Receiving a letter from HMRC notifying you that your tax return is under inquiry is unsettling, but it does not automatically mean you have done anything wrong. HMRC opens investigations for a wide range of reasons, from sophisticated data matching that spots an inconsistency in your return to a completely random selection that has nothing to do with any risk factor at all. What matters is how you respond. A professional, organized, and timely response to an HMRC inquiry is far more likely to result in a swift closure than a delayed, disorganized, or adversarial one. This guide explains what triggers an investigation, the different types of inquiries HMRC uses, how far back it can go, what your rights are, and exactly what to do if you receive that letter.
HMRC inquiries and investigations
In the UK, HMRC does not typically refer to its compliance activity as an audit, as in other jurisdictions. Instead, it uses the term inquiry for formal reviews of Self-Assessment tax returns under section 9A of the Taxes Management Act 1970, and the term investigation for more serious cases involving suspected fraud or criminal conduct.
An inquiry can cover income tax, corporation tax, VAT, PAYE, Capital Gains Tax, or any combination of these. It can focus on a single line of a return, a specific category of expenses, or the entirety of an individual’s or business’s financial affairs. The scope depends on what triggered the review and what HMRC finds during its review.
The types of HMRC inquiries
As a matter of law, all inquiries into a tax return are inquiries into the full return. While HMRC uses the terms “full inquiry” and “aspect inquiry” for internal classification purposes, this distinction carries no legal weight even where HMRC focuses only on a specific aspect in practice.
Aspect inquiry
An aspect inquiry is a targeted review of one or more specific items in a tax return. HMRC has identified something that does not look right, perhaps a high level of expenses relative to income, an unusual deduction, or a figure that does not match data HMRC has received, for instance, employment income reported by an employer under Real Time Information (RTI) or interest reported by a bank or building society. HMRC will want an explanation. Aspect inquiries are the most common form and are often resolved relatively quickly, where the taxpayer has good records.
Full inquiry
A full inquiry covers the entire return and, potentially, the underlying books and records of the business. HMRC’s starting point is a comprehensive review rather than a single item. Full inquiries are more time-consuming, more intrusive, and more expensive to defend against. They are typically opened when HMRC has identified multiple inconsistencies, when a business operates in a high-risk sector, or when an aspect inquiry has uncovered issues suggesting wider problems.
Discovery assessment
Where the standard twelve-month inquiry window has closed, HMRC can still raise a discovery assessment if it discovers information suggesting a loss of tax that it could not reasonably have been aware of from the return itself. Discovery assessments are subject to specific time limits discussed below.
COP9 civil investigation of fraud
Code of Practice 9 is HMRC’s most serious civil investigation procedure, used where it suspects deliberate tax fraud. Under COP9, HMRC offers the taxpayer the opportunity to make a full disclosure under the Contractual Disclosure Facility (CDF). The taxpayer has 60 days from receipt of the offer to accept or reject it; failure to respond within that period constitutes a rejection. Accepting the offer protects from criminal prosecution in exchange for complete cooperation and full disclosure. Rejecting it, whether expressly or by non-response, removes that protection, and HMRC may proceed with a civil or criminal investigation without the benefit of the taxpayer’s cooperation. Anyone facing a COP9 investigation must take specialist legal advice immediately.
Criminal investigation
HMRC reserves criminal investigation for the most serious cases where it intends to prosecute. These are relatively rare but carry the possibility of imprisonment. Criminal investigations are typically preceded by a civil inquiry or COP9 process that has broken down, or by intelligence suggesting serious and deliberate fraud.
What triggers an HMRC investigation
The Connect system and data matching
HMRC operates a sophisticated data analytics system called Connect, which cross-references information from thousands of sources, including bank accounts, the Land Registry, Companies House, the DVLA, social media, property rental platforms, online marketplaces, financial institutions, and overseas tax authorities. Connect compares the data it holds about an individual or business against what has been declared on tax returns and flags discrepancies automatically.
HMRC is actively cross-referencing platform data with Self Assessment returns. If your platform sales exceeded £1,000 and were not declared, a nudge letter or formal inquiry is a realistic outcome. This applies to income from Vinted, eBay, Airbnb, Etsy, Uber, Deliveroo, and other digital platforms that are now legally required to report user income directly to HMRC.
Inconsistencies in the return itself
Returns that contain figures that do not add up internally, that show unusual year-on-year swings in income, expenses, or profit margins without an obvious explanation, or that show results very different from comparable businesses in the same sector are more likely to be flagged for review.
Common inconsistencies that attract attention include sudden large increases in expenses relative to income, a significant drop in declared profits despite stable or growing turnover, pension contributions or dividend payments that appear disproportionate to reported income, and expenses in categories that are unusual for the stated trade.
High-risk sectors and trades
HMRC maintains intelligence on sectors where cash transactions are common or where tax compliance is historically lower. Hospitality, construction, taxi and private hire, hair and beauty, and some retail sectors are subject to heightened scrutiny. Operating in these sectors does not mean you will be investigated, but it does mean your return is more likely to receive closer attention than a comparable return from a lower-risk trade.
Third-party information
HMRC receives information from a wide range of sources beyond platform operators. Mortgage lenders report income declared on applications. Financial institutions report interest paid. Employers report wages via PAYE. Pension providers report payments. Land Registry reports property transactions. The Valuation Office Agency reports rental values. All of this data flows into Connect and is compared against declared income.
Whistleblower reports also trigger investigations. A disgruntled former employee, a business partner, or a competitor who reports suspected tax evasion can prompt HMRC to open an inquiry. HMRC has a dedicated reporting line and takes these reports seriously.
R&D tax relief claims
R&D claims have been subject to significantly increased scrutiny in recent years, with a growing number being flagged for inquiry. HMRC has expanded its R&D compliance activity in response to concerns about inflated and fraudulent claims. If a claim was intentionally inflated or fraudulent, the inquiry can escalate into a serious COP9 investigation. Businesses making R&D claims should ensure their Additional Information Form is thorough, technically precise, and supported by detailed documentation.
Random selection
A small proportion of inquiries arises from random selection rather than any specific risk factor. A randomly selected return may be entirely correct, and the inquiry may close quickly with no adjustment. The existence of random selection means that compliance is the only reliable protection, since no taxpayer can be certain they will never be selected, regardless of how accurately they file.
Data suggesting a mismatch between lifestyle and declared income
Where available data suggests a mismatch between a taxpayer’s apparent lifestyle and their declared income, property ownership, vehicle, foreign travel, or expenditure may prompt further review. This is typically a result of Connect’s data matching and third-party information rather than a standalone trigger, but it is a meaningful factor in how HMRC prioritizes cases.
How far back can HMRC go?
The time limits for HMRC investigations are set by the Taxes Management Act 1970. As a general rule, HMRC has 12 months from the date the return was actually filed to open a formal inquiry. Beyond that window, it can go back 4 years for innocent errors, 6 years for careless errors, and up to 20 years for deliberate behavior.
These time limits apply as follows:
- 12 months from the date the return was actually filed to open a standard inquiry under section 9A. If HMRC does not open an inquiry within this window, the return is generally finalized unless a discovery assessment applies.
- Four years from the end of the relevant tax year for discovery assessments where there is no carelessness or deliberate conduct, but HMRC has discovered information suggesting a loss of tax that it could not reasonably have been aware of from the return itself.
- Six years from the end of the relevant tax year, where the loss of tax was brought about by a careless error on the part of the taxpayer or their agent.
- 20 years from the end of the relevant tax year, where the loss of tax was brought about by deliberate behavior.
- 12 years for offshore matters where the loss of tax involves offshore income, assets, or activities, and the standard time limits do not apply.
A significant Upper Tribunal decision in HMRC v Harte [2026] UKUT 00112 (TCC), handed down on 11 March 2026, confirmed that HMRC must consider each discovered insufficiency separately when raising a discovery assessment. Where a return contains multiple errors arising from different degrees of culpability, HMRC cannot apply the 20-year time limit across all errors simply because one was deliberate. The Upper Tribunal ruled that the facts attributable to each error must be considered independently to determine whether an assessment has been raised on a valid basis, creating a binding precedent for future disputes. This is an important taxpayer protection, particularly for anyone entering a COP9 disclosure process.
What to do when you receive an HMRC inquiry notice
Do not ignore it
HMRC inquiry notices carry statutory deadlines for response. Ignoring a notice does not make it go away. It gives HMRC the right to issue information notices, impose penalties for non-compliance, and draw adverse inferences from the lack of cooperation. The first step when you receive a notice is to read it carefully, note the response deadline, and take action immediately.
Appoint a tax adviser or specialist if you do not have one.
An HMRC inquiry is not a matter to handle alone unless it is a clearly straightforward aspect inquiry covering a single, easily explained point. For anything beyond that, appointing a qualified accountant, tax adviser, or tax solicitor is the most important step you can take. They can communicate with HMRC on your behalf, assess the scope and risk of the inquiry, manage the flow of information, and ensure you do not inadvertently provide more information than is required or legally requested.
If HMRC has written to your accountant as your authorized agent, your accountant will typically contact you immediately. Work with them closely and respond promptly to their requests for information.
Gather and review your records.
Before responding to any HMRC request, gather all relevant records for the period under inquiry. Bank statements, invoices, receipts, mileage logs, contracts, and any other documentation supporting the figures in your return should be located and reviewed. Understanding your own position before HMRC asks detailed questions puts you in a much stronger position than scrambling to find records after the fact.
If records are missing or incomplete, it is better to know this before responding to HMRC than to discover it mid-inquiry.
Respond accurately and completely, but only to what is asked
HMRC has the right to request information and documents that are reasonably required for the inquiry. You are legally obliged to provide what is properly requested within the timeframe given or an agreed extension. You are not obliged to provide information that goes beyond what has been formally requested.
A common mistake is to volunteer additional information in an attempt to appear cooperative. This can inadvertently widen the scope of the inquiry into areas that were not originally under review. Answer what is asked, clearly and completely, and take advice before providing anything that was not specifically requested.
Do not amend your return unless advised to do so
Amending a return without professional advice during an open inquiry can be interpreted as an admission and may complicate the resolution. If you believe there is an error in your return, discuss this with your adviser before making any amendments.
Understand what HMRC can and cannot require.
HMRC can issue formal information notices under Schedule 36 of the Finance Act 2008, requiring the production of documents and information. These notices are legally enforceable, and failure to comply carries penalties. However, HMRC’s powers are not unlimited. It cannot require the production of documents protected by legal professional privilege. It cannot require information that is not reasonably required for the inquiry. It cannot go on a fishing expedition into years long past without demonstrating that the conditions for a discovery assessment are met.
If you receive a Schedule 36 information notice that you believe goes beyond HMRC’s legitimate powers, it can be appealed. Appeals must be made promptly, and professional advice should be sought immediately.
Penalties
If an inquiry results in a finding that additional tax is due, penalties may be charged in addition to the tax and interest. The penalty regime is behavior-based.
The standard domestic penalty ranges are as follows:
Where the error involves offshore income, assets, or activities, significantly higher penalties apply. The standard penalty for offshore non-compliance is up to 200% of the tax due, which can be reduced by the quality of disclosure to a minimum of 100% where disclosure is voluntary, or to 150% where it is not.
The quality of disclosure can reduce penalties within each range. HMRC applies a three-part test when assessing the quality of a taxpayer’s cooperation: telling, helping, and giving access. These elements apply both to domestic and offshore matters. In practice, this means:
- Telling — proactively informing HMRC of the error and its nature, without waiting to be asked.
- Helping — actively assisting HMRC to quantify the tax at stake, for instance by providing calculations and explanations.
- Giving access — making records, documents, and other relevant information available to HMRC without unnecessary delay or resistance.
Unprompted disclosure, where the taxpayer comes forward before HMRC raises the issue, attracts the lowest penalty within each range. Prompted disclosure, where the taxpayer cooperates fully after HMRC has already raised the issue, attracts a higher but still reduced penalty. Minimal cooperation across all three elements attracts penalties at the top of the applicable range.
HMRC also charges interest on unpaid tax from the date the tax was originally due. In a long-running inquiry covering multiple years, interest charges alone can be substantial.
Reducing your risk
No taxpayer can guarantee they will never face an HMRC inquiry. Random selection alone makes that impossible. But the overwhelming majority of triggered investigations arise from identifiable risk factors that can be managed.
- File all returns accurately and on time. Consistent compliance reduces your risk profile. Late or inaccurate filings signal to HMRC’s risk systems that a return warrants closer attention.
- Keep thorough records for at least six years. For most taxes, this covers the standard careless error time limit. Good records are the difference between an inquiry that closes quickly and one that drags on.
- Declare all income, including platform income, rental income, overseas income, and income from informal or occasional work. HMRC’s data-matching capabilities mean that undeclared income is increasingly likely to be identified.
- Take advice before making unusual claims. Large or unusual deductions, significant changes in expenses, R&D claims, and losses used to offset other income are all areas that attract attention. Having a professional review before filing is far cheaper than defending against an inquiry afterward.
If you have undeclared income or tax errors from previous years, consider using one of HMRC’s voluntary disclosure facilities before HMRC finds the issue itself. Unprompted disclosure carries the lowest penalties and, in most cases, avoids criminal investigation.
Unprompted disclosure, where the taxpayer comes forward before HMRC raises the issue, attracts the lowest penalty within each range. Prompted disclosure, where the taxpayer cooperates fully after HMRC has already raised the issue, attracts a higher but still reduced penalty. Minimal cooperation across all three elements attracts penalties at the top of the applicable range.
HMRC also charges interest on unpaid tax from the date the tax was originally due. In a long-running inquiry covering multiple years, interest charges alone can be substantial.
Conclusion
An HMRC investigation is not an accusation of fraud. Most inquiries arise from data matching, statistical analysis, or random selection, and many are closed without any adjustment to the tax position. What determines the outcome is usually the quality of your records, the completeness and accuracy of your original return, and the professionalism of your response once an inquiry is opened. The Upper Tribunal’s decision in HMRC v Harte has strengthened the position of taxpayers facing discovery assessments by confirming that time limits must be applied on an issue-by-issue basis, not in the round. Taxpayers who file accurately, keep thorough records, and take professional advice at the first sign of an inquiry are in the strongest possible position. Those who ignore the notice, provide incomplete information, or attempt to handle a serious inquiry without professional support face a significantly worse outcome.
