The three kinds of deadline in an HMRC letter

Last reviewed 1 September 2026 · How Tax Letters checks its facts

Every frightening HMRC letter contains a date, and almost nobody tells you what kind of date it is. There are three kinds, they behave completely differently, and knowing which one is looking at you changes how the next fortnight feels. This page is the reference the rest of this site leans on.

1. Administrative windows: set by the letter

The "please respond within 30 days" in a nudge letter is HMRC's own working deadline, not the law's. For the offshore letters that enclose a Certificate of Tax Position, the Chartered Institute of Taxation records HMRC agreeing there is no legal obligation to complete the enclosed certificate, that replies by ordinary letter are accepted, and that extension requests tend to be agreed when reasonable.

The other half of the truth: ignoring an administrative window has real consequences. Follow-up, risk assessment, possibly a compliance check, and a later disclosure treated as prompted, which raises the penalty range. Flexible is not the same as optional.

2. Statutory deadlines: set by law

Appeal rights against penalties and assessments usually run 30 days from issue. The window to query a Simple Assessment runs 60 days from the date on the letter (Taxes Management Act 1970, section 31AA(3)), though HMRC can allow longer. The window to amend a Self Assessment return runs 12 months from the filing deadline (Taxes Management Act 1970, section 9ZA). Reporting a UK residential property sale with tax to pay runs 60 days from completion. These are fixed by law. Asking nicely does not move most of them, and they quietly expire while people deliberate about the flexible kind.

3. Facility terms: set by the deal

Enter a disclosure facility and its own clock starts: 90 days to disclose after acknowledgement under the Worldwide Disclosure Facility and the Let Property Campaign, with complex WDF cases able to request 90 more, 180 in total. Payment within 30 days of a crypto disclosure. Not statute, but terms of the route you chose, and the route works by keeping them.

How to tell which is looking at you