What Triggers an HMRC Investigation — And How to Check Your Risk

How does HMRC choose who to investigate?

HMRC uses a sophisticated risk profiling system called CONNECT — a database that cross-references your tax return against third-party data from banks, Land Registry, Companies House, online platforms, and even social media. If your declared income doesn't match what CONNECT expects, you are more likely to be selected.

Investigations fall into three categories: full enquiries (a complete review of your tax affairs), aspect enquiries (focused on one specific area), and random checks (which can happen to anyone, regardless of risk level).

The top red flags that trigger HMRC investigations

1. Unusually high expenses relative to income

If your expenses are significantly higher than other businesses in your sector, HMRC will notice. A sole trader claiming 80% of income as expenses when the industry average is 30% will attract scrutiny. What to do: only claim legitimate business expenses and keep all receipts.

2. Consecutive years of losses

Reporting a loss for one year is understandable. Three or more consecutive years of losses while continuing to trade is a major red flag — HMRC may question whether this is a genuine commercial activity. What to do: be prepared to demonstrate commercial intent and keep detailed records.

3. High vehicle and entertainment costs

Claiming 100% of car costs when the vehicle is also used privately, or large entertainment expense claims, are common investigation triggers. HMRC expects a business-use proportion to be applied. What to do: keep a mileage log and apply a realistic business-use percentage.

4. Cash-based businesses with low declared income

Builders, market traders, hairdressers, and other cash-heavy businesses are statistically more likely to be investigated. HMRC uses industry benchmarks — if your profit margin is well below the sector average, you'll stand out. What to do: use a business bank account for all transactions and record all sales.

5. Significant lifestyle vs. income mismatch

HMRC's CONNECT system cross-references public data. If you've declared £20,000 income but bought a £400,000 house, posted luxury holidays on social media, or registered an expensive car — these can trigger an investigation. What to do: be prepared to explain any assets or purchases that exceed your declared income.

6. Large home office claims

Claiming a disproportionate percentage of your home's running costs as a business expense is a known trigger. HMRC expects the claim to reflect only the proportion of space and time used exclusively for business. What to do: use the £6/week flat rate (no receipts needed) or calculate the actual proportion carefully.

What happens during an HMRC enquiry?

HMRC will write to you (always by post) opening a formal enquiry and requesting specific information or documents. You must respond within the timeframe given — typically 30 days. You have the right to be represented by an accountant or tax adviser throughout the process.

Most enquiries are resolved with additional information and, sometimes, a payment of tax owed plus interest. Only a small minority escalate to formal investigations with penalties.