Key facts
- The main claim deadline is 2 years from the end of the accounting period.
- Pre-notification is required for first-time claimants (or those who haven’t claimed for 3+ years) within 6 months of the period end.
- The Additional Information Form must be submitted before or at the same time as the CT600.
- An amended CT600 can be filed within 2 years of the period end to add an R&D claim.
- There is no extension or appeal if you miss the 2-year window — the relief is lost.
Key Deadlines at a Glance
| Deadline | Time Limit | Who It Applies To |
|---|---|---|
| Pre-notification | 6 months after end of accounting period | First-time claimants or those who haven’t claimed in 3+ years |
| Additional Information Form | Before or with the CT600 | All R&D claimants |
| CT600 (original or amended) | 2 years after end of accounting period | All R&D claimants |
The Two-Year Claim Window
The most important deadline is the 2-year time limit for submitting your R&D claim. This is measured from the end of the accounting period to which the claim relates.[1]
For example:
| Accounting Period Ends | Claim Deadline |
|---|---|
| 31 March 2024 | 31 March 2026 |
| 31 December 2024 | 31 December 2026 |
| 31 March 2025 | 31 March 2027 |
| 30 September 2025 | 30 September 2027 |
The 2-year deadline applies whether you are submitting an original CT600 with an R&D claim or amending a previously filed CT600 to add a claim. There is no extension or appeal mechanism — if you miss it, the relief is lost forever for that period.
Important: The 2-year window for R&D claims is separate from the normal CT600 filing deadline (12 months after the period end). You can amend your CT600 specifically to add an R&D claim up to 2 years after the period end, even if the original return has already been filed.
Pre-Notification Rules
Pre-notification was introduced to prevent fraudulent or speculative claims. You must pre-notify HMRC if:[2]
- Your company has never claimed R&D tax relief before, or
- Your company has not made an R&D claim in any of the previous 3 accounting periods
The notification must be submitted within 6 months of the end of the accounting period for which you intend to claim. It is submitted online through HMRC’s digital service and requires:
- Your company’s name and Unique Taxpayer Reference (UTR)
- The start and end dates of the accounting period
- A brief description of the R&D activities
- Contact details for the senior officer
Tip: If you are unsure whether your activities will qualify, it is safer to pre-notify anyway. Pre-notification does not commit you to making a claim — it simply preserves your right to do so. Missing the 6-month window is irreversible.
Practical Timeline: First-Time Claimant
Here is a worked timeline for a company making its first R&D claim for a 31 March 2025 accounting period:
| Date | Action |
|---|---|
| 1 April 2025 | Accounting period ends |
| By 30 September 2025 | Submit pre-notification to HMRC (6-month deadline) |
| During 2025/26 | Prepare technical report, calculate costs, complete AIF |
| Before CT600 filing | Submit Additional Information Form digitally |
| By 31 March 2026 | File CT600 (12-month normal deadline) or by 31 March 2027 if amending |
| Absolute deadline: 31 March 2027 | Last date to submit or amend CT600 with R&D claim |
Amended Returns for Prior Periods
If you have already filed a CT600 for a prior period without an R&D claim, you can file an amended CT600 to add the claim, provided the 2-year window is still open.[3]
To amend a prior-period return:
- Submit the Additional Information Form for the relevant period
- File an amended CT600 including the R&D claim figures
- Include the AIF confirmation reference with the amended return
If pre-notification was required and you did not pre-notify within 6 months, you cannot add a claim by amendment — the pre-notification deadline is a hard requirement.
Consequences of Missing Deadlines
| Deadline Missed | Consequence |
|---|---|
| Pre-notification (6 months) | Cannot claim R&D relief for that period |
| AIF (before CT600) | R&D claim on CT600 is rejected; must file AIF and resubmit |
| CT600 / amendment (2 years) | R&D relief for that period is permanently lost |
Frequently Asked Questions
What if I miss the pre-notification deadline?
If you are required to pre-notify (first-time claimant or no claim in 3+ years) and miss the 6-month deadline, you cannot claim R&D relief for that accounting period. The pre-notification deadline is absolute. You may still be able to claim for the next accounting period if you pre-notify in time.
Can I claim R&D relief for periods before I knew about it?
Yes, as long as the 2-year deadline has not passed. Many companies discover R&D tax credits years after they first became eligible. You can file amended CT600 returns for any open periods (within the 2-year window) to claim the relief retrospectively.
Does the 2-year deadline apply to all aspects of the claim?
Yes. The entire claim — including the CT600, the Additional Information Form, and any supporting documents — must be submitted within 2 years of the end of the accounting period. Do not leave your claim until the last minute, as the AIF must be submitted before or with the CT600.
What if my accounting period does not end on 31 March?
The 2-year deadline runs from the end of your accounting period, regardless of the tax year. If your accounting period ends on 31 December 2024, your claim deadline is 31 December 2026. If it ends on 30 June 2024, the deadline is 30 June 2026.
Further Reading
- How to Claim R&D Tax Credits — the full step-by-step process
- HMRC Additional Information Form — the mandatory digital form
- Advance Assurance for R&D — getting confirmation before you claim
- HMRC R&D Enquiries — what to do if HMRC reviews your claim
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