What Happens If HMRC Opens a Self Assessment Enquiry?
Receiving a letter from HMRC opening an enquiry into your Self Assessment return is unsettling, but it doesn’t automatically mean HMRC thinks you’ve done something wrong — some enquiries are opened entirely at random. This guide from Filing Accounts UK explains the different types of Self Assessment enquiry, the strict time limits HMRC must follow, what happens at each stage, and your rights throughout, using only verified facts based on the Taxes Management Act 1970 and current HMRC practice.
At Filing Accounts, we support clients through HMRC enquiries as part of our tax enquiry service. If you’ve received an enquiry letter, always seek advice before responding directly to HMRC.
What Is a Self Assessment Enquiry?
A Self Assessment enquiry — also called a compliance check — is a formal examination by HMRC of the accuracy of a tax return you’ve already submitted. An enquiry can only be opened once you’ve actually filed a return; HMRC has no power to “enquire” into a return that doesn’t yet exist. HMRC must open an enquiry formally, in writing — a phone call or informal email query alone does not constitute a valid enquiry opening.
Types of Self Assessment Enquiry
Aspect Enquiry (Most Common)
An aspect enquiry is a targeted look at one specific item on your return — a particular expense claim, a single source of rental income, a capital gain, or a specific relief you’ve claimed. These are typically triggered by data-matching inconsistencies: HMRC cross-references your return against data from banks, Land Registry, Companies House, DVLA, and other sources, and an aspect enquiry often follows when something doesn’t line up. While legally an enquiry technically covers the whole return, in practice HMRC will usually limit its questions to the specific area flagged — unless something else comes to light during the process.
Full Enquiry
A full enquiry is a comprehensive examination of your entire tax return and the records behind it — all income sources, reliefs, and claims. HMRC will typically request full access to accounts, bank statements, invoices, and payroll records. Full enquiries are more common where HMRC suspects wider inaccuracies, or where a previous aspect enquiry has uncovered issues suggesting the whole return needs a closer look.
Random Enquiry
HMRC carries out a proportion of enquiries on a purely random basis each year, regardless of whether there’s any specific reason to suspect an error, partly to help HMRC estimate the overall scale of inaccurate returns across the tax system. If your return is selected at random, HMRC is not legally required to tell you why it’s been chosen. If the opening letter doesn’t specify whether it’s a full or aspect enquiry, it’s worth asking HMRC to confirm the scope in writing rather than assuming.
Discovery Assessment
A discovery assessment isn’t a “type” of enquiry in the same sense — it’s a separate power HMRC can use to assess additional tax outside the normal enquiry window, where new facts come to light suggesting tax has been underpaid. This is covered in detail further down, as it operates under much longer time limits than a standard enquiry.
Code of Practice 9 (COP9) — Serious Fraud Investigation
COP9 is reserved for cases where HMRC suspects serious tax fraud. It operates through the Contractual Disclosure Facility, under which the taxpayer is offered the chance to make a full disclosure in exchange for HMRC not pursuing a criminal prosecution for the specific matters disclosed. This is a significantly more serious process than a routine enquiry, and specialist legal and tax advice is essential if you receive a COP9 letter.
Time Limits: How Long Does HMRC Have to Open an Enquiry?
| Situation | HMRC’s Enquiry Window |
|---|---|
| Return filed on time | 12 months from the date HMRC received it |
| Return filed late | Up to the next quarter day (31 Jan, 30 Apr, 31 Jul, or 31 Oct) after the first anniversary of filing |
| Amended return | 12 months from the end of the quarter in which the amendment was filed |
Example: if you filed your 2024/25 return online on 15 November 2025, HMRC generally has until 15 November 2026 to open a standard enquiry. After that date passes, HMRC can no longer open a routine enquiry into that return — its only remaining option is a discovery assessment, and only where specific conditions are met.
Discovery Assessments: When HMRC Can Go Back Further
Once the normal enquiry window has closed, HMRC can still assess additional tax using a discovery assessment — but only where it can show it couldn’t reasonably have known about the shortfall from the information already provided. How far back HMRC can go depends on the taxpayer’s behaviour:
| Behaviour | Time Limit |
|---|---|
| Innocent error (reasonable care taken) | 4 years from the end of the relevant tax year |
| Careless error | 6 years from the end of the relevant tax year |
| Deliberate understatement | Up to 20 years from the end of the relevant tax year |
This tiered structure is exactly why keeping good records matters well beyond the standard enquiry window — see our guide on what records to keep for HMRC.
What Happens During an Enquiry: Step by Step
Step 1: The Opening Letter
HMRC sends a formal notice confirming an enquiry has been opened, generally indicating whether it’s a full or aspect enquiry and, unless it’s a random selection, some indication of what’s being examined.
Step 2: Information and Document Requests
HMRC has the legal right to ask for information or documents it reasonably requires to check your tax position, sometimes formalised through a statutory information notice. HMRC cannot demand records that genuinely don’t exist — but failing to have kept adequate records in the first place is itself a separate penalty matter.
Step 3: Meetings and Further Correspondence
Depending on the complexity, HMRC may request a meeting or exchange several rounds of correspondence. You are not obliged to attend a meeting in person, though cooperation generally helps resolve matters faster.
Step 4: Closure Notice
Once HMRC is satisfied, it issues a closure notice, either confirming no changes are needed or amending your return and setting out any additional tax, interest, and penalties due. If an enquiry appears to be dragging on unreasonably without progress, you can apply to the tax tribunal for a direction requiring HMRC to issue a closure notice within a specified period.
Step 5: Right of Appeal
If you disagree with HMRC’s conclusion, you generally have the right to appeal, first through HMRC’s internal review process and, if unresolved, to the independent tax tribunal.
Penalties: How Much Could an Enquiry Cost?
Penalties for an inaccuracy found during an enquiry are directly linked to the behaviour behind the error, not just the amount of tax involved:
- Reasonable care taken, genuine mistake: No penalty, though the underpaid tax and interest are still due
- Careless inaccuracy: A percentage-based penalty, reduced significantly for unprompted disclosure and full cooperation
- Deliberate inaccuracy: A substantially higher penalty
- Deliberate and concealed inaccuracy: The highest penalty band, particularly where offshore matters are involved
In every case, cooperating early and disclosing an error yourself, before HMRC identifies it, generally leads to a significantly lower penalty than if HMRC uncovers the same issue independently.
Why HMRC Selects a Return for Enquiry
HMRC uses a combination of methods to select returns:
- Data matching — cross-referencing your return against third-party data from banks, Land Registry, Companies House, DVLA, and other sources
- Risk-based criteria — figures that fall outside typical patterns for your industry or income level
- Random selection — a proportion of returns are checked with no specific trigger at all
- History of errors or late filing — a track record of previous inaccuracies or missed deadlines raises perceived risk
The majority of Self Assessment returns are processed each year with no enquiry at all.
Enquiry Types at a Glance
| Type | Scope | Typical Trigger |
|---|---|---|
| Aspect enquiry | One specific item on the return | Data-matching inconsistency |
| Full enquiry | Entire return and underlying records | Suspected wider inaccuracy |
| Random enquiry | Varies — no specific reason given | Statistical selection |
| Discovery assessment | Specific underpaid amount identified | New information after the enquiry window closed |
| COP9 | Full financial affairs | Suspected serious fraud |
Common Mistakes to Avoid
Assuming an Enquiry Means You’re Being Accused of Wrongdoing
Many enquiries — particularly random ones — carry no implication of suspected wrongdoing at all.
Responding to HMRC Without Checking the Scope First
If the letter doesn’t specify whether it’s a full or aspect enquiry, confirm the scope in writing before providing information, rather than volunteering more than necessary.
Ignoring the Letter or Missing Response Deadlines
Failing to engage can escalate a straightforward aspect enquiry into something broader, and can affect the penalty behaviour category applied later.
Not Distinguishing Between the Normal Enquiry Window and Discovery Assessments
Being outside the standard 12-month window doesn’t necessarily mean you’re completely safe — a discovery assessment can reach back much further depending on behaviour.
Not Seeking Advice Early
Getting professional support at the opening letter stage, rather than after the enquiry has escalated, generally leads to a faster and less costly resolution.
Frequently Asked Questions
What’s the difference between an aspect and a full enquiry?
An aspect enquiry looks at one specific item on your return; a full enquiry examines the entire return and all underlying records.
How long does HMRC have to open a Self Assessment enquiry?
Generally 12 months from the date you filed, if filed on time. Late or amended returns extend this to the next quarter day.
Can HMRC still investigate after the 12-month window closes?
Yes, via a discovery assessment, but only where it can show it couldn’t reasonably have known about an underpayment from the information already provided, and within time limits based on behaviour (4, 6, or 20 years).
Does HMRC have to tell me why my return was selected?
Not if it’s a random enquiry — HMRC has no legal obligation to explain the reason in that case.
Will I definitely face a penalty if HMRC finds an error?
Not necessarily. If you took reasonable care and made a genuine mistake, no penalty applies, though the underpaid tax and interest are still owed.
What is COP9?
Code of Practice 9 is HMRC’s process for suspected serious fraud, offering a chance to make a full disclosure via the Contractual Disclosure Facility in place of criminal prosecution for the disclosed matters.
Can I make HMRC close an enquiry that’s dragging on?
Yes. You can apply to the tax tribunal for a direction requiring HMRC to issue a closure notice within a set period, where the enquiry appears to be taking unreasonably long.
Facing an HMRC Enquiry? Talk to Filing Accounts UK
Whatever type of enquiry you’re facing, how you respond in the first few weeks often shapes the entire outcome. At Filing Accounts, we support clients through HMRC Self Assessment enquiries, from an initial aspect enquiry through to full investigations, helping you respond correctly and minimise any penalty.
You may also find these related guides helpful: