What does "check of your Self Assessment tax return" mean?

When a letter says HMRC is carrying out a "check of your Self Assessment tax return," it means HMRC has opened a formal enquiry under section 9A of the Taxes Management Act 1970. This gives HMRC the power to ask questions and request documents relevant to the figures on that specific return.

The wording sounds neutral because it is; opening this kind of check does not require HMRC to suspect wrongdoing. It can be triggered by something as simple as an unusually large expense claim, a change from your previous year's pattern, or a completely random selection.

The letter should tell you which tax year is under review and, ideally, what aspect of the return has prompted the check, though HMRC doesn't always specify a precise reason.

Self Assessment relies on you calculating and reporting your own income and gains, so HMRC's checking process exists to test the accuracy of what's been submitted. Triggers can include income that doesn't match data HMRC receives from banks, employers, or property records, or expense claims that look high relative to your reported income.

Directors of limited companies are sometimes selected because dividend income, benefits in kind, or director's loan account entries on their personal return don't align cleanly with the company's own filings. This is one reason it's worth keeping personal and company tax positions consistent and well documented.

HMRC also runs a proportion of entirely random checks, unconnected to any specific risk indicator, as a way of maintaining overall confidence in the system.

HMRC can normally only open a formal enquiry into a Self Assessment return within a set window, generally 12 months from the date the return was filed, or from the filing deadline if the return was submitted late. If you filed on time and haven't heard anything by that point, HMRC generally cannot open a new full enquiry into that return, other than through a discovery assessment in more limited circumstances.

This means the timing of your filing matters. Filing early doesn't extend the deadline for HMRC to act, but filing at the last minute, or late, can extend the period during which HMRC could still open a check.

HMRC will typically ask for supporting information: invoices, bank statements, or an explanation of how a particular figure was calculated. You should respond within the timeframe given, providing accurate information and flagging anything you're unable to locate.

Once HMRC is satisfied, either that the figures were correct or that any necessary corrections have been agreed, it issues a closure notice, which formally ends the enquiry and confirms whether any amendment to your tax bill is needed.

A well-organised response usually resolves a Self Assessment check without additional cost beyond your time. Poor record-keeping or delayed replies tend to extend the process and increase any professional fees involved.

Growth plan clients get tax investigation insurance included as standard, which covers professional costs if a Self Assessment check becomes a longer-running enquiry. See /fees.

Don't assume the check means an error has definitely been found; many close with no change.

Don't miss the response deadline; ask for more time in writing if you genuinely need it.

Don't submit amended figures without checking them carefully first, since a hasty correction can itself trigger further questions.