R&D Tax Credit Claim Deadlines
Written and reviewed by the R&D Tax Accountants editorial team. Last reviewed 8 August 2026.
R&D relief has two deadlines and the well-known one is not the dangerous one. The claim deadline gives most companies two years. The notification deadline, which applies to first-time and lapsed claimants, closes eighteen months earlier and cannot be extended.
Companies discover the relief late all the time and most of them can still claim. The ones who cannot are almost always the ones who were inside the claim window and outside the notification window without knowing the second one existed.
How Far Back a Claim Can Go
Where the period of account is 18 months or less, a claim has to be made or amended within 24 months of the last day of the period of account. In practice that means a company with a 31 March year end has until 31 March two years later for that period.
Because the window runs on each period separately, a company that has never claimed can usually pick up two open periods at once rather than one. That is worth checking before assuming only the most recent year is available.
Long and Short Periods of Account
Where the period of account is longer than 18 months, the deadline is 42 months from the first day of the period of account instead. Long periods arise on incorporation, on a change of year end, and around group reorganisations, and the different rule catches people who apply the 24 month figure by habit.
A period of account can also contain more than one accounting period for Corporation Tax, which means one set of accounts can generate two claims with their own computations. Establishing the period dates properly is the first step, not a formality.
Where the Notification Window Closes First
For accounting periods beginning on or after 1 April 2023, a first-time or lapsed claimant has to submit a claim notification form. That window opens on the first day of the period of account and closes six months after the end of it.
Set against a 24 month claim deadline, the notification window closes eighteen months earlier. A company that finds the relief a year and a half after its year end may be comfortably inside the claim window and permanently outside the notification window. There is no late notification and no reasonable excuse route: the claim is invalid.
The lapsed-claimant limb catches established businesses, not just start-ups. If the last claim was made more than three years before the end date of the claim notification period, notification is required again. A company with irregular project work can drift into that without noticing.
What Cannot Be Recovered
A missed notification cannot be fixed. Neither can a period that has passed its 24 or 42 month deadline. What can sometimes be fixed is a claim that was made but made badly, because an amendment inside the window is still an option, and that is worth a review while time remains.
The other recoverable situation is the reverse: a claim that was too large. Where the return can no longer be amended there is an HMRC disclosure route for overclaimed relief, and penalties are typically reduced where the disclosure is unprompted. HMRC sets out the claim time limits in its guidance on making a claim on the company tax return, and the British Business Bank covers the basics of qualifying.