Chris Stuttle
Senior Consultant
Businesses claiming R&D tax relief under the UK’s merged RDEC scheme are being advised to review their filing processes following a recent update to HMRC’s CT600L form, which is already causing issues for R&D tax claims.
On 6th April 2026, HMRC introduced new reporting requirements within the CT600L supplementary pages, adding six new boxes (L71, L71A, L72, L72A, L73 and L73A). These changes are intended to capture additional data relating to the PAYE and National Insurance (NIC) cap under the merged R&D scheme.
The update reflects the rules introduced under the new merged R&D regime, which applies to accounting periods starting on or after 1st April 2024, and replaces the former SME and RDEC schemes. Under this regime, companies are now required to calculate and evidence their exposure to the Step 3 PAYE & NIC cap, which limits the payable R&D credit based on employment taxes.
HMRC has acknowledged that some tax and accounting software providers are yet to update their systems to accommodate the new CT600L fields and are therefore facing some issues when submitting their claims. As a result of their software not being up to date, a number of companies and agents are unable to submit RDEC claims for cases where the new boxes are required but not yet available. Early reports show that this is causing validation errors when filing returns, missing CT600L fields in certain platforms, and delays in the preparation and submission of R&D claims.
The new boxes are mandatory for anyone submitting or amending an RDEC claim relating to accounting periods starting on or after 1st April 2024. They are being used to either report PAYE and NIC liabilities relevant to the cap, or, confirm that the company qualifies for an exemption.
If the new fields are not completed, returns cannot be submitted successfully, thereby risking a missed deadline.
HMRC has indicated it may allow late claims where submission issues are caused by missing CT600L functionality. However, this is not guaranteed, and companies will be required to provide evidence that supports their missed deadline.
Companies are expected to:
“We have been advised by some customers that they are unable to submit a merged scheme R&D expenditure credit (RDEC) claim, due to issues related to new boxes added to the CT600L from 6 April 2026. For accounting periods beginning on or after 1 April 2024, the merged scheme R&D expenditure credit (RDEC) introduced a new step 3 PAYE & NIC cap (s.1112B CTA09) and new boxes L71, L71A, L72, L72A, L73 and L73A were introduced to the CT600L on 6 April 2026 to collect information related to this. Those boxes need to be completed where a company is making or amending an RDEC claim for any accounting period beginning on or after 1 April 2024, to either provide information related to PAYE & NICs, or confirm the exemption to the cap applies.
If you are due to submit an RDEC claim you should check your software provider has made the changes needed to the CT600L to add the new boxes. If the new boxes are not available for you to complete your claim and as a consequence you are unable to submit an RDEC claim by the statutory deadline, then the company should reference this when applying to HMRC to exercise its discretion under paragraph 83E(5) of Schedule 18 to the Finance Act 1998 to allow a claim to be made late. Please retain any evidence (including any error message) that demonstrates that the issue arose due to the new boxes on the CT600L not being available, and any communication from your software provider that relates to this. Please provide this evidence alongside any representations under Statement of Practice 5/01 in relation to the lateness of the claim.”
Companies that are looking to submit R&D claims over the next few months should take a few simple steps to stay ahead of any issues that may arise. It’s worth checking in with your software provider to make sure the CT600L updates have been rolled out, and giving your submission process a test run early. This is especially important if you’re dealing with amended claims. If you do run into any problems, ensure you keep a record of them and don’t leave it too late, giving you a chance to get some advice sooner rather than later.
If you would like support with submitting your R&D tax credit claim, send us a message or book a free 1-2-1 consultation using our links below.
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