You can usually add an R&D claim to a company tax return that has already been filed, by amending the return within roughly two years of the end of the accounting period. For accounting periods that began before 1 April 2024, which sit under the old SME and RDEC schemes, the final standard amendment deadlines fall around 30 and 31 March 2027. Whether a backdated claim is still possible depends on more than the amendment window, though: the claim notification rule has already closed the door for many first-time claimants, quietly and permanently.
This page covers the windows, the trap, the historical rates, and how to judge whether a backdated claim is worth making.
How far back can you claim R&D tax relief?
Roughly two years from the end of the accounting period. An R&D claim is made in the company tax return, so backdating means amending a return already filed, and the standard amendment window runs to around the second anniversary of the period end.
At the time of writing in July 2026, that means for standard twelve-month periods:
- Year end 30 September 2024. Amendable until around 30 September 2026, so the runway is short.
- Year end 31 December 2024. Amendable until around 31 December 2026.
- Year end 31 March 2025. Amendable until around 31 March 2027, but check the regime: a period that began on or after 1 April 2024 falls under the current schemes, not the old ones.
Exact dates depend on your own accounting dates and filing history, so confirm the deadline for each period rather than working from the pattern. If you are unsure which regime a period falls under, which R&D scheme applies to your company settles it by date.
Why does March 2027 matter?
Because it is when the old schemes finally close. The merged scheme and ERIS apply to accounting periods beginning on or after 1 April 2024; every earlier period sits under the old SME scheme or old RDEC. The last periods to begin under the old rules ended, for standard twelve-month periods, in late March 2025. Add the roughly two-year amendment window and the final standard deadlines fall around 30 and 31 March 2027.
After that, no new old-scheme claim can be made through the standard amendment route. Each company’s own deadline is set by its own year end, and for most companies it falls well before March 2027. Treat the March date as the end of the runway, not the date to aim for.
How does the claim notification rule kill backdated claims?
Silently, and before the merits are ever considered. For accounting periods beginning on or after 1 April 2023, a company claiming R&D relief for the first time, or that has not claimed in the previous three years, must have submitted a claim notification within six months of the end of the period of account. There is no late route and no appeal.
The arithmetic is unforgiving. By mid 2026, every standard twelve-month period still inside the amendment window began on or after 1 April 2023, so the notification regime touches all of them. And because any period worth backdating ended more than six months ago, its notification window has already closed. A first-time claimant with a 31 December 2024 year end had until 30 June 2025 to notify; without that notification there is no claim for the year, even though the return itself can be amended until the end of 2026.
The practical consequence is that the runway to March 2027 mainly serves companies with a claim history. There is a wrinkle even for them: a claim made by amendment after 1 April 2023 for an earlier period does not count as a prior claim for the three-year test, so making a backdated claim does not by itself remove the need to notify for later periods.
What were the old scheme rates?
The table below is historical reference only. These rates apply to accounting periods beginning before 1 April 2024 and matter now only for backdated claims and open enquiries. Current claims use the merged scheme or ERIS rates.
| Old scheme | Expenditure before 1 April 2023 | Expenditure from 1 April 2023 |
|---|---|---|
| SME scheme: additional deduction | 130% | 86% |
| SME scheme: payable credit (surrender) rate | 14.5% | 10%, with 14.5% retained for R&D-intensive loss-makers |
| RDEC | 13% | 20% |
Two features of the old SME scheme deserve particular care in backdated claims. Grant funding and other subsidies could restrict old SME relief, a rule the current schemes have abolished; the contrast is set out in grant funding and R&D tax relief. And the old subsidy and subcontracting rules were contested territory, now shaped by two First-tier Tribunal decisions that HMRC chose not to appeal; they are covered in our guide to HMRC R&D enquiries.
When is a backdated claim worth making?
When the qualifying spend is material, the evidence survives, and the procedural gates are open. We test four things before recommending one:
- The gates. Notification status for the period, and the Additional Information Form, which has been mandatory for every claim since 8 August 2023, amendments included.
- The definition. The work must meet the same statutory test as a current claim: an advance in a field of science or technology, sought through resolving uncertainty a competent professional in the field could not readily resolve.
- The evidence. Backdating means reconstructing project narratives and cost workings for work finished a year or more ago. If the competent professionals have moved on or the records are thin, defensibility suffers, and HMRC checks roughly one in six R&D claims.
- The economics. The value depends on your tax position and the historical rates above. A claim that clears the first three tests is usually worth making; one that scrapes past them sometimes is not, and we will say so.
A backdated claim faces exactly the scrutiny a current one does. Preparing one thinly because the work is old is how enquiries, clawbacks and penalties happen.
What should you do before March 2027?
Three steps, in order:
- Map the open periods. List each year end still inside the amendment window and note its deadline.
- Settle the notification question for each period before investing in anything else, because it decides whether a claim can exist at all.
- Leave time to prepare properly. A defensible claim needs competent professional interviews, cost reconciliation and a carefully prepared AIF. None of that compresses well into the final weeks before a deadline.
Talk it through with a chartered adviser
Backdated claims reward early, honest assessment: some are clearly worth making, some are already impossible, and an early view stops you spending time on the ones that cannot succeed. LimestoneGrey is a firm of chartered tax advisers and chartered accountants specialising in R&D tax relief regulated by ICAEW, every claim is signed off by a chartered adviser, and enquiry support is included as standard, which matters more than usual when a claim revisits an old period.
If you think a past year holds a claim, get in touch before the window shortens further. For the wider picture, start with our R&D tax relief guide.
Written by Matthew Jones ACA CTA. Last reviewed July 2026.
Sources
- CIRD81800: SME claim time limits — the roughly two-year amendment window.
- Tell HMRC you plan to claim — the six-month notification window that can close a backdated claim.
- SI 2023/813: claim notification & AIF regulations — the statutory content requirements.
- Additional information form guidance — the AIF, mandatory for every claim including amendments since 8 August 2023.
- R&D relief for SMEs — the old SME rates of 86% / 10% / 14.5%.
- CIRD89710: RDEC rate — the RDEC rate rising from 13% to 20%.