You've filed your CT1 and included your R&D tax credit claim, then something comes up afterwards. Maybe a cost you forgot to include, an error in your workings, or new information about a project that changes the figures.
The short answer is yes, you can amend it, but only up to a point. Revenue gives you a fixed window to make changes, and once it closes, there's no way back for anything you've missed.
What is the deadline to make an R&D tax credit claim?
You have 12 months from the end of the accounting period the expenditure relates to: Revenue treats every R&D tax credit claim the same way, whether it's your first submission or a correction to one you've already filed. Revenue's own guidelines put it plainly:
"If a claim in respect of any given period is not made within the requisite 12 month period then no entitlement to any amount in respect of that period arises at any time."
This means that if you're trying to add a new cost or increase the credit for a period after that 12-month mark has passed, Revenue won't allow it, no matter how well documented the R&D itself is.
Take a company with an accounting period ending 31 December 2025. Its R&D tax credit claim deadline for that period is 31 December 2026, regardless of when it originally filed its CT1.
There is an additional deadline for a first-time claimant or one who hasn’t claimed in three years: you must submit a Pre-Filing Notification to Revenue at least 90 days before your claim.
You might still have time after your CT1 is already filed
The standard Corporation Tax filing deadline falls on the 23rd day of the ninth month after your accounting period ends, but Revenue gives R&D claims an extra three months on top of that. That gap matters, because it means you can file your CT1 on time and still come back later to add or adjust your R&D figures, right up until the 12-month mark.
For the same company above, with a 31 December accounting period end, the CT1 itself is due by 23 September the following year, but there's still until 31 December, three more months, to revisit the R&D figures included in that return.
Correcting an error that increases your claim
If you've simply underclaimed, missed a qualifying cost, or need to revise your figures upward, this is the straightforward case, provided you're still inside the 12-month window. You, or your adviser, can go back into your CT1 return on ROS, make the correction, and refile it as an amended return. Revenue treats this the same as an original claim: there's no separate approval process, no extra form, and no need to explain why the figures changed.
Once the 12 months has passed, that route closes for good. Any amount not claimed within that window is lost, and there's no appeal, extension, or discretion available, even where the underlying R&D work clearly qualifies.
Correcting an error that reduces your claim
A different situation is finding you've over-claimed, perhaps a cost that shouldn't have qualified, or a calculation error that inflated the credit. This isn't blocked by the 12-month rule, because you're not making or increasing a claim, you're correcting one, and Revenue expects you to fix it as soon as you become aware of it, whatever the reason it happened.
If you catch it within 12 months of your CT1 filing deadline, you can self-correct without penalty: write to Revenue, work out the correct tax position with statutory interest, and pay the difference along with your notification. This route only works if you get there first. It doesn't apply once Revenue has already opened a compliance check into your return.
Beyond that self-correction window, you can still come forward voluntarily through what Revenue calls an unprompted qualifying disclosure, which still reduces any penalty significantly compared with Revenue finding the error itself.
If Revenue has already been in contact about your claim, correcting it becomes a prompted disclosure instead, which carries a smaller reduction of your penalty, so the earlier you act, the better the outcome. For more on what a Revenue review of your claim actually looks like, see our guide: What Happens If Your R&D Tax Credit Claim Is Audited
Fixing an administrative mistake
Not every correction is about the credit amount. Sometimes it's a wrong accounting period, an incorrect reference number, or a transcription error that doesn't change what you're entitled to. These administrative slips sit outside the 12-month claim deadline like errors discussed above. But, since you're not changing the substance of the claim, they're generally best sorted by contacting Revenue directly through MyEnquiries rather than waiting for one to surface in a review.
How to actually make the amendment
- Log back into ROS and open the CT1 return for the relevant accounting period.
- Update the R&D tax credit fields, or the wider return if the error sits elsewhere, with the corrected figures.
- If you're inside the self-correction window and the change increases your tax liability, prepare a written notification to Revenue, recompute the tax due including statutory interest, and submit both together with payment.
- Refile the amended return, and keep a clear record of what changed and why, in case Revenue asks about it later.
Building in a check before you file
The real risk is running out of time to make a correction. A short review of your R&D figures a month or two before the 12-month deadline gives you room to catch anything you've missed while there's still time to fix it, rather than discovering the gap once nothing more can be done.
If you haven't worked through the full claim process yet, our guide on how to submit an R&D tax claim in Ireland covers each step in order.
Key takeaways
- You can amend your R&D tax credit claim up to 12 months after the end of the accounting period it relates to. After that, no new or increased claim is possible for that period, under any circumstances.
- Your CT1 filing deadline and your R&D claim deadline aren't the same date. The R&D deadline falls three months later, so you can amend a return you've already filed.
- Reducing an over-claimed credit isn't subject to the 12-month rule. Self-correct within 12 months of your CT1 filing deadline to avoid penalties entirely.
- Coming forward voluntarily, even outside the self-correction window, still gets you a better outcome than waiting for Revenue to find the error.
- Administrative errors that don't change the credit amount can usually be sorted directly with Revenue, without reference to the claim deadline.
If you think an earlier R&D tax credit claim needs correcting, or you'd like a second look before your deadline passes, get in touch and we'll help you work out where you stand. You can also sign up to Tax Cloud to get your next claim built right the first time.
