Companies claiming R&D tax relief for the first time, or for the first time in three years, must notify HMRC of their intention to claim within six months of the end of the period of account. Miss that window and the claim is invalid. There is no appeal, no discretion and no retrospective fix, however much qualifying R&D took place and even if the tax return amendment deadline is still open.
This requirement applies to accounting periods beginning on or after 1 April 2023, and it is now the single most common way we see genuine claims lost.
Who has to notify HMRC?
Broadly, companies that have not made a relevant R&D claim in the previous three years. There is also a wrinkle that catches people out: claims made by amendment after 1 April 2023 for earlier periods do not count as prior claims for this test, so a company that has claimed before can still find itself required to notify. The full rules are on our claim notification page, and our claim notification checker applies them to your year end in under a minute.
Why are start-ups losing the most?
Because the deadline runs out while founders are, quite reasonably, building. In the early life of a company the attention goes to the product and the technical problems, and R&D tax relief gets considered after the first big development milestone, often when a fundraise prompts a look at ERIS and the cash it returns to loss-making companies.
That early phase is usually where the technical uncertainty was greatest and the qualifying spend was highest. Arrive at an adviser after the notification window has closed on it, and the claimable work shrinks to current and future development, which by then is often iterative rather than fundamental. The claim survives; most of its value does not. For a pre-revenue biotech burning investor cash on its core science, that difference can be a material slice of runway.
What are we seeing in practice?
Matthew Jones: “We have spoken to companies seeking R&D tax credit advice, only to have to inform them that substantial historic R&D costs could not be claimed because notification was not submitted on time. Once the deadline passes, there is no route to appeal and no retrospective fix.
In many of these cases, businesses were working with accountants who do not offer specialist R&D tax credit services and, as a result, would not typically be equipped to advise on this highly specific legislative requirement.
At LimestoneGrey, we support companies from the outset, helping them meet notification obligations and track qualifying activities and expenditure, and providing ongoing guidance throughout the development lifecycle.”
What should you do now?
Three actions, in order. Check your position today with the claim notification checker: the answer depends only on your period of account and claim history. If the window is open, notify; it costs nothing and commits you to nothing. And if you are unsure whether your work qualifies at all, resolve that question early too, starting with what counts as qualifying R&D.
The one thing not to do is wait. Every week we speak to a company that assumed the two-year amendment window meant there was time. For first-time claimants, there is a much shorter clock running inside it.
If your year end was less than six months ago and you have never claimed, contact us this week, not this quarter.
Written by Matthew Jones ACA CTA. Last reviewed July 2026.
Sources
- Tell HMRC you plan to claim (claim notification) — the six-month notification window and the three-year look-back test.
- SI 2023/813 (claim notification & AIF regulations) — the statutory content requirements for the claim notification.
This article describes the rules as they stood at the review date above. The rules change: for the current position, start with our guides or talk to us.
