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: the legislation contains no reasonable-excuse provision and gives HMRC no discretion, so if HMRC removes the claim from your return you can make written representations within 90 days — but only on the ground that something it stated in its notice was wrong. Fairness, and not having heard of the rule, are not arguments the statute leaves room for.
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. A second exception works the same way: a claim HMRC rejected by removing it from the company tax return does not count as a prior claim either, so a company whose last claim was struck out has to notify again as though it had never claimed. 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 date arithmetic is mechanical once you have the right starting point. The deadline runs six months from the end of the period of account, so a period of account ending 31 May 2026 has a notification deadline of 30 November 2026, and one ending 31 December 2026 has 30 June 2027. Long and short periods of account are where it stops being obvious, and the claim notification deadline checker handles those.
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.
Sources
- Tell HMRC you plan to claim — the notification requirement, deadline and exceptions, including the two that override a recent claim: a claim HMRC removed from the return, and a pre-April 2023 period claimed by an amendment received on or after 1 April 2023.
- CTA 2009 s1042C, s1045A and s1054A — the requirement itself for each route into the relief, the three-year test measured to the last day of the claim notification period, and the subsection (2) exclusion for pre-April 2023 periods claimed by later amendment.
- CTA 2009 s1142A — the claim notification period: it begins with the first day of the period of account and ends six months after that period of account ends.
- FA 1998 Sch 18 para 83EB — removal from the return of claims made in error: the correction cannot be rejected, written representations run to 90 days on the ground that a matter stated in the notice was incorrect, and no new claim can be made for the same expenditure where a notification was required and none was made.
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.