Self Assessment Enquiry: A Complete UK Guide

A Self Assessment enquiry is opened by HMRC under Section 9A of the Taxes Management Act 1970, and it means a submitted tax return has been selected for a closer look rather than simply processed and filed. The opening letter usually sets out which parts of the return are in scope, though HMRC can extend that scope as the enquiry develops if new questions arise. Enquiries are opened either because something in the return looks inconsistent with information HMRC already holds — from employers, banks, letting agents, or other government data sources — or as part of HMRC's routine risk-based and random selection activity, which means an enquiry does not automatically imply wrongdoing.

What matters most in the early stages is establishing exactly what has been asked, protecting the statutory response deadline, and gathering the records that support the figures on the return before replying. A rushed or incomplete first response can broaden the scope of an enquiry rather than narrow it, so it is worth taking a short, considered pause to read the notice carefully rather than reacting immediately. If HMRC's questions reveal a genuine error, there are established routes to amend the position, and the enquiry can usually still be resolved by agreement rather than escalating further. The enquiry closes with a closure notice, which either confirms the return was correct or sets out the amendment HMRC believes is due, and either outcome carries its own appeal rights and deadlines.

Most Self Assessment enquiries proceed through a series of written exchanges, though HMRC can request a meeting in more complex cases, particularly where business records or a specific transaction need discussing in detail. There's no fixed overall time limit on how long an enquiry can run once opened, which means straightforward cases might close within a few months while more complex ones, especially those involving several years or a business with complicated affairs, can take considerably longer. Keeping responses focused and complete reduces the number of follow-up rounds needed and tends to shorten the overall process.

If it becomes clear during the enquiry that an amendment is needed, this can often be agreed informally between the taxpayer and HMRC before the formal closure notice is issued, which can be a faster route than waiting for HMRC to impose a figure. Where agreement can't be reached, the closure notice sets out HMRC's position formally, and from that point the normal appeal process applies in the same way as it would for any other HMRC decision, with its own deadline for challenging the outcome.