Does HMRC have to tell me why they're investigating me?

When HMRC opens a compliance check, whether under section 9A Taxes Management Act 1970 for self assessment or through a Schedule 36 notice, it should identify the tax, the period, and broadly the area of concern. This lets you understand what records and explanations are relevant without HMRC having to justify its suspicion in detail.

In practice, opening letters often say something like 'we are checking your Self Assessment tax return for the tax year ended 5 April 2024' or refer to a specific entry, such as rental income or a particular expense category. That's usually the extent of what's provided upfront.

HMRC's risk-assessment methods, including the data-matching tools and third-party information sources it uses, are deliberately not disclosed case by case. If HMRC revealed exactly what triggered every check, it would become far easier for people to structure their affairs to avoid detection rather than to comply properly.

This is consistent with how most regulatory and law enforcement bodies operate: the target of a check is entitled to know what's being examined, not the internal methodology that led to the examination.

Yes. HMRC can open a self assessment enquiry into any return within the statutory enquiry window without needing a specific suspicion, and some checks genuinely are selected at random or through general risk profiling rather than a targeted concern. You cannot infer wrongdoing is suspected simply because a check has started.

If your enquiry is random, cooperating promptly and providing clear, well-organised records is usually the quickest way through it, since there's no underlying issue to uncover.

It's common to suspect a check was prompted by something specific, such as a mismatch with third-party data or a tip-off, even where the opening letter is generic. Asking HMRC directly what prompted the check rarely gets a fuller answer, but asking what specific figures or entries it's focused on often does.

A more productive approach than pressing for the 'real reason' is to ask HMRC to confirm, in writing, exactly which entries or years are within scope, so you know what to focus your response on.

Yes. As a check develops, HMRC generally becomes more specific about what it's querying, particularly once it raises formal questions about individual transactions or issues a closure notice setting out its conclusions. The initial letter is deliberately broad; later correspondence should narrow considerably.

If HMRC reaches a formal decision, such as an amendment or assessment, it must give reasons for that decision, and those reasons can then be tested on appeal to the First-tier Tribunal.