To sort your tax code, check it first through HMRC’s online “Check your Income Tax” service or on your latest payslip, then update it either through that same service or by calling the Income Tax helpline on 0300 200 3300. Most corrections take under an hour, and HMRC usually notifies you and your employer of any change within 15 working days.
Check Your Code Before You Change Anything
Your tax code sits on every payslip next to your National Insurance number, and it also appears on your P60 (the end-of-year certificate your employer gives you after 5 April). That’s the quickest offline check.
For a fuller picture, sign in to the “Check your Income Tax” service on GOV.UK using a Government Gateway account. If you don’t have one, the system walks you through setting it up, and you may need a passport or driving licence to verify your identity the first time. Once you’re in, you can see your current code, your estimated income from each job or pension, the tax you’re expected to pay this year, and any recent changes.
What the Code Should Look Like
The numbers are your tax-free income with the last digit removed. So 1257 means £12,570 of tax-free pay, and 1257L is the standard code for someone with one job or pension and the full personal allowance.
The letter is what usually gives a wrong code away:
- L: standard personal allowance.
- BR: everything from that job is taxed at 20%, normally used for a second job where the allowance is already applied elsewhere.
- 0T: no personal allowance. Appears if your allowance is used up, if you earn over £125,140, or if your employer doesn’t have your details.
- K: your taxable benefits or unpaid tax outweigh your allowance, so the code adds to your taxable income instead of reducing it. Your employer can never take more than half your pre-tax pay under a K code.
- S: Scottish income tax rates apply.
- C: Welsh rates apply.
- NT: no tax deducted.
A BR code on your only job means you’re getting no personal allowance and losing roughly £200 a month more than you should. That’s the scale of what can be at stake.
Signs Your Code Is Wrong
Emergency Codes After a New Job
This is the most common reason people overpay. If your new employer doesn’t have your previous pay and tax details, they’ll put you on an emergency code. Look for W1 (paid weekly), M1 (paid monthly), or X (variable pay dates) after your code, or “NONCUM” on some payslips.
Emergency codes tax each pay period on its own instead of spreading your allowance across the year, and the result is almost always too much tax taken. Handing your P45 from your last job to your new employer straight away usually fixes this. HMRC normally updates your code within 35 days of you starting. Past that, you’ll need to update it yourself.
Life Changes HMRC Hasn’t Been Told About
Codes drift when circumstances change quietly. Common causes:
- A second job or pension where both sources apply the full allowance. You’ll underpay and face a bill.
- Company benefits like a car, private healthcare, or an interest-free loan. These are reported on a P11D and should show up in your code.
- Starting to receive the State Pension. It’s taxable but paid gross, so HMRC recovers the tax by adjusting the code on your employment or private pension.
- Income above £100,000. Your personal allowance drops by £1 for every £2 over that threshold and disappears at £125,140.
K Codes You Don’t Recognise
If HMRC issues a K code, it means the benefits or unpaid tax it’s built on outweigh your allowance. Every payslip compounds the effect, so if the underlying figures look wrong, check them in the online service without waiting.
Update Your Code Online
Go back to the “Check your Income Tax” service on GOV.UK. From there you can report changes affecting your code, update income estimates for each job or pension, and correct employer or pension provider details. Have your employer’s PAYE reference to hand; it’s on your payslips and P60.
Before you start, gather the numbers behind anything that changes your tax-free amount: the cash value of company benefits (from your P11D), professional expenses you pay for and aren’t reimbursed, and estimated annual income from all sources including savings interest and dividends above your allowances. Where the untaxed amounts are small enough, HMRC can collect the tax by adjusting your code instead of asking you to file a return.
Update Your Code by Phone
Call the Income Tax helpline on 0300 200 3300, Monday to Friday, 8am to 6pm, closed on bank holidays. From outside the UK, the number is +44 135 535 9022. Keep your National Insurance number and employer PAYE reference by the phone. Waits are long in January and around the tax year end, so online is usually faster.
What Happens After You Submit Changes
HMRC reviews your update and, if the code changes, notifies both you and your employer within 15 working days. You’ll receive a P2 “Notice of Coding” that breaks down how the new code was calculated. Read it against the figures you gave; a mistake in the P2 will follow you through every payslip until it’s corrected.
Your employer must apply whatever code HMRC sends. Expect it on your next monthly payslip, or by your third weekly one. If the old code is still there after two pay cycles, contact HMRC again because the update hasn’t landed.
Getting Money Back If You’ve Overpaid
When HMRC changes your code mid-year and you’ve overpaid, they’ll usually tell your employer or pension provider to refund the difference through your pay once the new code takes effect. You don’t have to do anything extra.
After the tax year ends, HMRC reconciles your records against actual earnings and tax paid. If there’s a difference, they send a P800 calculation. If you’re owed money, the letter tells you how to claim, and you can request a bank transfer through your personal tax account or the HMRC app, or ask for a cheque. Some cheques go out automatically. You’ll need the P800 reference number and your National Insurance number to claim online.
If the P800 says you’ve underpaid, don’t set it aside. HMRC usually collects the shortfall by adjusting next year’s code, spreading it across 12 months.
Marriage Allowance
If you’re married or in a civil partnership and one of you earns under the personal allowance, Marriage Allowance lets the lower earner transfer £1,260 of their allowance to the other, cutting the higher earner’s tax by up to £252 a year. The recipient must be a basic-rate taxpayer (income between £12,571 and £50,270), or in Scotland must pay the starter, basic, or intermediate rate (£12,571 to £43,662). Apply through GOV.UK; HMRC then adjusts both codes automatically. Claims can be backdated up to four years.
When You Need Self Assessment Instead
Adjusting your code through PAYE works for most employed people, but some situations mean filing a Self Assessment return instead. You’ll need one if any of these applied in the tax year:
- Self-employed income above £1,000 as a sole trader.
- You were a partner in a business partnership.
- You owed Capital Gains Tax on something you sold or disposed of.
- You owed the High Income Child Benefit Charge and it wasn’t collected through PAYE.
- You had significant untaxed rental, foreign, savings, or dividend income above your allowances.
You can’t use the “Check your Income Tax” online service if Self Assessment is your only route for paying Income Tax. HMRC’s checker on GOV.UK will tell you which side of the line you’re on if you’re not sure.