HMRC investigations fall into four main types. Each one has a different scope and a different level of risk. The lightest is an aspect enquiry. Then comes a full enquiry. Above those sit COP8 and COP9 investigations. Your letter should say which one HMRC has opened.
Knowing the type tells you how much is at stake and how far back HMRC can go. HMRC normally has 12 months to open an enquiry into a return. This guide explains each type and how serious it is.
Disclaimer: This article gives general information about HMRC enquiries. It isn’t tax or legal advice. If HMRC has written to you, speak to a qualified accountant before you reply.
What are the main types of HMRC investigation?
HMRC uses four main routes into your tax affairs. An aspect enquiry looks at one part of your return. A full enquiry looks at the whole return and the records behind it. COP8 and COP9 are fraud investigations, and COP9 is the most serious of the two.
HMRC publishes its rules on tax compliance checks. It writes or phones first to say what it wants to look at. The letter names the tax, the year and the scope.
| Type | What HMRC checks | Severity |
|---|---|---|
| Aspect enquiry | One or two named entries on your return | Lowest |
| Full enquiry | The whole return and the records behind it | Medium |
| COP8 | A large suspected tax loss, usually avoidance | High |
| COP9 | Suspected tax fraud | Highest civil level |
| Criminal investigation | Fraud, with prosecution in view | Highest |
The words "aspect" and "full" don’t appear in the tax law. HMRC opens Self Assessment enquiries under section 9A of the Taxes Management Act 1970. That one power covers your whole return. The two labels just describe how wide HMRC chooses to look.
What is an aspect enquiry?
An aspect enquiry looks at one part of your tax return. HMRC names the entry it wants to check. It might be one expense claim, one property, or one source of income. The rest of your return stays outside the enquiry for now.
HMRC names the entry in the letter itself. It might ask about one rental property. It might ask about your motor expenses. It might ask why your gross margin dropped.
An aspect enquiry does not mean HMRC suspects fraud. It means one figure did not sit right in its risk scoring. Records that answer the question end the enquiry.
HMRC can still add a penalty at the end. It looks at why you underpaid the tax. It checks whether you told HMRC as soon as you could. It also weighs how helpful you were during the check.
What is a full enquiry?
A full enquiry covers your whole tax return. HMRC checks every figure and asks for the records behind each one. Bank statements, sales invoices, supplier receipts and payroll records all come into scope. HMRC can ask to visit your home, your business or your adviser’s office.
A full enquiry takes longer and costs more to handle. You can have your accountant or a legal adviser with you at any visit.
HMRC can charge a penalty if you refuse a visit. The same applies if you ignore an information notice. That penalty falls away if you have a reasonable excuse. Serious illness counts. So does a death close to you.
HMRC writes to you with the result at the end. You get repaid with interest if you paid too much. You pay any extra tax within 30 days if you owe more. Interest normally runs from the date the tax fell due.
Can an aspect enquiry become a full enquiry?
Yes, HMRC can widen an aspect enquiry without sending a new notice. The law lets HMRC issue only one notice of enquiry per return. That single notice covers the whole return from the start. So the scope can grow while the letter stays the same.
This point catches people out. Most guides treat aspect and full enquiries as two separate events. They are not. An aspect enquiry is a door, not a boundary.
So treat the first letter as the start of the whole process. What you send in week one shapes what HMRC asks for in month six.
The window works in your favour too. If HMRC misses its 12-month deadline, it normally loses the right to enquire. The return and the self assessment are then final.
What is a COP8 investigation?
HMRC’s Fraud Investigation Service runs COP8 investigations. It uses COP8 where it believes a large tax loss has happened. Most COP8 cases involve avoidance schemes rather than dishonesty. HMRC does not say you’ve been dishonest when it opens a COP8 case.
The same HMRC team runs COP8 and COP9 cases. It uses Code of Practice 8 where the COP9 route does not fit.
COP8 mostly covers avoidance schemes and marketed tax planning. HMRC sometimes uses it for evasion cases too. That happens when the COP9 criteria do not apply for some reason.
A COP8 case often ends with penalties. HMRC never offers the Contractual Disclosure Facility in a COP8 case. That facility belongs to COP9 alone.
What is a COP9 investigation?
HMRC opens a COP9 investigation when it suspects tax fraud. The letter offers you a contract called the Contractual Disclosure Facility. You get 60 calendar days to accept or reject that offer. Accepting and disclosing in full stops HMRC prosecuting you for what you disclose.
The COP9 pack contains four items. An acceptance letter, a rejection letter, a disclosure form and a copy of COP9.
You have two choices and a deadline. Accepting the Contractual Disclosure Facility means you admit causing a tax loss on purpose. HMRC then agrees not to prosecute you for what you disclose. You still pay the tax, the interest and civil penalties.
You give outline detail within the 60 days. A fuller disclosure report follows after that period.
Rejecting the offer carries its own risk. HMRC can start a criminal investigation at any time. It can also use the letter you signed as court evidence.
Doing nothing is the worst option of the three. HMRC treats silence as a choice not to co-operate. It then starts a civil or criminal investigation anyway. HMRC extends the 60 days only in exceptional cases.
COP8 or COP9: which letter did you get?
| COP8 | COP9 | |
|---|---|---|
| What HMRC suspects | A large tax loss, usually avoidance | Tax fraud |
| Does HMRC allege dishonesty | No | Yes |
| Disclosure Facility offered | Never | Yes, with a 60-day clock |
| Protection from prosecution | Not on offer | Yes, for what you fully disclose |
COP9 sits at the top of the civil ladder. Both codes come from the same HMRC team. Both need specialist handling from the first reply. Our page on dealing with an HMRC investigation explains how we handle it.
How far back can HMRC go?
HMRC normally assesses four years back from the end of the relevant tax period. Careless behaviour extends the window to six years. Offshore income and offshore transfers can reach twelve years. Intentional behaviour lets HMRC assess twenty years of tax.
Two separate clocks matter here. The first is the enquiry window. The second is the assessing time limit.
HMRC normally has 12 months from the day your return arrives. File after the deadline and that window runs longer. It then closes on the quarter day after the first anniversary. The quarter days are 31 January, 30 April, 31 July and 31 October.
Once the window shuts, HMRC needs an assessment instead. Assessments have their own time limits. The normal limit is four years from the end of the tax period.
Careless behaviour pushes that to six years. The six-year limit covers income tax, capital gains tax and corporation tax. It also covers stamp duty land tax and inheritance tax.
The 12-year offshore limit gets missed by most guides. It applies to income tax, capital gains tax and inheritance tax. It covers offshore matters and offshore transfers. It matters if you hold any assets abroad.
Intentional behaviour takes HMRC back twenty years. That is the number people hear about most. It only applies where HMRC shows intent.
How serious is your letter?
Two details tell you most of it. Check which law or code of practice HMRC names. Then check whether HMRC asks about one entry or all your records. A named code of practice puts the letter near the top of the ladder.
A section 9A notice with one question is the lowest rung. A section 9A notice asking for every record is a full enquiry. A COP8 booklet means the Fraud Investigation Service holds your file. A COP9 pack with a 60-day clock is the most serious civil letter of all.
The type of letter follows the risk signal that flagged you. Our guide on what triggers an HMRC investigation covers those signals.
Not every letter from HMRC opens an enquiry. Our guide explains what an HMRC letter means and the deadlines inside it.
An enquiry is not a finding. HMRC writes to tell you the result at the end. Until that letter arrives, nothing has been decided.
Talk to us before you reply to HMRC
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Author: Raqeeb Marzook ACCA, Manager at DASA Accountancy. He works with UK businesses and sole traders on tax compliance. Last verified against HMRC guidance on 6 August 2026. For corrections, contact: raqeeb@dasaaccountancy.co.uk
FAQs
What are the types of HMRC investigation?
HMRC uses four main routes. An aspect enquiry looks at one part of your return. A full enquiry checks the whole return and all your records. COP8 and COP9 are Fraud Investigation Service cases, and COP9 is the most serious.
What is an HMRC aspect enquiry?
An aspect enquiry looks at one named part of your tax return. HMRC states the entry it wants to check. The rest of the return stays outside the enquiry for now. It does not mean HMRC suspects fraud.
Can an aspect enquiry turn into a full enquiry?
Yes. The law lets HMRC issue only one notice of enquiry per return. That notice covers the whole return from the start. HMRC can widen the scope without sending a second letter.
What is the difference between COP8 and COP9?
COP8 covers a large suspected tax loss, usually from avoidance, and HMRC does not allege dishonesty. COP9 means HMRC suspects tax fraud and offers the Contractual Disclosure Facility with a 60-day deadline.
How far back can HMRC go?
HMRC normally assesses four years back from the end of the tax period. Careless behaviour extends this to six years. Offshore matters can reach twelve years. Intentional behaviour lets HMRC go back twenty years.
How long does HMRC have to open an enquiry?
HMRC normally has 12 months from the day your return arrives. If you file late, the window closes on the quarter day after the first anniversary of filing. The quarter days are 31 January, 30 April, 31 July and 31 October.