HMRC enquiry window

~4 min read

Category: Tax investigations and HMRC powers · Reviewed by the Apex broking team · Last reviewed 2026-08-20

In short: The enquiry window is the statutory period during which HMRC may give notice of enquiry into a filed tax return — for a personal return delivered on time, broadly 12 months from the day the return was delivered, under section 9A Taxes Management Act 1970, with an equivalent regime for company returns in Schedule 18 Finance Act 1998. Once the window closes, HMRC can generally only reopen the year by discovery assessment.

Category: Tax investigations and HMRC powers Also known as: enquiry period, s.9A window Related concepts: discovery assessment, aspect enquiry

Definition

Self assessment operates on a “process now, check later” basis: returns are processed as filed, and HMRC’s opportunity to check them is framed by a statutory time limit for opening an enquiry. For personal returns the power is section 9A Taxes Management Act 1970; for company returns it is paragraph 24 Schedule 18 Finance Act 1998. The period in which a valid notice of enquiry can be given is known as the enquiry window.

Notice periods in outline

In outline: for a personal return delivered on or before the filing date, notice of enquiry may be given up to the end of the period of 12 months after the day the return was delivered. For a return delivered late, the window runs to the quarter day next following the first anniversary of delivery — the quarter days being 31 January, 30 April, 31 July and 31 October. Company returns follow the same pattern in outline: 12 months from delivery for a timely return, extended to the next quarter day after the first anniversary for a late one, with separate provision for companies in groups. The notice must be given within the window; the enquiry it opens can then run well beyond it.

Amendments

Amending a return affects the window. Where the taxpayer amends a return, the statute provides a further period — in outline, to the quarter day following the first anniversary of the amendment — for an enquiry so far as relating to the amendment. An amendment therefore does not reopen the whole return indefinitely, but it does extend HMRC’s opportunity to look at what was changed.

Closure notices

An enquiry, once opened — whether a aspect or full enquiry — is ended by a closure notice under section 28A Taxes Management Act 1970 (or the Schedule 18 equivalent). The closure notice states the officer’s conclusions and makes any amendments to the return that the officer considers necessary; the legislation also provides for partial closure notices dealing with discrete matters while the rest of the enquiry continues. A taxpayer who considers an enquiry has run its course can apply to the tribunal for a direction that a closure notice be issued.

Why it matters

The window is the line between HMRC’s routine right to check a return and the stricter conditions of a discovery assessment. Knowing when it closes for a given return tells a taxpayer when ordinary finality arrives — and an enquiry notice arriving inside the window, however routine, starts a process whose professional costs are the risk addressed by tax fee protection insurance.

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