HMRC doesn't pick investigation targets out of a hat. Its Connect system cross-references billions of data points, including bank interest, property records, online marketplace sales, and even social media, against what taxpayers declare.
When the numbers don't line up, a compliance check follows. Knowing what draws attention is the first step in not drawing it. Here are the ten triggers that come up again and again.
1. Figures That Don't Match Third-Party Data
Banks report your interest. Letting platforms report your rental income. Marketplaces like eBay, Etsy, Airbnb and Vinted now report seller income directly to HMRC. If your return says one thing and the data feed says another, the mismatch is flagged automatically. Declare everything, even income you assume is too small to matter.
2. Profit Margins That Look Wrong for Your Sector
HMRC benchmarks businesses against their industry. A takeaway declaring 15% gross margin in a sector averaging 60% stands out instantly. Margins genuinely vary, but if yours sit far from the norm, expect to be asked why, and keep the evidence that explains it.
3. Heavy Cash Trading
Restaurants, taxis, salons, market traders and the building trade attract structural suspicion because cash can vanish before it reaches the books. Daily till reconciliations and banking takings intact, rather than skimming expenses off first, are what make a cash business defensible.
4. Late Returns and a Messy Filing History
A business that files late, amends often, and pays later still is telling HMRC its records are chaotic, and chaotic records make attractive enquiry targets. The fix is boring: file on time, every time. Most of the record-keeping failures behind bad filings appear in our top 15 accounting mistakes UK small businesses must avoid.
5. Big Swings With No Explanation
Turnover halves, or expenses double, and the return offers no context. Real businesses have bad years, but unexplained volatility invites questions. The white space on a Self Assessment return exists for exactly this: a sentence explaining the fire, the lost contract, or the equipment purchase can stop an enquiry before it starts. How and where to disclose is part of our complete Self Assessment guide for UK taxpayers.
6. Expenses Out of Proportion to Income
Motor costs on a business with no obvious travel, entertainment claims that dwarf turnover, or a home-office claim covering half the house. Round numbers are their own red flag; £5,000 exactly for travel reads as a guess, and HMRC treats guesses as invitations.
7. Living Beyond Your Declared Means
Connect links Land Registry purchases, vehicle registrations and foreign property data to tax records. A director declaring £25,000 a year who buys a £900,000 house prompts an obvious question. If the money came from savings, inheritance or a spouse, the paper trail should already exist.
8. Repeated Losses That Never End
Claiming losses year after year while the business somehow keeps operating suggests either a hobby dressed up as a trade or income going unrecorded. Genuine loss-makers should keep the business plan, the invoices and the turnaround evidence close at hand.
9. VAT Claims That Break Pattern
A sudden large repayment claim, input VAT out of line with turnover, or flat-rate scheme errors all get picked up by automated checks before a human ever looks. VAT enquiries also have a habit of widening into full business enquiries once opened, which is reason enough to get returns right the first time.
10. Tip-Offs
Less glamorous than algorithms, but ex-employees, former partners and competitors report businesses to HMRC's fraud hotline in large numbers every year. You can't prevent a malicious report, but a clean set of books means a tip-off leads nowhere.
If the Brown Envelope Arrives
A compliance check letter is not an accusation, and panic responses make things worse. Don't ring HMRC unprepared, don't hand over more than the letter asks for, and don't sign anything you don't understand. Get representation before you reply: enquiries handled professionally from the first letter close faster and cheaper. HMRC investigation support and dispute resolution is one of our core specialist tax services at ASPIRE UK, and we've dealt with everything from aspect enquiries to full-scale investigations.
The better outcome is never receiving the letter. Clean records, consistent filings and declared income that matches the data HMRC already holds keep you off the list. If any trigger on this page felt uncomfortably familiar, talk to ASPIRE UK's tax team now, while it's still a tidy-up job rather than a defence.
