You’ve just opened a letter from HMRC. It says they’re looking into your tax affairs. Your stomach drops. You’re staring at the page, wondering what this means, how much it’s going to cost you, and whether you’re in serious trouble.
But the question is: what should you do next?
Should you call HMRC straight away? Should you ignore it and hope it goes away? Should you get an accountant? Or should you gather your records and respond yourself?
In this guide, I’m going to show you the 7 most important things to do after you’ve received an HMRC investigation letter — step by step. The most important thing: don’t ignore it, and don’t phone HMRC without preparing first. What you do in the first few days after opening that letter shapes how the entire enquiry plays out. For the full background on what HMRC investigations are and how they work, see our HMRC investigation hub.
Step 1: Read the letter carefully and identify what type of contact it is
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. The letter in front of you tells you which tier you’re in — if you read it carefully.
A compliance check (informal). This is the most common form of contact. HMRC writes or calls asking questions or requesting records to verify something on your return. It doesn’t require a formal notice, and most compliance checks close without escalating. The letter will usually say something like “we’re checking your tax return” and ask for specific information. According to HMRC’s guidance on compliance checks, HMRC will write or phone to say what they want to check — and they must enclose a copy of factsheet CC/FS1a, which explains your rights.
A 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. The letter will state that they’re opening an enquiry into your return under section 9A. According to HMRC’s Enquiry Manual (EM1551), the opening letter must include the notice of enquiry and a copy of the relevant factsheet. Once an enquiry is open, it stays open until HMRC issues a closure notice.
A COP9 letter (suspected serious fraud). This is the most serious. HMRC sends a COP9 letter when they suspect deliberate, material, and concealed tax fraud. It offers you a Contract Disclosure Facility — a guaranteed route to avoid criminal prosecution if you make a full and complete disclosure within 60 days. According to HMRC’s Code of Practice 9 guidance, if you receive a COP9 letter, you need professional representation immediately. Do not respond, sign anything, or contact HMRC until you’ve taken advice.
Read the letter twice. Note the type of contact, the tax year(s) they’re asking about, the specific information requested, and any response deadline. If the letter mentions COP9, suspected fraud, or the Fraud Investigation Service, go straight to Step 5.
Step 2: Check the enquiry window is valid
If the letter is a formal enquiry under section 9A, HMRC had a fixed time limit to open it. If they missed it, the enquiry may be invalid — and you can challenge it.
According to HMRC’s Self Assessment Manual (SAM31100), the enquiry window works like this:
- If you filed your return on time (by 31 January): HMRC has 12 months from the date you filed to open an enquiry. For a return filed on 31 January 2026, the enquiry window closes on 31 January 2027.
- If you filed late: the window extends to the quarter day following the first anniversary of the filing date.
Check the date on the enquiry letter against the date you filed your return. If the letter was sent outside the enquiry window, the enquiry may be invalid — contact an accountant or tax adviser before responding. Don’t assume HMRC has got it right; the deadline is statutory and HMRC does occasionally miss it.
If the letter is a compliance check (not a formal enquiry), there’s no fixed time limit for HMRC to ask questions — they can do that at any time. But there are time limits on what they can do if they find errors, which is covered in Step 6.
Step 3: Don’t ignore it and don’t phone HMRC immediately
The two worst reactions to an HMRC letter are the two most common ones: ignoring it, and phoning HMRC in a panic.
Ignoring it is the worst option. If you don’t respond, HMRC can issue formal information notices requiring you to provide documents — and failing to comply with a formal notice can result in penalties. Worse, if you don’t engage, HMRC will make their assessment based on the information they have, which is almost always worse for you than if you’d participated. They can also raise assessments for the tax they think is due, which you’ll then have to appeal. For the full breakdown of what happens if you don’t respond, see our HMRC investigation hub.
Phoning HMRC without preparing is nearly as bad. Anything you say in that call can be used in the enquiry. If you’re flustered, you might volunteer information they didn’t ask for, speculate about figures you’re not sure of, or inadvertently confirm something that isn’t accurate. HMRC officers are experienced at getting information; you are not experienced at giving it under pressure.
The right response is: read the letter, gather your records, and respond in writing within the deadline. If you want to speak to HMRC, do it through an accountant or tax adviser who knows what HMRC can and can’t demand — and who acts as a buffer between you and the officer.
Step 4: Gather your records for the period they’re asking about
Once you know what HMRC is checking, gather the records that cover it. The letter will tell you which tax year(s) and which specific areas they’re asking about. Don’t gather everything you’ve ever filed — focus on what they’ve actually requested.
For a compliance check asking about expense claims, that means:
- Invoices and receipts for the expenses in question
- Bank statements showing the payments
- Any supporting documentation (contracts, mileage logs, receipts for materials)
- Your Self Assessment return for the year in question
For a formal enquiry, HMRC can request information that’s “reasonably required” to check your return — but they can’t go on a fishing expedition. According to HMRC’s Self Assessment Legal Framework (SALF402), the courts have balanced HMRC’s information powers against your right not to be subject to fishing expeditions. If HMRC asks for something that seems disproportionate or irrelevant, you can push back — though this is where professional representation helps.
If your records are thin for the period they’re asking about, that’s a signal to get professional help before you respond. Don’t try to reconstruct records from memory; if you’re missing documentation, say so honestly. Inventing records after the fact is far worse than admitting you don’t have them. For the record-keeping basics that prevent this situation, see our first-year self-employed checklist.
Step 5: Decide whether you need professional representation
Not every HMRC enquiry needs an accountant. But some absolutely do, and knowing the difference is one of the most important decisions you’ll make after receiving the letter.
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 HMRC enquiry.
For the full decision framework and what professional representation costs, see our HMRC investigation hub.
Step 6: If you’ve made errors, consider voluntary disclosure
If you read the letter and realise you have made an error on your return — or several — the best thing you can do is come forward before HMRC finds it. This is called voluntary disclosure, and it’s the single biggest lever you have for reducing penalties.
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. According to HMRC’s compliance handbook, the penalty framework works on two axes: the behaviour that caused the error (genuine mistake, careless, deliberate, or deliberate and concealed) and the quality of your disclosure. A full, honest, well-documented voluntary disclosure gets you to the bottom of the penalty range — often 0% for a genuine mistake corrected before HMRC finds it. 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%.
To make a voluntary disclosure, follow HMRC’s guidance on making a disclosure or use their online service to tell HMRC about underpaid tax from previous years. If the undisclosed income is offshore, use the Worldwide Disclosure Facility. You notify HMRC first, then have 90 days to calculate what you owe and submit the full disclosure.
If you’ve already received an enquiry letter, you can still disclose — but the penalty reduction is smaller than if you’d come forward before HMRC contacted you. The key is to be open and complete in what you tell them. For the full process on correcting mistakes on previous returns, see our made a mistake on your Self Assessment guide.
Step 7: Respond in writing within the deadline
Once you’ve read the letter, gathered your records, decided on representation, and considered disclosure, it’s time to respond. Do this in writing — not by phone.
Your response should:
- Answer the specific questions asked. Don’t volunteer information about areas HMRC hasn’t asked about. Answer what they’ve asked, clearly and concisely.
- Include the records they’ve requested. Send copies, not originals, unless they specifically ask for originals. Keep a full copy of everything you send.
- Meet the deadline. If the letter gives a response deadline, meet it. If you need more time, write and ask for an extension before the deadline expires — HMRC is usually reasonable about this if you ask in advance.
- Be honest and complete. If you don’t have a document, say so. If a figure on your return was wrong, say so and explain why. Honesty at this stage reduces penalties; dishonesty discovered later increases them.
- Don’t speculate. If you’re not sure about something, say you need to check your records and will confirm. Don’t guess at figures under pressure.
If you disagree with HMRC’s findings after the enquiry, you have the right to appeal. According to HMRC’s guidance on disagreeing with a tax decision, you usually have 30 days from the date of the decision letter to either appeal or accept a review. You can also ask for a statutory review by a different HMRC officer, or appeal to the tax tribunal. For the full appeal process, see our how to appeal an HMRC penalty guide.
Throughout the enquiry, you also have the right to ask the tax tribunal to direct HMRC to close the enquiry if it’s dragging on unreasonably. This is a formal application — see HMRC’s tax tribunal guidance for the process.
The Bottom Line
What you do after receiving an HMRC investigation letter matters more than anything that led to the letter being sent. The enquiry process is designed to reward cooperation and punish stonewalling — and the penalty ranges reflect that.
The most important steps from this guide:
- Read the letter carefully — identify whether it’s a compliance check, a formal enquiry, or a COP9 letter. The type of contact determines everything that follows.
- Don’t ignore it and don’t phone HMRC without preparing — the two worst reactions are also the two most common ones. Respond in writing within the deadline.
- If you’ve made errors, come forward before HMRC finds them — voluntary disclosure is the single biggest lever for reducing penalties. The difference between 0% and 30% (or 70%, or 200%) is often whether you told them or they had to dig it out of you.
Most enquiries close with no or modest adjustments if you engage honestly and keep good records. The fear is almost always worse than the reality. But if the letter mentions COP9 or suspected fraud, get professional representation before doing anything else — that’s not a DIY situation.
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 background on what HMRC investigations are, the 3 tiers of scrutiny, COP9, when to get professional representation, and what an enquiry costs, see our HMRC investigation hub. For what triggers investigations 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 the full Self Assessment penalty regime, see our Self Assessment penalties guide.