HMRC Self Assessment Enquiry: What Happens and What to Do (2026/27)

Types of HMRC Enquiry

HMRC has several enquiry powers under the Taxes Management Act 1970 (TMA). Understanding which type you face tells you the scope of what HMRC is examining.

Aspect Enquiry (Partial Enquiry)

An aspect enquiry targets one or more specific items in your Self Assessment return. HMRC will identify the line items it wishes to examine — for example, your self-employment expenses, a property disposal, or untaxed income from a side business — and request documentation to support them. The rest of the return is not under scrutiny. Aspect enquiries are the most common type and are often resolved within a few weeks if records are clear.

Full Enquiry

A full enquiry examines the entire Self Assessment return, including all income sources, all deductions and all reliefs claimed. HMRC typically opens a full enquiry where it suspects material under-declaration of income, where a taxpayer has been subject to previous compliance activity, or where the overall profile of the return raises concerns that cannot be addressed by examining a single item. Full enquiries can last 12–24 months or longer in complex cases.

PAYE Compliance Check

A PAYE compliance check is separate from a Self Assessment enquiry. It focuses on whether your employer has operated PAYE correctly, whether benefits in kind (P11D items) have been properly reported, and whether National Insurance has been correctly accounted for. If you are an employee and receive a PAYE compliance check notice, it is usually directed at your employer — though if you have benefits in kind that were not declared on your Self Assessment return, it can have personal tax consequences.

COP9 — Civil Investigation of Fraud

COP9 (Code of Practice 9) is HMRC's most serious civil enquiry procedure. It is used where HMRC suspects deliberate tax fraud. The taxpayer is offered the Contractual Disclosure Facility (CDF): make a complete disclosure of all deliberate tax fraud and HMRC will not pursue a criminal prosecution. Refusing CDF opens the door to criminal investigation. COP9 cases are rare but result in very significant penalty settlements and settlement costs. If you receive a COP9 notice, instruct a specialist tax investigation solicitor or CIOT-qualified adviser immediately.

What Triggers an HMRC Enquiry

HMRC does not usually disclose why it has opened an enquiry. However, the most common triggers are well documented:

HMRC Connect — Data Matching

Connect is HMRC's analytical platform that cross-references over 30 data sources to identify inconsistencies. Connect ingests: bank and building society interest reports; land registry property transactions; Companies House directorships; online marketplace sales data (eBay, Etsy, Vinted, Airbnb, Booking.com); overseas bank accounts under the Common Reporting Standard (100+ countries); social media posts; DVLA vehicle records; student loan records; and credit reference data. If your declared income is inconsistent with your known assets, property transactions or lifestyle signals, Connect flags the discrepancy.

Underpayment or Income Drop

A declared income that is significantly lower than the previous year — particularly a drop of more than 25% in self-employment profit without an obvious explanation — raises a flag. Similarly, a tax payment that appears significantly less than expected based on third-party data (bank interest, share dividends, property rental income) will trigger review.

Non-Filing and Late Filing

Persistent non-filing or late filing of Self Assessment returns is a significant enquiry risk factor. HMRC views habitual late filers as higher risk. Returns filed very close to the deadline with large last-minute adjustments also attract scrutiny.

IR35 and Contractor Risk

Contractors operating through personal service companies (PSCs) are subject to enhanced scrutiny under the IR35 off-payroll rules. HMRC has dedicated IR35 investigation teams and uses Connect data to identify contractors who may be caught by the rules but have not been assessed as inside IR35.

Random Selection

HMRC runs a random enquiry programme. A proportion of returns are selected for full enquiry without any specific trigger — this is partly to maintain a deterrent effect and partly to gather statistical data about compliance levels across sectors.

Statutory Time Limits

The time within which HMRC can open or raise an assessment is strictly regulated by statute. These limits are your primary protection against very old years being reopened.

The 12-month enquiry window means that for a 2024/25 return filed on time (by 31 January 2026), HMRC can open a formal enquiry only until 31 January 2027. After that, HMRC needs to rely on its discovery powers and must show that a valid discovery exists — meaning new information has come to light that was not available at the time the return was filed or at the end of the standard enquiry window.

The 4-year limit for careless errors means HMRC can go back to 2022/23 (from the 2026/27 tax year). The 6-year limit covers 2020/21. For offshore or fraudulent cases HMRC can go back to 2006/07. This underscores the importance of retaining records for at least 6 years — and indefinitely if offshore income or assets are involved.

Your Rights During an HMRC Enquiry

HMRC has significant powers but taxpayers have important safeguards:

• Right to reasonable time to respond.HMRC's initial information notices typically allow 30 days. You can request additional time — HMRC is required to be reasonable. If HMRC refuses a reasonable request and you cannot comply in time, you may apply to the Tribunal to vary or set aside the notice.
• Right to representation. You can appoint any tax adviser, accountant or solicitor as your agent. Once appointed, HMRC must deal with them rather than directly with you (unless you agree otherwise). Correspondence is routed through your agent.
• Right to appeal.If HMRC raises an assessment or imposes penalties that you disagree with, you can appeal to the First-tier Tribunal (Tax Chamber) within 30 days of the decision. The Tribunal is independent of HMRC and can overturn or vary HMRC's decisions.
• Right to complain.If HMRC behaves unreasonably — for example by failing to follow its own procedures, causing unnecessary delay or treating you disrespectfully — you can complain to HMRC's complaints department and ultimately to the Adjudicator's Office (independent of HMRC).
• Privilege. Communications between you and a solicitor for the purpose of obtaining legal advice are protected by legal professional privilege and cannot be demanded by HMRC. Communications with non-solicitor advisers are not automatically privileged.

You are never legally required to self-incriminate. If HMRC's questions stray into territory that could lead to criminal prosecution, you should take specialist legal advice before responding.

Penalties and Co-operation Reductions

Where HMRC finds underpaid tax, it can charge penalties in addition to the tax and interest. The penalty regime under Schedule 24 FA 2007 distinguishes between levels of taxpayer behaviour:

“Potential Lost Revenue” (PLR) is the tax that should have been paid but was not. Penalties are then adjusted for the quality of the taxpayer's disclosure:

• Telling — informing HMRC of the error (unprompted vs prompted)
• Helping — assisting HMRC in quantifying the error and establishing the facts
• Giving access — providing records, accounts and documents

An unprompted disclosure (before HMRC contacts you) achieves the maximum reduction in each category. A prompted disclosure (after HMRC has made contact) achieves a smaller reduction. Full co-operation across all three categories can reduce a careless penalty to nil. For deliberate penalties, the floor is higher but co-operation still makes a material difference.

In addition to penalties, HMRC charges late payment interest at the official rate (currently Bank of England base rate + 2.5%) on unpaid tax from the original due date. Interest is not negotiable and cannot be suspended, though it stops accruing when the tax is paid. The combination of back-tax, interest and penalties in a serious multi-year enquiry can be significant — another reason why voluntary disclosure before HMRC contacts you is the most cost-effective outcome.

Common Areas HMRC Investigates

HMRC's enquiry work tends to concentrate on specific income types where under-reporting is historically common:

• Side income and the gig economy. Earnings from freelancing, online platforms, tutoring, content creation, dog walking, car boot sales and cash-in-hand work all form taxable income if they exceed the £1,000 trading allowance. HMRC receives data from major platforms (Uber, Deliveroo, eBay, Airbnb, Upwork) and cross-references it against declared self-employment income.
• Rental income.HMRC's Let Property Campaign targets landlords who have not declared rental income or who have over-claimed expenses (e.g. mortgage capital repayments, personal bills, non-allowable refurbishments). Property ownership data from the Land Registry is directly fed into Connect.
• Cryptocurrency. Disposing of crypto assets (selling, swapping, gifting or using crypto to pay for goods) triggers Capital Gains Tax. HMRC has received data from UK crypto exchanges and is increasingly targeting individuals who have not declared gains. The HMRC Crypto Assets Manual confirms that most individual crypto activity is subject to CGT.
• Expense claims. Unusually high expense ratios relative to turnover in self-employment returns, or home-office claims without a clear basis, attract scrutiny. Clothing (other than specialist protective equipment), commuting costs and entertainment are generally not allowable.
• Foreign income. UK residents are taxable on worldwide income. Bank accounts in EU countries, rental properties abroad, overseas pensions and offshore investment income are all reportable. Under the Common Reporting Standard, data from 100+ countries now flows automatically to HMRC annually.

Use the self-employed tax calculator to check that your declared profit aligns with realistic deductions, and the income tax calculator to verify your overall tax position before filing.

How to Respond to an HMRC Enquiry: 5 Steps

Getting a Tax Adviser

The HMRC enquiry process is formal and follows specific procedural rules. An experienced adviser will know those rules and use them to your advantage. When choosing:

• CIOT members (Chartered Institute of Taxation) hold the highest qualification in UK tax — the Chartered Tax Adviser (CTA) designation. They are regulated and carry professional indemnity insurance.
• ATT members (Association of Taxation Technicians) hold a strong practical tax qualification and are regulated. Many work alongside CTAs.
• Specialist investigation firms focus exclusively on HMRC enquiries and employment tax disputes. They often employ ex-HMRC inspectors who understand how HMRC approaches cases internally.
• Check whether the adviser offers a free initial consultation to assess the scope of your enquiry and whether your tax investigation insurance policy covers their fees.

The cost of professional representation is almost always outweighed by the penalty reduction and earlier settlement that a qualified adviser can achieve — particularly in deliberate or higher-value cases.

Official References

• gov.uk: Tax compliance checks
• gov.uk: Self Assessment tax returns
• HMRC: Penalties factsheet (CC/FS7a)
• gov.uk: Appeal a tax decision
• Chartered Institute of Taxation — find a member
• Association of Taxation Technicians — find a member