Most enquiries start with a computer comparing what you filed against what someone else reported about you. Banks, employers, platforms and foreign authorities all report.
Enquiries feel arbitrary from the outside and are mostly not. HMRC operates a large risk-analysis system that ingests third-party data and compares it against returns, and the majority of enquiries begin with a discrepancy it identifies. A smaller number are randomly selected, which exists precisely so that compliance cannot be gamed by looking unremarkable.
Where the Data Comes From
- Employers, through real-time payroll reporting on every payment.
- Banks and building societies, which report interest paid.
- Investment platforms and registrars, reporting dividends and distributions.
- The Land Registry, on property transactions.
- Online marketplaces and gig platforms, which now report seller and worker income.
- Foreign tax authorities, under automatic exchange of financial account information — the reason an undeclared overseas account is a visible mismatch rather than a quiet omission.
- Crypto exchanges, under reporting requirements that are being extended internationally.
Assume anything reported to a regulated institution is visible. The practical question is no longer whether HMRC could find something, but whether your return matches what it already holds. Where it does not, an explanation on the return itself is far better than a discrepancy left to be discovered.
What Raises the Score
| Pattern | Why it attracts attention |
|---|---|
| Income reported by a third party but not on the return | Direct mismatch — the most common trigger of all |
| Figures out of line with the sector | Margins and expense ratios are benchmarked |
| Large year-on-year swings without explanation | Invites the question of what changed |
| Round numbers throughout | Suggests estimates rather than records |
| Repeated late filing or amendments | Read as weak record-keeping |
| Lifestyle apparently inconsistent with declared income | Property and other data give a picture |
| A one-off transaction with no explanation | A large disposal with nothing said about it |
What Does Not Trigger One
Claiming reliefs you are entitled to does not, and the fear that it does costs people real money every year. Pension relief, Gift Aid, capital losses and employment expenses are all there to be claimed. Nor does having a complicated return: complexity is normal for anyone with several income sources. What matters is that the figures are supportable and that anything unusual is explained in the white space on the return, which is what that space exists for.
If One Arrives
- Read what it actually is. A request for information about one item is not a full enquiry, and a full enquiry into a return is not a suspicion of fraud. The letter says which it is.
- Note the deadline and ask for an extension early if the information will take time. Extensions are usually given; silence is not tolerated.
- Answer what was asked. Volunteering unrelated material widens the enquiry.
- Use your accountant if you have one. It is what the fee is for, and correspondence handled by an agent tends to be shorter.
- Correct genuine errors promptly. Penalties are heavily influenced by whether a disclosure was prompted or unprompted, and by co-operation.
- Do not guess. An answer given from memory that turns out to be wrong is worse than asking for time to check.
Disclosure Beats Discovery
Where you know something is wrong — an unreported source, a year not filed, foreign income never declared — the outcome is materially better if you raise it. Penalty regimes distinguish sharply between unprompted disclosure and being found, and HMRC operates specific disclosure facilities for exactly this. The instinct to wait and hope is understandable and it is the expensive choice. Our guides to Time to Pay and stopping filing properly cover the two situations that most often produce a backlog in the first place.
Frequently Asked Questions
How does HMRC decide who to enquire into?
Mostly by risk scoring returns against third-party data from employers, banks, platforms, the Land Registry and foreign tax authorities. A smaller number of enquiries are randomly selected.
Will claiming reliefs make an enquiry more likely?
No, and the fear that it does costs people money. Pension relief, Gift Aid, losses and employment expenses are there to be claimed. What matters is that figures are supportable.
Does HMRC know about my overseas accounts?
Very likely. Automatic exchange of financial account information means overseas institutions report to the country they sit in, which passes the data to HMRC. An undeclared account is a visible mismatch.
What should I do if I know something is wrong?
Disclose it. Penalty regimes distinguish sharply between unprompted disclosure and being found, and HMRC operates specific disclosure facilities. Waiting and hoping is the expensive option.
Related Guides
Keep reading with these related guides and calculators:
- Time to Pay — when disclosure creates a bill
- The P800 calculation — the routine version of the same data
- When to stop filing — how backlogs start
- Proving non-residence — the records an enquiry asks for
- Payments on account — the other Self Assessment surprise
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