A hard procedural rule introduced as part of HMRC's R&D compliance overhaul catches first-time claimants by surprise every year. Miss it and the entire R&D claim for that period is void. No appeal, no late submission, no partial credit. The rule is the claim notification requirement, and the clock is shorter than most founders expect.
The rule in short: 6 months from period end
First-time R&D claimants, and any company that has not claimed in the prior three years, must send HMRC a claim notification within 6 months of the end of the accounting period for which they intend to claim. Miss the window and the R&D claim is invalid for that period. The notification is a separate, earlier obligation from the Additional Information Form and the CT600 claim itself. Source: HMRC guidance on R&D claim notification.
Who is caught: the first-time and three-year lapse rule
You must submit a claim notification if either of the following applies to your company for the accounting period in question:
- First-time claimant. Your company has never made an R&D tax relief claim before.
- Three-year lapse. Your company made a valid R&D claim at some point in the past, but has not made one in any of the three accounting periods immediately before the period you are now claiming for.
If you have claimed continuously, year after year, without a gap of three years or more, you are not required to submit a notification again. But if you skipped even one claim and more than three years have passed since your last successful one, you are back in the notification pool.
Quick self-check:
- Has your company ever claimed R&D tax relief? If no, you must notify.
- Did you claim in the accounting period ending most recently? If no, go to step 3.
- Have you claimed in any of the three periods before the one you are claiming for now? If no, you must notify.
- If yes to a claim in any of those three periods, you are not required to notify for this period.
If you are unsure whether a prior submission constitutes a valid claim (for example, you submitted an AIF but the claim was later amended or withdrawn), confirm with us before assuming the notification is not needed.
The 6-month clock: it runs from the accounting period end, not the filing deadline
The most common mistake is treating the notification deadline as tied to the CT600 filing deadline. It is not. The 6-month window runs from the end of the accounting period. For most companies the CT600 is due 12 months after the period end. The notification deadline is therefore 6 months earlier than the filing deadline.
That gap is where claims are lost. A founder who builds their R&D analysis in the 9 months after year-end (well within the CT600 window) has almost certainly missed the notification.
Worked timeline (dates are illustrative):
| Event | Date | Notes |
|---|---|---|
| Accounting period ends | 31 March 2025 | The clock starts here |
| Claim notification deadline | 30 September 2025 | 6 months from period end. This is the hard deadline for first-time claimants. |
| CT600 filing deadline | 31 March 2026 | 12 months from period end. Later than the notification deadline by 6 months. |
| AIF submission | Before CT600 is filed | Must precede the CT600 but does not replace the notification. |
Substitute your company's actual accounting period end to calculate your notification window.
What happens if you miss it
The R&D claim is invalid for that accounting period. This is the consequence stated in HMRC's guidance. There is no mechanism to submit a late notification and recover the claim. The relief is lost for that year entirely.
This applies regardless of whether the R&D work genuinely qualifies under HMRC's definition, regardless of the size of the potential claim, and regardless of whether an AIF and CT600 are later submitted. Without a timely notification, those downstream steps have no valid foundation to rest on.
There is no appeal route for a missed notification. If the window has passed, the only recourse is to ensure the notification is submitted on time for future periods and to structure future years so that the three-year lapse rule does not apply.
What the notification needs and where it fits in the wider process
The claim notification is the first step in a three-stage process. All three stages are required. Missing or confusing them is the most common R&D procedural error. Source: HMRC guidance on the Additional Information Form.
| Step | What it is | When it is due | Consequence of missing |
|---|---|---|---|
| Claim notification | Tells HMRC you intend to make an R&D claim. Short online submission. Required for first-time claimants and those with a three-year lapse. | Within 6 months of the accounting period end | Claim is invalid for that period. No recovery route. |
| Additional Information Form (AIF) | Detailed breakdown of R&D projects, costs, and qualifying activities. Submitted via HMRC's online portal before the CT600. | Before the CT600 claim is submitted | HMRC removes the R&D claim from the CT600. Source: gov.uk AIF guidance. |
| CT600 R&D claim | The formal corporation tax return including the R&D relief or expenditure credit claim. Filed with HMRC as part of annual tax filing. | Within 12 months of the accounting period end | No relief claimed for the period. Standard late-filing penalties apply to the CT600 itself. |
For a detailed walkthrough of the AIF, see our guide to the Additional Information Form. For an overview of the merged scheme and how relief is calculated, see the merged R&D scheme explained.
Worked example: a software startup in its first R&D year
The following example is illustrative. It uses a software company to reflect the startup audience this site serves, and because software R&D is frequently the context in which founders first encounter the notification rule.
Scenario: A SaaS company incorporated in early 2024 has an accounting period running from 1 April 2024 to 31 March 2025. In the 2024-25 year the company's engineers spent substantial time resolving genuine technological uncertainty in the core product architecture. This is the company's first accounting period and its first time considering an R&D claim.
- Period end: 31 March 2025
- Notification deadline: 30 September 2025 (6 months from period end)
- CT600 filing deadline: 31 March 2026
The founders begin preparing their R&D analysis in July 2025 with the help of their accountant. That timing sounds comfortable relative to the CT600 deadline of March 2026. But the notification deadline of 30 September 2025 is only weeks away. If the founders wait until the analysis is complete before asking whether a notification is needed, they risk missing it.
The right sequence: as soon as you believe you may have a qualifying R&D claim, check whether you are in scope for the notification requirement and submit it promptly. The notification does not commit you to a final claim value. It simply reserves your right to claim.
Note: whether the R&D work in any given year genuinely qualifies depends on the nature of the uncertainty and the advance being sought. Routine software development does not qualify. See our guide on software R&D eligibility for the qualifying test. If your work is at the margin, the honest answer is to assess carefully before notifying, rather than notifying and filing a weak claim that draws scrutiny.
Notification, AIF, and CT600 are three different things
It is worth being direct about a confusion that arises regularly. The three steps in the R&D process are distinct obligations with different deadlines, different content, and different consequences for missing them.
The notification is not the AIF. The AIF is not the CT600. Submitting an AIF does not mean a notification was submitted. Submitting a CT600 with an R&D claim does not make a missed notification valid.
Some R&D advisers focus almost entirely on the AIF and CT600 (the substantive work) and treat the notification as an afterthought. For established companies that have claimed continuously, that is fine. For first-time claimants or companies with a three-year gap, it is the single most expensive oversight available.
If you are approaching your first R&D claim, or returning after a gap, speak to us early. The notification window opens the moment the accounting period ends. Acting before the period ends is even better: you can confirm your notification obligation before the clock starts.
For context on what else is changing in the R&D landscape, see the merged scheme explained and, if your company is loss-making and R&D-intensive, the ERIS route for intensive R&D spenders.