You do if you are claiming for the first time, or have not claimed in the three years ending with the notification deadline, for an accounting period beginning on or after 1 April 2023. The claim notification must reach HMRC within six months of the end of the period of account. Miss the window and the claim is invalid, even though the return amendment deadline typically stays open for around another eighteen months; there is no late route and no reasonable-excuse relief, and while an appeal can be made against HMRC removing the claim, it cannot succeed, because the tribunal has no power to disapply the statutory bar.
Who is caught
The rule is aimed at new claimants, but its reach is wider than it looks. The test asks whether the company made an R&D claim in the three years ending with the last day of the notification period — a window shifted six months later than “the last three years” suggests. The shift cuts both ways: a claim made after the year end, during the notification window itself, can exempt the company, while a claim that felt recent at the year end may already have aged out, because the three years run to the deadline rather than from today. One wrinkle catches established claimants: a claim that appears in a return only because of an amendment made on or after 1 April 2023, for an accounting period that began before that date, does not count as a prior claim for this test. A company that has claimed for years, but always by amendment, can find that none of its recent claims counts, and that it needed to notify like a first-timer. Groups need care too: the statute frames every test at the level of the single company, so a subsidiary claiming for the first time is caught even where a sister company claims routinely.
The rule bites hardest on start-ups, because the companies most likely to be caught are the ones least likely to have heard of it. A young company typically discovers R&D relief when it first talks to an adviser about its accounts, which is often more than six months after the period ended, and by then the window on the most valuable period is often already shut.
What notifying involves
The notification is submitted online and covers the company’s details, the periods, an overview of the R&D, the officer responsible for its accuracy and the details of every agent involved; it commits you to nothing. HMRC’s guidance confirms that if you notify and then decide not to claim, you need do nothing further — so where a period might contain qualifying R&D, notifying protects the option. Two timing details are easy to miss: the window opens on the first day of the period of account, so a notification cannot validly be made before the period begins, and filing the claim itself early — in a return or amendment HMRC receives before the notification deadline — also satisfies the requirement without a separate notification. The claim then needs the Additional Information Form. Deciding not to bother, or deciding late, is the way to get this wrong.
Where to go next
The claim notification deadline checker tests your dates in under a minute, including the awkward cases: long periods, short periods and the amendment wrinkle. The full claim notification guide works through examples by year end. If your deadline has not yet passed and R&D is even a possibility, notify first and decide the claim afterwards.
Written by Matthew Jones ACA CTA. Last reviewed July 2026.
Sources
- Tell HMRC you plan to claim — the notification requirement, deadline and exceptions.
This page describes the rules as they stood at the review date above, as general information rather than advice on your circumstances. For how that distinction works, see our terms; for an answer on your own facts, talk to us.