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Decoding HMRC Letters: Every Reference Code and What to Do Next (UK 2026)

A letter from HMRC lands on your doormat and your stomach drops. What does SA302 mean? What's a UTR? What happens if you ignore it? This plain-English guide maps every common HMRC reference code to its real meaning — and tells you exactly what to do next.

The Clarify Team

— 10 min read
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⚡ 2026 update: HMRC has extended its Making Tax Digital rollout to sole traders and landlords earning above £20,000 from April 2026. If you receive any HMRC letter referencing MTD, this guide explains what it means and what action you need to take.

At a glance

  • Your UTR (Unique Taxpayer Reference) is a 10-digit number that identifies you to HMRC — every Self Assessment letter will show it.
  • SA302 is your tax calculation for the year; lenders and letting agents often ask for it as proof of income.
  • A PAYE reference (in the format 123/AB45678) identifies your employer, not you personally.
  • Letters marked P800 mean HMRC thinks you've overpaid or underpaid tax — you may be owed a refund.
  • A Coding Notice (P2) tells you your tax code has changed; get it wrong and you could pay too much tax all year.
  • Not legal or financial advice: This article explains HMRC terminology in plain English. For your specific tax situation, speak to a qualified accountant, tax adviser, or call HMRC directly on 0300 200 3300.

A brown envelope from HMRC has a unique ability to ruin a morning. Even if you've done nothing wrong, the reference numbers, form codes, and official language can make the most straightforward letter feel like a threat.

The good news: HMRC sends millions of routine letters every year. Most of them are informational — they're telling you something has changed, asking you to confirm a detail, or letting you know about a refund. Understanding the reference codes is the first step to knowing whether you need to act urgently, act eventually, or simply file the letter away.

This guide covers every common HMRC reference code and form number, tells you what each one means in plain English, and explains what to do when you receive it.

Your Unique Taxpayer Reference (UTR)

Your UTR is a 10-digit number that looks like 1234567890. It's specific to you and never changes. HMRC uses it to identify your tax records, so you'll see it at the top of almost every Self Assessment letter.

If you're self-employed, a company director, or you receive rental income, you almost certainly have a UTR. If you've never filed a Self Assessment return and you're employed exclusively through PAYE, you may not have one.

Where to find your UTR

  • Previous Self Assessment tax returns
  • Any letter from HMRC addressed to you personally
  • Your HMRC online account at gov.uk/personal-tax-account
  • Your accountant (if you use one)

What to do: Keep your UTR safe. You'll need it every time you contact HMRC about Self Assessment, and it's required on your tax return. Never share it publicly.

PAYE Reference Numbers

A PAYE reference looks like 120/A46753 or 475/WX12345. Unlike your UTR, a PAYE reference identifies your employer, not you. The number before the slash is the HMRC office number; the code after it is your employer's registration.

You'll see this on payslips, P60s, P45s, and any PAYE-related correspondence. If you change jobs, your new employer will have a different PAYE reference.

What to do: If a PAYE letter arrives in your name but refers to an employer you don't recognise, contact HMRC — it may indicate an error or, rarely, a fraud attempt using your National Insurance number.

Tax Codes and the P2 Coding Notice

Your tax code tells your employer how much tax-free income you're entitled to in a tax year. The most common is 1257L, which simply means your Personal Allowance is £12,570 — so your employer won't deduct tax on the first £12,570 you earn.

A P2 notice (officially called a "Notice of Coding") is a letter HMRC sends when your tax code changes. You'll also see your tax code on your payslip each month. Common tax code letters include:

  • L — standard Personal Allowance (most employees)
  • M — you've received the Marriage Allowance transfer from your spouse
  • N — you've transferred part of your allowance to your spouse
  • T — HMRC needs more information before fixing your code
  • BR — all income from this source taxed at 20% (basic rate); often applies to a second job
  • D0 — all income taxed at 40% (higher rate)
  • D1 — all income taxed at 45% (additional rate)
  • 0T — no Personal Allowance applied; every pound taxed
  • K code — a negative allowance, meaning you owe tax from a previous year or have a taxable benefit. The K appears before the number (e.g. K497).
  • NT — no tax to be deducted (rare; usually for non-UK-resident situations)
  • W1 or M1 — emergency code; tax calculated on a week-by-week or month-by-month basis rather than cumulatively. Often applied when you start a new job without a P45.

What to do: When a P2 arrives, check the code makes sense for your circumstances. If your code has dropped (meaning you'll pay more tax) or you see an unexpected adjustment, call HMRC or use your personal tax account online to query it. An incorrect code left uncorrected costs you money all year.

P800: The Tax Calculation Letter

A P800 is sent after the end of the tax year when HMRC has reconciled your actual income with the tax collected through PAYE. It tells you one of two things:

  • You've overpaid tax — HMRC will offer a refund, usually by cheque or directly into your bank account if you claim online at gov.uk/claim-tax-refund.
  • You've underpaid tax — HMRC will collect the extra either through your next year's tax code (spreading the cost) or, if the amount is over a certain threshold, by a separate demand.

P800s are common if you changed jobs during the year, started or stopped receiving a taxable benefit (like a company car), or received income from multiple sources.

What to do: Read the calculation carefully. If you think the figures are wrong — for example, HMRC has used the wrong income figures — you can contact them to dispute it. If you're owed money, claim it promptly; the link on the letter or your personal tax account both work.

SA100 to SA900: Self Assessment Forms

Self Assessment forms all begin with SA. Here are the most common:

  • SA100 — the main Self Assessment tax return form. Most people complete this online; the paper version is SA100.
  • SA102 — supplementary pages for employment income (if you're also employed)
  • SA103S / SA103F — the self-employment pages. S is the short version (turnover under £85,000); F is full.
  • SA105 — rental income (UK property)
  • SA106 — foreign income
  • SA108 — capital gains (e.g. if you've sold a second property or shares)
  • SA302 — this is not a form you fill in; it's a document HMRC generates showing your tax calculation for a specific year. Mortgage lenders and letting agents often request it as proof of income for self-employed applicants. You can download it from your HMRC online account.
  • SA316 — the reminder letter HMRC sends telling you your Self Assessment return is due.
  • SA370 — a penalty notice for late filing or late payment.

What to do: If you receive an SA316, check your return deadline (31 January for online returns). If you receive an SA370, the penalty is usually £100 for filing up to three months late, rising significantly after that. You can appeal a penalty if you have a reasonable excuse.

Confused by a letter that references a code you don't recognise?

Upload it to Clarify and ask in plain English. Clarify reads the actual text of your letter and explains every reference code, deadline, and required action — with citations pointing back to the exact lines that matter.

Try Clarify free → getclarify.co.uk

NTC and CT: Company and Trust References

If you run a limited company, you'll also deal with:

  • CTR (Corporation Tax reference) — your company's unique reference for Corporation Tax. Format: 1234567890 (10 digits, different from your personal UTR).
  • CT600 — the Corporation Tax return your company files with HMRC each year.
  • Unique Taxpayer Reference for the company — sometimes shown as "CT UTR"; separate from the director's personal UTR.

If HMRC sends a letter to your company address about Corporation Tax, it's addressed to the company, not to you personally as an individual.

VAT Registration Numbers

If your business is VAT-registered, your VAT registration number (format: GB 123 4567 89) appears on all VAT correspondence. HMRC also uses a VAT return reference for each quarterly (or annual) submission.

Letters from the VAT Compliance team may reference your VAT periods (e.g. "period ending 30 June 2025") and any discrepancies in your returns.

Common HMRC Letter Types and What They Mean

"We need to check your tax return" (compliance check)

HMRC routinely checks a percentage of returns. This doesn't necessarily mean you've done something wrong — it may be a random enquiry. You'll be given a deadline to respond and asked to provide records. If you have an accountant, tell them immediately. If you don't, Citizens Advice can signpost you to free tax advice services.

"You owe us money" (payment demand)

HMRC will specify the tax year, the amount, and the deadline. If you can't pay in full, call HMRC's Payment Support Service on 0300 200 3835 before the deadline — they can often arrange a Time to Pay arrangement, spreading the debt over months.

"You may be entitled to a refund" (repayment notice)

Check the calculation, then claim online. Refunds are typically paid within five working days once claimed via the personal tax account.

Making Tax Digital (MTD) notices

From April 2026, sole traders and landlords with income above £20,000 must keep digital records and submit quarterly updates using MTD-compatible software. HMRC is sending letters to those affected. If you've received one, you have until April 2026 to get the right software in place.

What Happens If You Ignore an HMRC Letter?

Most HMRC letters have a deadline. Ignoring them rarely makes things better:

  • Late filing penalties start at £100 and increase the longer your return stays outstanding.
  • Late payment interest currently accrues at the Bank of England base rate plus 2.5 percentage points (around 7.5% in 2026).
  • Compliance checks have a response window — miss it and HMRC may issue a "closure notice" with a formal assessment that you'd have to appeal.
  • Debt collection escalates from letters to enforcement action (including deductions from wages or, in extreme cases, county court action) if payment demands are ignored.

The key point: HMRC is generally flexible if you engage with them early. The same amount of debt is much harder to resolve once enforcement has started.

How to Contact HMRC

  • Self Assessment helpline: 0300 200 3310 (open Monday–Friday 8am–6pm)
  • PAYE/income tax: 0300 200 3300
  • VAT: 0300 200 3700
  • Online: Your personal tax account at gov.uk/personal-tax-account handles most queries, including updating contact details, viewing correspondence, and making claims.
  • Post: Use the address on the letter; different HMRC departments have different postal addresses.

When you call, have your UTR, National Insurance number, and the reference number from the letter ready. HMRC agents will ask security questions before discussing your account.

Got an HMRC letter you're not sure about?

Upload it to Clarify. Within seconds, you'll get a plain-English breakdown of every code and deadline — so you know exactly whether to act now, act later, or simply file it away. No accountant needed for the initial read.

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Frequently asked questions

What is a UTR number and do I need one?

A UTR (Unique Taxpayer Reference) is a 10-digit number that identifies you in HMRC's Self Assessment system. You need one if you're self-employed, a company director, or you have income that can't be collected through PAYE — such as rental income or significant savings interest. If you're exclusively employed and your employer handles all your tax through PAYE, you may not have one.

My tax code changed and I don't know why. What should I do?

Check the P2 Coding Notice that HMRC would have sent explaining the change. Common reasons include a change in income, a new taxable benefit from your employer, or HMRC updating their records after you filed a return. If the explanation doesn't make sense for your situation, contact HMRC on 0300 200 3300 to query it — an incorrect code costs you money throughout the year.

I received a P800 saying I owe tax. Is this definitely right?

Not necessarily. P800s are calculated automatically from information HMRC holds about your income. If any source of income has been reported incorrectly — for example, your employer reported the wrong salary — the P800 will be wrong. Compare the figures on the letter against your own payslips and P60. If there's a discrepancy, contact HMRC before paying.

What is an SA302 and how do I get one?

An SA302 is a tax calculation document generated by HMRC showing your income and tax liability for a specific Self Assessment year. Mortgage lenders and landlords often request it as proof of income for self-employed applicants. You can download it from your HMRC online account at gov.uk/personal-tax-account, or ask your accountant to print one if they submit your returns on your behalf.

I've received an HMRC letter about Making Tax Digital. Do I need to do something now?

If you're a sole trader or landlord with income over £20,000, MTD for Income Tax applies to you from April 2026. Letters HMRC is currently sending are advance notices — you don't need to act immediately, but you should choose MTD-compatible accounting software (such as QuickBooks, Xero, or FreeAgent) before April 2026 and start keeping digital records. HMRC's MTD service is at gov.uk/guidance/use-making-tax-digital-for-income-tax.

What does "BR" mean on my payslip tax code?

BR stands for Basic Rate — it means all income from that employment is taxed at 20% with no Personal Allowance applied. This typically happens if you have a second job and HMRC has applied your full Personal Allowance to your main employment. If BR is applied to your only job, or if you think your allowance is in the wrong place, contact HMRC to correct it.

HMRC says I owe money I don't think I owe. What are my options?

You have the right to appeal most HMRC decisions. The letter will usually include details of how to appeal and the deadline (typically 30 days). For Self Assessment penalties, you can appeal online through your HMRC account or by writing to HMRC explaining your reasonable excuse. For tax assessments you think are wrong, you can request a statutory review. If the amount is significant, consider engaging a tax adviser or accountant — the cost of advice is often worth it.

Can I check all my HMRC correspondence online?

Yes — your HMRC personal tax account at gov.uk/personal-tax-account holds most of your correspondence digitally. You can view tax codes, P800 calculations, Self Assessment returns, and some other notices. However, not every letter is available online, and HMRC may still send paper copies for formal notices even if you've opted for paperless communications. Set up the personal tax account if you haven't already — it's free and reduces the chance of missing something.

This article is for general information only and does not constitute financial or tax advice. Tax rules can change and individual circumstances vary. For advice specific to your situation, speak to a qualified accountant or tax adviser, or contact Citizens Advice (citizensadvice.org.uk) or HMRC directly (gov.uk/contact-hmrc).

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