Self-assessment: do you need to file? The full checklist

Plenty of people file a tax return who do not need to, and plenty who should have never do. Here is who HMRC actually expects one from for 2025/26.

You usually need to file if

  • You were self-employed and turned over more than 1,000 pounds.
  • You had untaxed income: rent, tips, savings interest or dividends above the allowances.
  • You or your partner claimed Child Benefit and one of you earned over the charge threshold.
  • You made gains above the capital gains allowance.

You might be surprised

Being employed and taxed through PAYE does not always exempt you. A side income, a lodger beyond the Rent a Room limit, or income from abroad can all pull you in. And if HMRC sends you a notice to file, you must, even if you owe nothing. Guessing wrong here is expensive, because the penalties start the day after the deadline whether or not there is any tax to pay.

Check your own position with the free tool on GOV.UK, which is the authoritative answer.

Written by Rachel Oduya. General tax information, not financial advice; figures current for 2026.…

What your tax code actually means, and how to tell in two minutes whether it is wrong

A plain brown envelope and a folded blank sheet of paper on a wooden kitchen table beside a mug of tea and reading glasses in morning light

Here is what to do first, before any of the explanation. Find your tax code on your most recent payslip. If it is 1257L and you have one job, no company car and no untaxed income, it is almost certainly right and you can stop reading. If it is anything else, read the line in the table below that matches it, then check the three questions further down. If your code is wrong, you tell HMRC, not your employer, and the fastest route is your personal tax account or the HMRC app.

Your tax code is the instruction HMRC gives your employer about how much tax to take off your pay. Your Personal Allowance is the amount you can earn in a year before paying income tax, and for 2026/27 it is £12,570.

What the numbers mean

The number is your tax-free income for the year from that job, with the last digit removed.

GOV.UK explains the calculation: HMRC starts with your Personal Allowance, takes off any income you have not paid tax on, such as untaxed interest or part-time earnings, and any other deductions such as company benefits or the High Income Child Benefit Charge. It then replaces the final digit with a letter.

So 1257L means £12,570 of tax-free pay. If HMRC knows you have a company car worth £3,000 in taxable benefit, your allowance drops to £9,570 and your code becomes 957L.

What the letters mean

Code What it means
L You get the standard tax-free Personal Allowance
M Marriage Allowance: you have received 10% of your partner’s allowance
N Marriage Allowance: you have transferred 10% of your allowance to your partner
0T Your allowance is used up, or a new employer does not yet have your details
BR All income from this job or pension is taxed at the basic rate
D0 All income from this job or pension is taxed at the higher rate
D1 All income from this job or pension is taxed at the additional rate
NT No tax is taken from this income
K Your untaxed income or deductions are greater than your Personal Allowance
S or C Scottish or Welsh rates apply

Source: GOV.UK, what your tax code means.

BR is the one most often misread as a mistake. It is correct and normal on a second job, because your whole Personal Allowance is already being used by the first. It is a problem only if it is on your main job, or your only job, in which case you are paying 20% on income that should be tax free.

K codes, and the 50% protection

A K code is the opposite of a normal code. Instead of giving you tax-free pay, it adds notional income on top of your actual pay so that tax can be collected on something else.

GOV.UK lists the usual reasons: you are paying off tax owed from an earlier year through your wages, you receive the State Pension or a taxable state benefit, you have company benefits such as a car, or you earn more savings interest than your Personal Savings Allowance covers.

There is one protection worth knowing. Employers and pension providers cannot take more than half of your pre-tax wages or pension when using a K code. If the arithmetic of your K code would take more, the deduction is capped and the balance rolls forward.

Diagram showing how a tax code number is built, starting from the 12,570 pound Personal Allowance, subtracting untaxed income and benefits, then dropping the final digit to give 1257L

Emergency codes

Your code is an emergency code if it ends in W1, M1 or X. Some payroll software shows NONCUM instead.

Normally your tax is worked out on your total income so far in the tax year, so the system self-corrects. On an emergency code it is worked out on that week or month alone, as though you were paid that amount every period of the year. That is why a first payslip in a new job, or a large one-off payment, can produce a tax deduction that looks absurd.

Emergency codes usually resolve themselves once HMRC has your full details. The one to watch is an emergency code that is still there three or four pay periods later.

The three questions that catch most errors

  1. Do you have one job and a code that is not 1257L? If so, something has been deducted from your allowance. Check your personal tax account for what HMRC thinks you owe tax on.
  2. Has a benefit you no longer receive stayed in your code? Company cars and medical insurance are the common ones. They are not removed automatically when they end.
  3. Has your code changed mid-year without a letter? HMRC will usually contact you to explain a change. If your code moved and nothing arrived, check it.

The one worth checking deliberately

Marriage Allowance is claimed by fewer couples than are eligible. It transfers £1,260 of Personal Allowance from the lower earner to the higher, and reduces the recipient’s tax by up to £252 in a year. The lower earner normally needs income below the £12,570 Personal Allowance and the recipient must be a basic rate taxpayer.

You will see it in the codes: the person who gave the allowance away gets an N, the person who received it gets an M.

The counterweight

A wrong tax code is not a fine and usually not a disaster. If you have paid too much, HMRC reconciles after the tax year ends and refunds you. If you have paid too little, it is generally collected through next year’s code rather than demanded at once.

What a wrong code does do is move money between months, and if you are budgeting tightly, a code that takes £180 a month too much from April to March is a real problem even if you get it all back in July. That is the reason to check it now rather than wait for the system to correct itself.

Frequently asked questions

What does 1257L mean? £12,570 of tax-free pay from that job for the year. It is the code used for most people with one job or pension.

Is BR wrong? Not usually. On a second job it is correct, because your Personal Allowance is already used by the first. On your only job it is wrong and worth querying.

Why does my code start with K? Because untaxed income or deductions exceed your Personal Allowance, so tax is collected on that through your pay. No more than half your pre-tax pay can be taken this way.

Who do I tell if my code is wrong? HMRC, not your employer. Your employer only applies the code it is given. Use your personal tax account or the HMRC app.

Updated on 11 September 2026.

Sources

This article explains how tax codes work. It is not personal tax advice, and a complicated situation is worth putting to HMRC or an accountant.…

Smart Investment Finance Strategies for Long-Term Growth

Long-term investment finance is centered on making thoughtful decisions that can support financial growth over an extended period. Rather than attempting to predict every market movement, long-term investors often focus on building diversified portfolios and maintaining consistent investment habits. This approach can provide a structured framework for managing wealth while reducing the temptation to react emotionally to short-term market changes.

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Reviewing and Improving Investment Decisions

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