your employment or private pension code, so the employer or pension provider deducts enough tax to cover both income streams.2GOV.UK. Income Tax Rates and Personal Allowances
For someone with a small part-time job alongside the full new State Pension, the pension can consume nearly all of the personal allowance. That leaves almost the entire job salary taxable at the basic rate, which comes as a shock to people who expected their wages to be tax-free. The same logic applies to rental income, untaxed savings interest above your savings allowance, or dividends above the dividend allowance. HMRC estimates the untaxed amount and builds it into your code.
If your rental income drops or a payment stops, tell HMRC through your online account so they can adjust the code. Leaving an outdated estimate in place means you’ll overpay tax all year and wait months for a refund.
You’re On an Emergency Tax Code
Starting a new job, receiving a company benefit for the first time, or beginning to draw the State Pension can all trigger an emergency tax code. You’ll spot one by the suffix on your payslip: W1 (weekly paid), M1 (monthly paid), or X (variable pay dates). Some payslips show “NONCUM” instead.8GOV.UK. Emergency Tax Codes
Emergency codes calculate tax on the pay period in isolation, as if you’ll earn that amount every week or month for the full year. Normal PAYE is cumulative and smooths tax across the year using your total earnings so far. Emergency codes ignore that, and the usual result is too much tax being deducted, especially if you started the job partway through the year. Your allowance hasn’t technically changed, but the emergency code stops it being applied properly.
These codes usually resolve once your new employer receives your details from HMRC or processes your P45. If weeks pass with no correction, contact HMRC through your personal tax account to push it along. Any overpaid tax will be refunded, but it can drag on until the end of the tax year if no one flags it.
You Have a K Tax Code
A K code appears when the deductions against your allowance are larger than the allowance itself. It happens when your benefits in kind, State Pension, and any prior-year underpayment together exceed £12,570.3GOV.UK. Understanding Your Employees Tax Codes – What the Letters Mean Instead of giving you tax-free income, a K code adds a taxable amount to your salary. A code of K500 means £5,000 is added to your taxable income for the year.
The safeguard is that tax collected through a K code in any pay period cannot exceed half your pre-tax pay. If everything owed can’t be collected within the year, the balance carries into the next year’s code.
Marriage Allowance Has Stopped or Changed
Marriage Allowance lets a lower-earning spouse or civil partner transfer £1,260 of their personal allowance to their partner, cutting the recipient’s tax bill by up to £252 a year.9GOV.UK. Marriage Allowance – How It Works The transferring partner must earn less than the personal allowance, and the recipient must be a basic rate taxpayer, or a starter, basic, or intermediate rate taxpayer in Scotland.10GOV.UK. Marriage Allowance Transfer Form
If you’d been getting the Marriage Allowance boost and your tax-free allowance has now dropped, something in that arrangement has ended. The transferring partner may have started earning above the personal allowance, or the recipient may have moved into the higher rate. Divorce, dissolution of a civil partnership, or cancelling the transfer all end it. Once it stops, the recipient’s allowance reverts from £13,830 to £12,570, and the monthly tax deduction rises accordingly.
How to Check and Fix Your Tax Code
HMRC’s online personal tax account is the fastest way to see exactly what’s driving your current code. You can check your personal allowance, see the deductions applied, update your income details, and report changes in your circumstances.11GOV.UK. Check Your Income Tax for the Current Year If you spot a benefit you no longer receive or an estimated income figure that looks too high, you can correct it there.
Your P2 Notice of Coding shows the calculation behind the code. It lists your personal allowance at the top, then every item that reduces it: benefits in kind, estimated untaxed income, prior-year underpayments. Read each line against what you actually receive. HMRC often carries forward last year’s benefit figures, and if your circumstances have changed the numbers may be wrong. Catching an error in April saves months of overpaying.
If you can’t use the online service, or if the account won’t let you change what needs changing, call HMRC’s income tax helpline with your National Insurance number and most recent payslip. Corrections made early in the tax year spread across the remaining months; corrections made late can produce a large one-off adjustment in your final payslip of the year.