A tax code change can quietly cost you money. What triggers one, how to read your P2 notice, and how to query it with HMRC in the 2026/27 tax year.
Most people meet their tax code once, on their first payslip, and never think about it again. Then one month the number is different, the take-home is different, and there is no obvious reason why.
A tax code change is HMRC telling your employer to tax you differently from now on. Sometimes it is a correction you asked for. Sometimes it is an assumption HMRC made on your behalf, based on data that may or may not be right. Either way, your payroll team will apply it without question, because they have to. Checking it is your job.
HMRC has revised how much of your tax-free personal allowance you get for the rest of this tax year, and told your employer to spread the new figure across your remaining pay periods.
That is the whole mechanism. Your code is not a rate and it is not a judgement about you. It is an instruction about one number: the slice of your pay that is protected from income tax. For the 2026/27 tax year the standard allowance is still £12,570, which is where the common code 1257L comes from. Every code above or below that is the standard allowance with something added or taken away.
This is the part that trips people up. When HMRC wants to collect £400 of underpaid tax from you through your payslip, they do not reduce your allowance by £400. They reduce it by the amount of income that would produce £400 of tax at your rate.
Worked example. You earn £34,000, you are a basic-rate taxpayer, and you underpaid £400 last year.
Now the same logic in reverse. You start getting private medical insurance from your employer worth £600 a year. That benefit is taxable, so HMRC reduces your allowance by the full £600, your code becomes 1197L, and you pay 20% of £600 across the year, roughly £10 a month. The benefit is not deducted from your wages, it is taxed through your code.
One more, because it is the one that alarms people most. Your code goes from 1257L to 967L with no letter you remember reading. That is £2,900 of allowance removed, which at 20% costs you £580 a year, about £48 a month. Something worth £2,900 has been added to your taxable income: a company car, a fuel allowance, or an estimate of untaxed income from elsewhere.
If your code changed and you can work out the gap in pounds, you can usually work out the cause.
Six things account for the overwhelming majority of them.
You started a new job. Until your new employer has your P45 or your starter details, HMRC may put you on an emergency code with W1 or M1 after it. That is temporary and it usually resolves itself, but it often overtaxes you first.
A benefit in kind started or stopped. Company car, fuel card, private medical, gym membership, interest-free loan over £10,000. Anything your employer gives you that has a cash value gets taxed through your code.
HMRC is recovering underpaid tax. A P800 calculation, an unreported side income, or bank interest above your savings allowance. HMRC can collect up to £3,000 of underpaid tax this way for most people, which is why an unexpected drop in your code is so often last year catching up with you.
You claimed or received Marriage Allowance. The partner transferring £1,260 of allowance moves to a code ending in N. The partner receiving it moves to a code ending in M.
You have a second income. A second job, a pension alongside a salary, or rental income HMRC has picked up. The second source is usually taxed at BR, or your main code is reduced to cover it.
Your estimated pay changed. If HMRC thinks you will earn more this year than last, they may adjust your code in advance to avoid an underpayment. This one is worth watching, because HMRC’s estimate is built on last year’s figures and can be badly wrong for anyone with variable hours or seasonal shifts.
That last point matters most to anyone on shift work. If you did a run of overtime in one quarter, HMRC may extrapolate it across the whole year and tax you as though the overtime is permanent.
The change is probably right if you can point at the event behind it. You did start a job. You did get the car. You did claim Marriage Allowance. A code that moves because a real thing happened is doing its job.
Treat it as suspect if any of these are true:
A K code is not automatically an error, but it is always worth reading line by line, because it means HMRC believes you owe more than your allowance covers.
Accept it. If the figures on the P2 match your real circumstances, do nothing. The code flows to your employer automatically.
Update the underlying figures yourself. This is the option most people skip and it is usually the fastest. In the HMRC app or your Personal Tax Account you can correct an estimated income, remove a benefit you no longer receive, or tell HMRC about a job that ended. The code recalculates from your corrections, often within a few days, with no phone call.
Query it with HMRC. If the notice is built on something you cannot correct yourself, or you simply cannot work out where a figure came from, ring 0300 200 3300. Have your National Insurance number and the notice in front of you, and ask them to talk you through each line of the calculation. You are entitled to that explanation.
What you cannot do is ask your employer to change it. Payroll has no authority to override an HMRC code, and asking them to puts them in an awkward position. The only thing worth raising with payroll is a code that HMRC has already issued but your payslip is not yet showing.
The personal allowance is unchanged at £12,570 and remains frozen to April 2028. The higher-rate threshold is still £50,270. So the arithmetic behind any tax code change this year works exactly as it did last year.
What did change is the delivery. Since April 2026 HMRC issues most P2 coding notices digitally rather than by post. The notice lands in your HMRC online account, and the heads-up is an email or an app notification that is very easy to miss in a busy inbox.
This is the practical consequence: a lot of people in 2026 have had a tax code change they were properly notified about and never saw. If your pay moved and no letter arrived, that is not evidence that nothing happened. Open the app.
Here is what I would check first: pull up your last three payslips and find the code on each one. If it moved, the size of the move tells you most of what you need to know before you speak to anyone.
If you want to see what a new code does to your actual take-home before it hits your bank account, the NetPay app works it out across income tax, National Insurance, pension and student loan. Free to download.
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The most common triggers are a new job, a benefit in kind starting or stopping, HMRC recovering underpaid tax from a previous year, a Marriage Allowance claim, or a second income HMRC has learned about. Every one of these changes how much of your £12,570 personal allowance is available for the rest of the year.
No. Roughly half the changes HMRC makes give people more allowance, not less. A code going up usually means a benefit ended, an expenses claim was approved, or an earlier underpayment has been cleared. A code going down usually means something new is being taxed through your payslip.
Call HMRC on 0300 200 3300 with your National Insurance number and the P2 notice to hand, or use the HMRC app to update the estimates the code is built on. If your only problem is a wrong estimate of your pay or your benefits, updating it yourself in the app is usually faster than the phone.
HMRC sends the new code to your employer electronically, usually within a few days of issuing your P2. Whether it lands in this month's pay or next depends on where your employer is in their payroll cycle. If two full pay periods pass and nothing has changed, ring your payroll team before you ring HMRC.
Want to see your actual take-home pay?
NetPay UK works out your real net pay after tax, NI, pension and salary sacrifice, for hourly, shift and variable-income workers. Free to download.
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A note on financial advice: NetPay UK calculates take-home pay based on official HMRC tax rules. This article reflects rules in force at the time of publication (23 September 2026). Tax rules change. For complex situations, consult a qualified UK accountant or visit gov.uk/income-tax.
