A 1255L tax code means your employer or pension provider has been told to treat £12,550 of your income as tax-free for the year. That’s £20 less than the standard personal allowance of £12,570, which is what the usual 1257L code delivers. Somewhere in HMRC’s records, a small adjustment has been made to your allowance, and the code is how that adjustment reaches your payslip.
How the Code Number Is Built
Every PAYE tax code has a number and a letter. The number is your tax-free amount with the last digit dropped, so 1257 means £12,570 and 1255 means £12,550. The letter L means you’re entitled to the standard personal allowance with no special category applied.1GOV.UK. Tax Codes: What Your Tax Code Means
The mechanics behind a 1255L code are simple. HMRC starts with the £12,570 allowance, subtracts a £20 adjustment, and divides by 10 to produce the code number. Your employer’s payroll then spreads £12,550 evenly across your pay periods so each payslip includes a proportionate slice of tax-free pay. Everything above that is taxed at the appropriate band rate.
Under Pay As You Earn, this happens automatically: HMRC tells your employer or pension provider the code, and tax comes out of each payment before it reaches you.2GOV.UK. How You Pay Income Tax – Section: Pay As You Earn (PAYE) You don’t see the adjustment itself, only the code that carries it.
Why HMRC Reduced Your Allowance by £20
A £20 reduction is small, which usually means one of three things is going on.
The most common cause is a minor taxable benefit from your employer. If you receive something like private medical insurance or another small perk, HMRC can collect the tax owed on it gradually by shaving a matching amount off your tax-free allowance, rather than sending you a separate bill.3GOV.UK. Tell HMRC About Changes to Your Employer Paid Medical Insurance A benefit worth £20 in taxable value produces exactly a 1255L code.
The second common cause is a small underpayment from a previous tax year. If PAYE didn’t quite collect enough from you last year, HMRC can recover the shortfall this year by adjusting your code downwards. Small underpayments, generally under £3,000, are usually handled this way instead of demanding a lump sum.4GOV.UK. Tax Overpayments and Underpayments
The third is estimated untaxed income. If HMRC believes you’re receiving a small amount of income that isn’t already being taxed at source — for example, bank interest sitting above your savings allowance — the estimated tax on that income can be pulled through your code.
Any of these can produce the same £20 nudge. The only way to know which one applies to you is to look at your Notice of Coding.
What a 1255L Code Costs You Compared with 1257L
For the 2025/26 tax year, income above the personal allowance falls into the following bands for England and Northern Ireland taxpayers:5GOV.UK. Income Tax Rates and Personal Allowances
- Basic rate (20%): taxable income from £12,571 to £50,270
- Higher rate (40%): taxable income from £50,271 to £125,140
- Additional rate (45%): taxable income over £125,140
On a £30,000 salary with a 1255L code, your first £12,550 is tax-free and the remaining £17,450 is taxed at 20%, giving £3,490 in income tax for the year. With a 1257L code, only £17,430 would be taxed, so 1255L costs you £4 more across the year. If you’re a higher-rate taxpayer, the same £20 allowance difference costs £8. Small either way — but the reason it’s worth checking is that codes drift. What starts as a £20 adjustment for one year’s benefit can quietly persist after the benefit ends, or stack on top of a new adjustment, and the numbers grow.
Payroll applies your code cumulatively across the tax year, checking each pay period whether the tax collected to date matches what you should have paid on your earnings so far. That’s how any mid-year code change works itself through without leaving a big shortfall or surplus at year end.
How to See What’s Behind Your Code
Your code appears on your payslip, your P60 at the end of the tax year, and your P45 if you’ve recently left a job. But the document that actually explains it is your P2 Notice of Coding. It lists each allowance and deduction HMRC has applied and shows the arithmetic that produced the code. If your code is 1255L, the P2 will name the £20 adjustment and where it came from.6GOV.UK. Tax Codes: If You Think Your Tax Code Is Wrong
The fastest way to review your code and the details behind it is through HMRC’s “Check your Income Tax” online service, also available in the HMRC app.7GOV.UK. Check Your Income Tax for the Current Year You sign in with a Government Gateway account and can see the income, benefits, and estimates HMRC holds on file. If anything is wrong — a benefit you no longer receive, an estimate of untaxed income that’s too high, an old underpayment already settled — you can update it there.
Once HMRC processes a change that affects your code, they issue a new code and notify both you and your employer, typically within 15 working days.6GOV.UK. Tax Codes: If You Think Your Tax Code Is Wrong Keep the new P2 in case you need to check the figures against future payslips.
If you can’t use the online service, the Income Tax helpline can review your code by phone. Have your National Insurance number and a recent payslip ready so the agent can locate your record via your employer’s PAYE reference.
Watch for Payrolled Benefits
Some employers now handle benefit tax directly through payroll instead of routing it through your code. When benefits are payrolled, the taxable value appears on your payslip each period and your code stays at 1257L.8GOV.UK. 9GOV.UK. If You Cannot Pay Your Tax Bill on Time
If Your Code Has Been Wrong in Past Years
After each tax year ends on 5 April, HMRC reconciles your PAYE tax against what you actually owed. If you overpaid, a P800 tax calculation is sent, usually between June and the following March, and tells you how to claim the refund. Claim online and the money typically arrives within a few weeks; wait past 45 days and HMRC issues a cheque automatically.4GOV.UK. Tax Overpayments and Underpayments
You have four years from the end of the tax year in which the overpayment happened to claim a refund. A 2022/23 overpayment must be claimed by 5 April 2027, and a 2025/26 overpayment by 5 April 2030. If you suspect the adjustment behind your 1255L code has been running longer than it should — or was never right in the first place — check the earliest years first, because those windows close soonest.