There is a persistent idea that an HMRC tax investigation works like being pulled aside at random at airport security.
You file your return, HMRC picks your name out of a hat, and a brown envelope arrives. That isn’t how modern compliance work operates.
An HMRC tax investigation is almost always risk-led. HMRC compares your return against data it already holds from banks, employers, Companies House, online platforms and overseas tax authorities, and opens an enquiry when something doesn’t add up unusual margins, mismatched figures, a tip-off, or a sector it is currently examining.
Understanding the HMRC tax investigation procedure helps you avoid unnecessary risk and respond calmly if a letter arrives.
Key Takeaways
- An HMRC tax investigation is usually triggered by data mismatches, unusual figures, a tip-off or a targeted sector not random selection.
- HMRC’s Connect system cross-checks your return against banks, Companies House, payment platforms and overseas records; that is where the HMRC tax investigation procedure starts.
- Roughly 93% of enquiries are triggered by something specific; only 7% are genuinely random.
- HMRC’s compliance yield reached £48.0 billion in 2024–25, up from £41.8 billion the HMRC tax investigation procedure is intensifying, not easing off.
- How far HMRC can go back depends on behaviour: 4 years for innocent errors, 6 for careless ones, up to 20 where fraud is suspected.
- Good records, consistent cross-tax reporting and prompt correction of errors are your best defense.
- If a letter arrives, work out what HMRC is asking before you respond and talk to your accountant first.
What Is an HMRC Tax Investigation?
HMRC’s own term is usually “compliance check” or “enquiry” a formal review by HM Revenue & Customs to check that the right amount of tax has been declared and paid. It can touch Self Assessment, Corporation Tax, VAT, PAYE, Capital Gains Tax or the Construction Industry Scheme, and can cover one figure on a single return or several years of history. HMRC sets out your rights in its compliance checks guidance on GOV.UK.
An HMRC tax investigation is HMRC’s formal process for checking that a tax return, claim or set of business records is accurate. It does not automatically mean HMRC believes you have done something wrong many enquiries close with no further tax due. What varies is scope: some look at one figure, while the most serious, Code of Practice 9 (COP9), apply where HMRC suspects deliberate tax fraud.
What Is the HMRC Tax Investigation Procedure?
Owners are often unsure what actually happens once HMRC decides to look closer, so it helps to set out the HMRC tax investigation procedure step by step.
- Risk assessment: HMRC’s systems, including Connect, flag a return on data mismatches, unusual ratios, sector risk or third-party information.
- Opening letter: HMRC confirms which tax and accounting period are under review and what it wants to see the formal start of the HMRC tax investigation procedure.
- Information gathering: HMRC may request records, bank statements or explanations under its statutory powers (Finance Act 2008, Schedule 36).
- Review and dialogue: You or your adviser respond, provide evidence and answer HMRC’s questions; further correspondence often follows.
- Proposed findings: HMRC sets out its conclusions, any additional tax and any proposed penalty, and invites comments.
- Closure: The enquiry ends by agreement, or you request an independent review or appeal to the First-tier Tribunal.
The HMRC tax investigation procedure runs from a risk-based trigger, through an opening letter and evidence-gathering, to proposed findings and closure by agreement or on appeal. Aspect enquiries often resolve within months; full enquiries and COP8 or COP9 cases run far longer.
The process is sequential: a precise, well-evidenced reply at stage three often stops stages four and five expanding, while a vague one guarantees another round of questions. Our tax investigation support service exists for exactly this.
Types of HMRC Investigation
Not every enquiry is the same, and the right response depends on which type you’re facing.
| Type | What it covers | Typical trigger | Seriousness |
|---|---|---|---|
| Compliance check / aspect enquiry | One item on a return (an expense or relief) | Data mismatch, unusual figure | Low |
| Full enquiry | The whole return and underlying records | Repeated inconsistencies, higher risk | Medium |
| VAT or PAYE compliance check | VAT repayments, RTI/payroll data, CIS | Unusual repayment pattern, employment status query | Low–Medium |
| Code of Practice 8 (COP8) | Complex planning or avoidance, no fraud alleged | Avoidance structures, large or unusual transactions | High |
| Code of Practice 9 (COP9) | Suspected deliberate fraud, Contractual Disclosure Facility offered | Tip-off, serious discrepancy, concealment | Very High |
| Criminal investigation | Prosecution as the objective, conducted under PACE | Serious or organised fraud | Highest |
10 Red Flags That Trigger an HMRC Tax Investigation
An HMRC tax investigation ranges from a narrow compliance check into one expense claim, through a full enquiry covering an entire return, up to COP8 and COP9 civil fraud work and, in the worst cases, criminal investigation. Establishing which type you face is the first step in deciding how to respond. There are usually identifiable reasons why one return attracts attention and another doesn’t.
Here are ten of them.
1. HMRC increasingly chooses enquiries using data
HMRC has spent years building its ability to compare information from different sources, and the best-known part of this is Connect. Rather than looking at a tax return entirely in isolation, HMRC can compare information reported by the taxpayer against data available elsewhere.
- Banks and financial institutions;
- PAYE records;
- Companies House;
- Property transactions;
- Online payment platforms;
- Employment records;
- VAT;
- Overseas financial accounts;
and other government-held information
Years ago, an incorrect figure could sit on a return with nothing drawing attention to it; today another data source almost certainly exists against which it can be tested. That is why mismatches remain the most common opening point in the HMRC tax investigation procedure.
Data-matching through Connect is the biggest single driver of a modern HMRC tax investigation far more so than random selection.
2. Your margins look unusual for the business you’re running
Imagine two businesses, both turning over £1 million. One reports gross profit of £450,000, the other £120,000. Neither is automatically wrong. But HMRC doesn’t only ask “Is this possible?” it also asks “Does this make sense?”
- Gross profit percentages;
- Wages compared with turnover;
- Expenses compared with previous years;
- Directors’ remuneration;
- Drawings;
- Subcontractor costs;
- Or other figures that move significantly away from previous patterns.
Unusual doesn’t mean incorrect: a contractor might have had one extraordinary project; a retailer might have cleared old stock at a loss. What matters is being able to explain it, and a short disclosure on the return can settle the point early.
Margins sitting well outside your sector norm are a classic red flag; explaining unusual figures on the return itself can head off an HMRC tax investigation before it starts.
3. What you report doesn’t match information HMRC already has
Suppose your return shows one amount of bank interest but HMRC holds information suggesting another. Or a property transaction appears in external records with no matching treatment on the return. Or Real Time Information payroll data doesn’t sit comfortably with the figures reported elsewhere. Discrepancies don’t necessarily mean evasion.
They arise from:
- Bookkeeping mistakes
- Forgotten accounts
- Incorrect coding
- Duplicated transactions
- Misunderstanding what should be declared
- Poor communication between advisers
- Or simply using the wrong figure
The same applies to platform income: an owner may see Stripe or PayPal as a way of collecting money, but HMRC is interested in the income underneath.
A mismatch between your return and third-party data is among the most reliable triggers of an HMRC tax investigation, whatever the innocent explanation behind it.
4. The return itself contains unusual patterns
Sometimes the trigger isn’t outside information but what’s on the return. A business reporting repeated losses while apparently supporting the owner’s lifestyle invites examination.
So do large round-sum expenses if somebody claims exactly £20,000 of a cost year after year without supporting detail, HMRC will want to know how it was reached.
- Sudden movements in turnover;
- Unexplained falls in profitability;
- Substantial changes in expenses;
- Repeated late filing;
- Frequent amendments;
- Unusual transactions involving directors;
- And movements through directors’ loan accounts that don’t appear to fit the wider picture.
None of it proves anything businesses have strange years, as Covid showed. But if the figures tell an odd story, expect someone to ask you to explain it.
Internally inconsistent figures can prompt a full HMRC tax investigation even where there is no external data mismatch at all.
5. VAT and PAYE can create their own warning signs
An enquiry doesn’t always start with Corporation Tax or Self Assessment sometimes VAT or payroll opens the door. Repayment claims are the obvious example: a repayment isn’t inherently suspicious, but where the pattern shifts HMRC may want evidence.
Other areas that can generate questions include:
- Unusual effective VAT rates;
- Changes involving partial exemption;
- PAYE inconsistencies;
- Employment status;
- Off-payroll working;
- IR35;
and mismatches within the Construction Industry Scheme
Which is what our payroll and PAYE services are built around.
Taxes also don’t live in separate boxes. Something found during a VAT check can raise questions about Corporation Tax, PAYE or the underlying records, so a narrow query widens easily.
VAT and PAYE checks are frequently the entry point into a much wider HMRC tax investigation, because findings in one tax can legitimately open questions in another.
6. Somebody tells HMRC about you
Not every enquiry begins with data analysis sometimes somebody picks up the telephone or completes an online form. HMRC provides routes for suspected fraud or evasion to be reported directly.
Potentially:
- A former employee;
- An ex-business partner;
- An estranged spouse;
- A competitor;
- A customer;
- A supplier;
- Or somebody else who believes they know something about the business.
A tip-off isn’t evidence that anything has gone wrong people fall out, and divorces aren’t famous for dispassionate assessments of financial affairs. But a specific allegation gives HMRC something to examine, and you don’t want to be reconstructing old records from memory.
Informant tip-offs remain a real, if less discussed, way an HMRC tax investigation begins and contemporaneous records are your best protection if one lands.
7. Your business falls within an area HMRC is examining
HMRC doesn’t examine every taxpayer with equal intensity. Compliance resources are directed towards particular sectors and perceived risk.
Over recent years this has included areas such as:
- Landlords;
- Offshore income and assets;
- Cryptocurrency;
- E-commerce;
- Online trading;
- R&D claims;
- And higher-risk or hidden wealth cases.
The important point isn’t to try to second-guess HMRC’s next campaign.
The point isn’t to second-guess the next campaign, but to recognise that something ordinary for your business may become interesting because it sits inside an area under scrutiny. If your affairs are documented that shouldn’t frighten you but hoping HMRC never asks is rarely a strategy.
Sitting in a targeted sector raises your statistical chances of an HMRC tax investigation even where your own affairs are entirely correct.
8. You receive a nudge letter
A nudge letter isn’t a formal enquiry, but it doesn’t belong in the recycling either. HMRC increasingly uses One-to-Many campaigns: where its information suggests a group of taxpayers should check something, it writes asking them to review their position rather than opening the HMRC tax investigation procedure in every case.
The letter might concern overseas income, a share disposal, property income or an R&D claim. The sensible response is to establish why it was sent, check the records and correct anything that needs it. The dangerous one is “it’s probably nothing” ignoring a letter doesn’t make a discrepancy disappear.
A nudge letter is a warning shot rather than a full HMRC tax investigation but ignoring one is a reliable way to turn it into one.
9. Not all HMRC enquiries are the same
The phrase covers very different situations. At one end HMRC enquiry a straightforward question about one figure; at the other you may be facing suspected deliberate fraud under COP9, where the Contractual Disclosure Facility is offered.
That is why the first thing we establish when somebody brings us an HMRC letter is what, precisely, is being asked. Don’t respond to the enquiry you fear you’ve received respond to the one you actually have.
An HMRC tax investigation can mean anything from a single-question compliance check to a COP9 fraud case, and identifying which determines everything about how you should respond.
10. Poor records make everything harder
This is the least exciting point here and one of the most important. Good records won’t stop HMRC asking questions, but they make them far easier to answer.
Suppose HMRC asks why your gross margin dropped three years ago: there is an enormous difference between producing contemporaneous documentation and saying “I vaguely remember it was a difficult year.”
Clean bookkeeping, bank reconciliations, receipts, payroll records, contracts and supporting calculations all matter, and where a return involves judgement, documenting the reasoning at the time is invaluable.
Weak records don’t cause a compliance check by themselves, but they turn a manageable HMRC tax investigation into a stressful, drawn-out one.
Does an HMRC Tax Investigation Mean HMRC Thinks I’ve Done Something Wrong?
No. That letter does not automatically mean HMRC believes you’ve committed fraud or deliberately underpaid it means HMRC wants to check something. Sometimes the check ends with no additional tax; sometimes errors are found; sometimes both sides simply disagree.
An HMRC tax investigation is a verification exercise, not an accusation. Many enquiries close with no additional tax and no penalty, so don’t decide the outcome before the process has started.
What Should I Do If HMRC Contacts Me?
First, read the letter properly. That sounds obvious, but people react to the HMRC logo rather than the words underneath.
Establish:
- What tax HMRC is looking at;
- Which accounting period is involved;
- What information has been requested;
- What deadline applies;
- And whether HMRC has identified a particular concern.
Then speak to your accountant before firing off an explanation, particularly where judgement, a technical position or incomplete records are involved.
Trying to be helpful can backfire: an owner sends an unrequested bundle of documents, writes three pages from memory and creates new questions inside what should have been a simple enquiry.
Should I Speak to HMRC Myself?
You can, and for a simple factual question you may not need anyone else. But representation becomes more valuable as amounts and complexity increase.
An experienced adviser establishes what HMRC is entitled to request, identifies the real issue, assembles the evidence and keeps the correspondence from wandering.
You may deal with HMRC yourself, but representation pays once amounts or complexity rise, because an adviser keeps the HMRC tax investigation procedure focused on the question actually asked.
Can I Reduce the Chance of an HMRC Tax Investigation?
You can’t eliminate it, and you shouldn’t distort legitimate decisions because they might look unusual.
But you can keep accurate records and reconcile the bank; file on time; keep Corporation Tax, VAT and payroll submissions consistent; investigate odd balances rather than rolling them forward. Document significant tax decisions; and correct errors instead of waiting to see whether HMRC notices.
You cannot rule out an HMRC tax investigation, but consistent cross-tax reporting, timely filing, clean records and prompt voluntary correction reduce the risk and shorten any enquiry that does arrive.
One Final Point About Tax Investigation Fee Protection
Even a routine enquiry involves substantial professional time, including where the taxpayer has done nothing wrong. That is why fee protection is worth considering but, like house insurance, it must be arranged before the problem arrives.
The Real Defense Against an HMRC Tax Investigation
After more than 25 years running an accountancy practice, I’ve seen enquiries ranging from straightforward questions to long, stressful disputes. The common thread isn’t that every business examined had done something wrong it hadn’t. The ones that cope best are those where the records are sound and somebody can explain why the numbers are what they are.
Modern compliance is built on information. HMRC has more of it, your accountant should have good information too, and so should you. If something in your accounts doesn’t make sense, don’t wait for HMRC to ask why. Talk to Carter Collins & Myer Accountants. Whether you want a second pair of eyes on your records or help with an HMRC tax investigation that has already started, our tax team works with owner-managed businesses across Rochdale.
Received a Letter From HMRC? Talk to Us Before You Respond
If HMRC has opened an HMRC tax investigation, sent a nudge letter or asked questions about your tax affairs, don’t panic and don’t ignore it. Carter Collins & Myer will review the correspondence, establish what HMRC wants and help you decide on the right response at every stage of the HMRC tax investigation procedure.
We work with owner-managed businesses across Rochdale, Greater Manchester, Lancashire and Cheshire and regularly handle HMRC enquiries and disputes. Get in touch with our tax team the earlier we see the correspondence, the more useful we can be.