Many businesses that could have claimed R&D tax credits in previous years simply didn't know they were eligible. Others started looking into it partway through a financial year and assumed it was too late for periods that had already closed. In most cases, that assumption is wrong.
HMRC allows companies to go back and claim for up to two prior accounting periods. For many companies, that represents a great opportunity for your cashflow.
How far back can you claim?
You can amend a submitted corporation tax return to include an R&D tax credit claim up to two years after the end of the accounting period in question.
In practice, this works as follows: a company has 12 months from the end of its accounting period to submit its original corporation tax return. It then has a further 12 months to amend that return. So, for an accounting period ending on 31 March 2024, the window to amend the return and include an R&D claim runs until 31 March 2026.
That is the standard position. HMRC has discretion to consider claims outside this window in exceptional circumstances, but this is genuinely rare and should not be planned around.
For example, Company A has an accounting period that ends 31 March and has been doing qualifying R&D for several years but didn’t claim for the last couple years. In April 2026, it could still submit claims for accounting periods ending 31 March 2024 and 31 March 2025. The period ending 31 March 2023 would now be outside the window.
What changed with the advance notification requirement?
For accounting periods beginning on or after 1 April 2023, HMRC introduced an additional requirement: a company must notify HMRC of its intention to make an R&D claim within six months of the end of the accounting period it wants to claim for.
This is the Advance Notification Form (ANF), sometimes called a claim notification form. It is submitted separately from the corporation tax return and must be lodged within the six months after your accounting period ends.
The ANF rule only applies to companies claiming for the first time or those who haven’t made a claim in the last three years.
What this means for backdated claims is straightforward but important. If your accounting period began on or after 1 April 2023, and you did not submit a pre-notification within six months of that period ending, your claim for that year will not be accepted by HMRC, even if it is still within the two-year amendment window.
Example: a company with an accounting year running from 1 July 2024 to 30 June 2025 has until 31 December 2025 to submit its pre-notification for that period. If it misses that deadline, it cannot make a valid R&D claim for that year, regardless of whether the two-year amendment window is still open.
The pre-notification requirement does not apply to accounting periods beginning before 1 April 2023. For those earlier periods, the standard two-year window applies without the additional step.
Do I need to submit a completely new return?
No. A backdated R&D claim is made by amending your existing corporation tax return for the relevant period, not by filing a new one.
The amendment process is relatively straightforward: the R&D claim is added to the CT600 return via the relevant supplementary pages, and the amended return replaces the original. HMRC then processes the amendment and either applies the credit against the company's outstanding corporation tax or initiates a cash repayment. You can read more about how HMRC pays out the R&D tax credit in our blog.
As with all R&D tax credit claims, you should also submit an Additional Information Form (AIF) before submitting the amended tax return. As part of this submission, you’ll need to gather all the financial records from the relevant period to support your submitted figures: payroll data, supplier invoices, contractor agreements. The older the period, the more important it is to locate and organise this material before starting the claim, as HMRC may raise questions and your records need to support the figures you submit.
What does a backdated claim actually involve?
HMRC expects the same quality of claim regardless of whether it relates to the current period or one that ended two years ago. That means:
Technical narrative. You need to be able to describe what qualifying R&D activity took place in the period, what technological or scientific uncertainties were being addressed, and how the work went beyond routine development. This information is sent with the AIF, but you should have evidence of the work you carried out, in case HMRC asks to see it.
Cost calculations. Staff costs, contractor fees, software, and consumables must be correctly apportioned to qualifying projects, using the allocation methods appropriate for the period being claimed. If HMRC queries the claim, they will want to see the reasoning behind your apportionments.
Eligibility check under the rules of the time. The Merged R&D Scheme applies from April 2024. Claims for periods before that date fall under the old SME or RDEC rules. The scheme that applies to your claim is determined by when your accounting period began, not when you submit the amendment.
If you received any grant funding during the period you are claiming for, the old SME rules applied restrictions to subsidised expenditure that the merged scheme has since removed. For periods before April 2024, grant-funded expenditure may still reduce the amount of relief available.
The rules around subcontracted work have also changed: large companies can now claim for subcontracted expenditure under the Merged Scheme and foreign third-party costs are now ineligible.
Is it worth it?
For most businesses doing genuine qualifying R&D, the answer is yes. The credit rates represent a meaningful return on expenditure that has already been incurred. You are not spending additional money; you are recovering a proportion of what you have already spent.
However, there are some practical considerations. If you haven’t submitted your ANF in time (if this is your first time claiming ever or after a break), then unfortunately, that R&D tax claim is lost to you.
If the ANF doesn’t apply to you (or you’ve already submitted it), the next thing you need to consider is whether you have sufficient records from the period to make a solid claim. You need to be able to justify your work was qualifying and that your costs were properly apportioned.
HMRC has increased their scrutiny of R&D tax claims in recent years in order to crack down on poor quality, spurious or even fraudulent claims. Backdated claims should be prepared with the same care and robustness as current-year submissions, as they are more likely to sound alarm bells at HMRC. If you receive a compliance check into your claim, you’ll need to prove that every element is qualifying, or risk spending considerable time and resources in defending your claim.
Key takeaways
- You can amend a corporation tax return to add an R&D claim up to two years after the end of the accounting period.
- For periods beginning on or after 1 April 2023, an ANF must have been submitted to HMRC within six months of the period end for companies claiming for the first time or after a break of at least 3 three years. Without it, the claim is invalid.
- Backdated claims use the rules that applied at the time; check your accounting period and the rules and rates of the era.
- Records from the period are essential. Technical narratives and cost allocations must be prepared to the same standard as a current claim.
If you think your business may have unclaimed R&D tax credits from a previous year, the best starting point is working out which periods are still within window and whether you need an ANF. Get in touch with the Tax Cloud team and we'll work through your position with you.
