HMRC Letters Decoded: What Each One Means and How Long You Have

Most HMRC letters fall into one of five families: a tax calculation (P800), a bill you must pay without filing a return (Simple Assessment, PA302), a prompt to check something HMRC already knows about you (a "nudge" letter), a Self Assessment reminder or penalty notice, or a coding notice telling you how your tax code is built (P2). The code is usually printed at the top or bottom of page one, and it tells you what the letter is and how urgent it is.

The deadline matters more than the tone. Some letters have a hard statutory clock — 30 days to appeal a penalty, 60 days to query a Simple Assessment — while others carry no legal deadline at all but still start a compliance trail if you ignore them.

Key points

• A P800 is a calculation, not a demand. It shows a refund or an underpayment for a tax year that has already ended.
• A PA302 Simple Assessment is a bill. Pay by 31 January after the end of the tax year, or within three months of the letter date if that is later.
• Nudge letters have no statutory deadline, but HMRC normally asks for a reply within 30 days and keeps a record either way.
• Penalty notices carry a 30-day appeal window from the date on the notice.
• HMRC's figures are estimates built from third-party data. Check them before you pay — our underpaid tax check works through the usual causes.

Which letter have you got? A quick index

If your letter is not in that list, put the code into the HMRC letter decoder and it will tell you what the reference means and what happens next.

P800: the tax calculation letter

A P800 is sent after the end of a tax year when HMRC's records show the PAYE system did not collect exactly the right amount. It lists your income, the tax deducted, your allowances, and a bottom line that is either a repayment or an underpayment. It is not a bill and it is not a final decision — it is HMRC's arithmetic based on what employers, pension providers and banks have reported.

Why P800s happen

• Two jobs or a job plus a pension, where the Personal Allowance of £12,570 was applied twice or not at all.
• Savings interest above the Personal Savings Allowance — £1,000 for basic-rate taxpayers, £500 for higher-rate and nothing for additional-rate. Banks report interest automatically, so HMRC often catches this after the year ends.
• A company car, medical insurance or other benefit reported on a P11D that was not in your code.
• Starting or leaving a job mid-year, or a period on an emergency code.
• Taxable state benefits, or a state pension that started part-way through the year.

If the P800 shows a refund

You can normally claim it online through your Personal Tax Account and have it paid to a bank account within about five working days. If you do nothing, HMRC posts a cheque — that takes several weeks longer. You never need to pay anyone to claim a P800 refund.

If the P800 says you owe tax

Small underpayments are usually collected by reducing your tax code for a later year rather than demanding a lump sum. Here is how that maths works.

Worked example — a £600 underpayment coded out. Ayesha is a basic-rate taxpayer in Leeds. Her P800 for 2025/26 shows £600 of tax underpaid because untaxed savings interest was not in her code. HMRC collects it through her 2027/28 code:

1. Tax to collect: £600.
2. Her marginal rate: 20%.
3. Allowance reduction needed: £600 ÷ 0.20 = £3,000.
4. Her code drops from 1257L to 957L.
5. Extra tax taken: £600 ÷ 12 = £50 a month.

A Scottish taxpayer whose top slice of income falls in the 21% intermediate band would need a slightly smaller adjustment — £600 ÷ 0.21 = £2,857 — because each pound of allowance is worth more to them. Scottish rates apply to non-savings, non-dividend income only, so savings interest is still taxed at UK rates wherever you live.

Coding out only works if you have enough PAYE income and the amount is within HMRC's limit for collecting debts through a tax code. Above that limit, or if you have no PAYE source, HMRC asks you to pay directly.

Simple Assessment (PA302): a bill, not a return

Simple Assessment is HMRC's way of taxing people who owe something but do not need the full Self Assessment machinery — commonly pensioners whose State Pension exceeds the Personal Allowance, or people with untaxed income HMRC can already see. The letter is a PA302 and it sets out the calculation and the amount due.

The payment deadline is 31 January following the end of the tax year, or three months from the date of the letter if the letter arrives late. If you disagree, you have 60 days from the date on the letter to contact HMRC and explain why. That window is the reason these letters should not sit on the hall table.

Worked example — State Pension above the allowance. Ron's only income in 2026/27 is the full new State Pension at £241.30 a week, plus £3,000 of interest from a fixed-rate bond.

• State Pension: £241.30 × 52 = £12,547.60, which is just under the £12,570 Personal Allowance.
• Remaining allowance: £12,570 − £12,547.60 = £22.40, which covers part of the interest.
• The rest of the interest falls in the £5,000 starting rate band for savings and the £1,000 Personal Savings Allowance, so no tax is due.

Add a small occupational pension and the picture changes quickly, which is exactly when a PA302 lands. There is more detail in our guide to Simple Assessment.

Nudge letters: HMRC knows something and wants you to act

These are "one to many" letters, sent in batches to everyone who matches a data pattern. They are not formal enquiries and there is no legal obligation attached, but HMRC records who replied, and a non-reply can shorten the path to a compliance check.

Common nudge campaigns

• Online marketplace and side-hustle income. Digital platforms report seller data to HMRC. If your trading income was under the £1,000 trading allowance you may owe nothing, but you may still need to say so.
• Cryptoasset disposals. Exchanges share data. Gains above the £3,000 annual exempt amount are taxable at 18% or 24% depending on your band.
• Offshore income and accounts. Driven by international information exchange.
• Undeclared rental income. Often prompted by deposit scheme or land registry data. Remember the £1,000 property allowance and the £7,500 Rent-a-Room limit.
• High Income Child Benefit Charge. Triggered where PAYE data shows income above £60,000 in a household receiving Child Benefit.

Worked example — HICBC nudge. Tom has adjusted net income of £70,000 and his partner claims Child Benefit for two children.

1. Child Benefit for the year: (£27.05 + £17.90) × 52 = £2,337.40.
2. Income above the £60,000 threshold: £10,000, which is halfway to the £80,000 full-clawback point.
3. Charge: 50% × £2,337.40 = £1,168.70.

A pension contribution reduces adjusted net income, which reduces the charge — the arithmetic is the same whichever way you approach it.

The "certificate of tax position"

Some nudge letters enclose a certificate asking you to tick a box and sign a declaration. There is no legal requirement to complete it, and signing an inaccurate one carries real risk. A plain written reply setting out your position is the usual alternative.

Self Assessment reminders and penalty notices

If HMRC has issued you a notice to file (SA316), the return is legally due even if you owe nothing. The key dates for the 2025/26 tax year are:

How the penalties stack

• £100 as soon as the return is a day late, even with no tax to pay.
• £10 a day from three months late, up to 90 days (£900 maximum).
• At six months and again at twelve months, 5% of the tax due or £300, whichever is greater.
• Separate late payment penalties apply to the tax itself, plus daily interest from the due date.

You have 30 days from the date on a penalty notice to appeal, and HMRC can accept a late appeal with a reasonable excuse. Appeals can be made online or on form SA370.

Making Tax Digital letters

From April 2026, Making Tax Digital for Income Tax applies where gross self-employment or property income is over £50,000. HMRC has been writing to affected taxpayers directly. If you get one of these letters, it is telling you that quarterly updates through compatible software replace the old once-a-year rhythm — it is not a demand for tax.

Coding notice (P2): the letter worth reading closest

A P2 shows how your tax code was built: the Personal Allowance, minus deductions for benefits, untaxed interest or an earlier underpayment, plus any expenses relief. Your employer gets the code, not the explanation, so you are the only person who will spot an error.

What the letters in the code mean:

• 1257L — the standard code, the £12,570 Personal Allowance spread across the year.
• S prefix — Scottish rates apply; C prefix — Welsh rates apply.
• BR, D0, D1 — all income from that source taxed at basic, higher or additional rate, normal for a second job.
• K prefix — deductions exceed your allowance, so the code adds income rather than subtracting it.
• W1/M1/X — an emergency, non-cumulative code, which frequently overtaxes.

There is no formal deadline to challenge a P2, but a wrong code costs you every payday until it is fixed. Estimated untaxed interest is the single most common cause of a surprise deduction — you can sanity-check the numbers with the savings interest tax calculator and the income tax calculator.

Deadlines to respond: the clock on each letter

Is the letter genuine?

Real HMRC letters quote a reference such as your UTR, National Insurance number or PAYE reference, and they never ask for card details or threaten immediate arrest. If a letter demands payment to an unfamiliar account or asks you to phone a number you cannot verify, check it against HMRC's published list of genuine contacts on GOV.UK before doing anything. You can also log in to your Personal Tax Account and see whether the same message appears there.

What ignoring a letter actually does

Nothing dramatic happens in week one. Over months, though, interest accrues daily on unpaid tax, penalties escalate on a fixed timetable, an unanswered nudge can become a formal enquiry, and unpaid debt can be passed to HMRC's debt management teams or collected through your tax code. Replying — even to say you need more time or you disagree — resets the relationship and is almost always cheaper than silence.

What to do next

Start with the reference on the letter. Run it through the HMRC letter decoder to confirm what it is and what HMRC expects, then use the underpaid tax check to test whether the figures actually stack up before you pay anything. If the letter is a PA302, read the Simple Assessment guide first — the 60-day query window is the one deadline people most often miss.