A letter from HMRC saying they’re looking into your tax affairs is one of the most frightening things a sole trader or landlord can receive. The fear is understandable — but most of what people imagine about HMRC investigations comes from rumour, not reality. Most contact from HMRC is a question, not an accusation, and most enquiries close with no or modest adjustments if you engage honestly and keep good records.
This hub maps what’s actually happening when HMRC contacts you, the three tiers of scrutiny they can apply, what your rights are at each stage, and when to get help. It links to the detailed guide for each specific scenario — what triggers an investigation, how to come clean on undeclared foreign income, and what to do when platform-reported payments get flagged.
1. The 3 tiers of HMRC scrutiny
Not all HMRC contact is the same thing. There are three distinct tiers, and which one you’re in determines what HMRC can do, what your rights are, and how seriously you should take it.
Tier 1 — Compliance check (informal). HMRC writes or calls asking questions or requesting records to verify something on your return. This is the most common form of contact and doesn’t require a formal notice — HMRC can ask questions at any time. Most compliance checks are resolved by you providing records or an explanation, and they close without escalating. You’re not under formal enquiry at this stage, but you should treat the questions seriously because failing to answer satisfactorily can trigger the next tier.
Tier 2 — Formal enquiry (s9A TMA 1970). HMRC opens a formal enquiry under section 9A of the Taxes Management Act 1970. This gives them statutory powers to demand information and documents, and they must notify you in writing within 12 months of your filing date (if you filed on time). Once an enquiry is open, it stays open until HMRC issues a closure notice — there’s no fixed time limit, but you can ask the tribunal to direct closure if it drags on. Most enquiries are “aspect enquiries” looking at one or two specific areas; a smaller number are “full enquiries” reviewing everything.
Tier 3 — Criminal investigation. Reserved for suspected deliberate, large-scale fraud. This is rare — HMRC’s published policy is that criminal prosecution is for cases involving fraud that’s “material, deliberate, and concealed.” If you’re at this tier, you’ll know: you’ll be interviewed under caution, possibly arrested, and you need a lawyer (not just an accountant) immediately. The good news is that the vast majority of sole traders and landlords who hear from HMRC never reach this tier — and there’s a formal route (COP9, see §6) designed to keep you out of it even when HMRC suspects serious fraud.
2. How HMRC decides who to investigate
Most HMRC contact isn’t random — it’s triggered by Connect, HMRC’s data-matching system that cross-references your return against 30+ external sources including banks, Land Registry, DVLA, digital platforms, and overseas tax authorities. The biggest single trigger is income that doesn’t match third-party data: undeclared rental income, platform sales, bank interest, or foreign income that someone else already told HMRC about. Other triggers include expense claims out of line with industry benchmarks, significant profit fluctuations, repeated late filings, and cash-heavy businesses. For the full breakdown of every trigger and how Connect works, see our what triggers an HMRC investigation guide.
3. What happens during an enquiry — the stages
If HMRC opens a formal enquiry, it follows a recognisable sequence. First, you receive a written notice of enquiry and informal requests for information or documents. Second, you provide records and answer questions — HMRC can request anything “reasonably required” but can’t go on a fishing expedition. Third, if HMRC finds potential issues, they’ll discuss their preliminary findings with you (often called a “findings meeting”). Fourth, they issue an assessment or amendment to your return showing the tax they think is due. Fifth, you either accept the assessment or appeal to the tax tribunal. Throughout, you have the right to be treated fairly, to representation, to appeal, and to ask the tribunal to close the enquiry if it’s taking too long. For the detail on the enquiry window, aspect vs full enquiries, your rights, and what happens if they find errors, see our what triggers an HMRC investigation guide.
4. If the issue is undeclared foreign income
If HMRC’s enquiry relates to income you earned abroad and didn’t declare, the stakes are higher — offshore penalties run from 100% to 200% of the tax due, compared to 0–70% for onshore errors. But the framework is the same: come forward voluntarily before HMRC contacts you for the lowest penalties, declare the income on the Self-employment pages (SA103, not SA106), claim Foreign Tax Credit Relief for tax already paid abroad, and get professional help because the Double Taxation Treaty analysis is not simple. For the full step-by-step on declaring foreign income, the penalty structure, and how far back HMRC can go, see our undeclared foreign income guide.
5. If the issue is platform-reported payments
If HMRC has written to you saying a platform (PayPal, eBay, Airbnb, Vinted) reported payments that look like income — but the money was actually personal transfers, loans between friends, or round-trips — you’re likely facing a compliance check rather than a formal enquiry. The key point is that a platform reporting payments to HMRC is not the same as HMRC deciding those payments are taxable income. A genuine loan is not income, regardless of how short the term is, and money you receive and immediately return is not a gain. The fix is paperwork-heavy but not complicated: assemble a complete audit trail showing every pound in matched a pound out, net zero, and respond in writing. For the full process, see our PayPal payments flagged as income guide.
6. COP9: when HMRC suspects serious fraud
If HMRC suspects serious tax fraud — deliberate underpayment that’s material and concealed — they can open an investigation under Code of Practice 9 (COP9). COP9 is the formal route designed to keep you out of criminal prosecution by offering a Contract Disclosure Facility (CDF). Here’s how it works:
HMRC sends you a COP9 letter explaining they suspect fraud and offering you a CDF. You have 60 days to sign and return the CDF, in which you agree to make a full and complete disclosure of all tax irregularities — across all taxes and all years. If you do, and your disclosure is genuinely complete, HMRC guarantees that the case will be dealt with civilly (penalties and back tax) rather than criminally (prosecution). If you refuse the CDF or make an incomplete disclosure, HMRC can pursue a criminal investigation.
The penalties under COP9 are significant — up to 200% of the tax due for deliberate and concealed behaviour, plus interest. But the alternative (criminal prosecution) is worse, and the CDF is the only mechanism that gives you a guaranteed route away from prosecution.
If you receive a COP9 letter, get professional representation immediately. This is not a DIY situation. You need a tax adviser or accountant experienced in COP9 cases, and potentially a lawyer. Do not respond, sign anything, or contact HMRC until you’ve taken advice. The 60-day deadline is real, but the first step is getting help — not panicking and phoning HMRC.
7. When to get professional representation
Not every HMRC enquiry needs an accountant. Here’s the honest decision framework:
You can probably handle it yourself if: it’s a compliance check (not a formal enquiry), the questions are about one specific area you have records for, the amounts involved are small, and you’re confident you can explain your position clearly in writing. If HMRC is asking for invoices to support expense claims and you have them, you don’t need to pay someone to send them.
You should get an accountant or tax adviser if: the enquiry is formal and full (not just one aspect), HMRC is asking about areas where your records are thin, the potential tax at stake is significant (roughly: more than a few thousand pounds), you’re not sure whether you’ve done something wrong, or you’re not confident you can answer HMRC’s questions without inadvertently making things worse. An experienced adviser knows what HMRC can and can’t demand, can negotiate on penalties, and acts as a buffer between you and the officer.
You need a lawyer (not just an accountant) if: HMRC mentions COP9 or suspected fraud, you’re interviewed under caution, the case has been referred to the Fraud Investigation Service, or there’s any possibility of criminal prosecution. Accountants are not legally privileged — communications with an accountant are not confidential in the same way as communications with a lawyer. If there’s any criminal risk, you need legal representation.
If you can’t afford an accountant, TaxAid offers free, confidential tax advice to people on low incomes, including help with enquiries. For complex cases where you can afford help, use our Find an Accountant service and mention you’re under enquiry.
8. What cooperation actually means — and why it saves you money
Penalties for tax errors aren’t fixed — they’re ranges, and where you land in the range depends heavily on how you behave when HMRC finds the problem. The penalty framework under HMRC’s compliance handbook works on two axes: the behaviour that caused the error (genuine mistake, careless, deliberate, or deliberate and concealed) and the quality of your disclosure.
Quality of disclosure has three components:
- Telling — you tell HMRC about the non-compliance and explain what happened, ideally before they have to dig it out of you.
- Helping — you give HMRC reasonable help to quantify the underpayment: providing records, bank statements, and explanations that let them calculate what’s owed without a long investigation.
- Giving access — you allow HMRC to check your records freely, rather than making them issue formal information notices.
A full, honest, well-documented voluntary disclosure gets you to the bottom of the penalty range. A grudging, incomplete response after HMRC has found the problem gets you to the top. The difference is often enormous: on a careless error, the range is 0–30% of the tax due — full cooperation can mean 0%, while stonewalling can mean 30%. On a deliberate error, it’s 20–70%. On offshore matters, it’s 100–200%. For the specific penalty ranges by behaviour type, see our what triggers an HMRC investigation guide; for the offshore penalty structure specifically, see our undeclared foreign income guide.
9. What an enquiry costs (beyond the tax)
People focus on the tax and penalties, but an HMRC enquiry has other costs that are rarely discussed:
- Professional fees. An accountant handling an aspect enquiry might charge £500–£2,000; a full enquiry can run to £5,000–£20,000+ in fees depending on complexity and duration. COP9 cases are more. If you have fee protection insurance (often included with accounting subscriptions or professional membership), check whether it covers enquiry defence.
- Time. An enquiry is not a single meeting — it’s months of gathering records, responding to letters, attending meetings, and answering follow-up questions. For a sole trader, that’s time away from earning. Budget for it.
- Emotional toll. This is the cost people underestimate most. An open enquiry sitting in the background for months creates persistent anxiety, affects sleep, and can spill into your work and relationships. Naming it helps — and knowing the process (§3) and your rights reduces the fear of the unknown.
- Interest. Even if you agree the tax owed, interest accrues from the original due date until payment, currently at 7.25%. On a large assessment over several years, this compounds significantly.
The practical implication: if HMRC offers a settlement, factor in all of these costs — not just the headline tax figure — when deciding whether to accept or appeal. Dragging out an enquiry to save £500 in tax while spending £2,000 in fees and 6 months of stress is not always the right call.
10. How to reduce your risk of being selected
You can’t eliminate the risk — HMRC runs random compliance checks, and even a perfect return can be selected. But you can dramatically reduce your risk profile: declare all your income (especially income third parties already know about), keep accurate organised records, file on time every time, claim only legitimate expenses with supporting documentation, correct mistakes promptly through voluntary disclosure, and use MTD-compatible software so your digital records are consistent. For the full risk-reduction checklist and the detail behind each lever, see our what triggers an HMRC investigation guide.
The bottom line
- Not all HMRC contact is an investigation — there are 3 tiers (compliance check, formal enquiry, criminal), and most people only face the first two.
- Most enquiries are triggered by Connect, HMRC’s data-matching system — the biggest trigger is undeclared income that someone else already told HMRC about.
- You have rights — to appeal, to representation, to ask the tribunal to close the enquiry, and to be treated fairly.
- Cooperation saves you money — penalties are ranges, and full honest disclosure gets you to the bottom of the range.
- Get help if the enquiry is full, the amounts are large, or COP9 is mentioned — and get a lawyer, not just an accountant, if there’s any criminal risk.
- The best defence is declaring all your income and keeping good records. If Connect can find it, you should have declared it. If HMRC asks about it, your records should explain it.
If you’ve received an enquiry notice or a COP9 letter, professional representation is strongly recommended — especially for full enquiries or anything involving suspected fraud. Find an accountant through our free matching service — mention you’re under HMRC enquiry and we’ll match you with someone who handles investigation defence.
For the full list of HMRC contact types, investigations, and what to do when HMRC contacts you, see our HMRC contacted me hub.
For the full breakdown of what triggers an HMRC investigation and how the Connect system works, see our what triggers an HMRC investigation guide. For coming clean on undeclared foreign income, see our undeclared foreign income guide. For when platform-reported payments get flagged as income, see our PayPal payments flagged as income guide. For the full Self Assessment penalty regime, see our Self Assessment penalties guide.