Written By Simba Mareverwa, Head of Consulting and Tax Compliance
A practical issue has emerged for R&D tax claims under the merged scheme, particularly where an SME is claiming RDEC but has no ERIS expenditure to surrender.
The difficulty is not simply a data-entry error. It appears to be a circular validation problem within the CT600L.
In order to, complete Box L75, which relates to the RDEC PAYE/NIC cap, agents may be forced into completing parts of the SME and ERIS section, including boxes L166 to L169 and L175. However, once those boxes are partly completed, the software then expects SME loss surrender information that does not apply to the claim.
This creates an impossible loop:
Complete the SME section to clear the Box L75 issue, then reverse that same entry to clear the SME loss surrender errors.
The result is that a valid RDEC-only claim can become unfileable, not because the claim is wrong, but because the form logic does not appear to accommodate this fact pattern.
This issue sits alongside wider CT600L challenges following HMRC’s 6 April 2026 update, which introduced additional reporting requirements for accounting periods beginning on or after 1 April 2024 under the merged R&D scheme.
A practical fix would be to allow Box L75 to be completed independently where no ERIS claim is being made. Alternatively, a simple decision box asking whether ERIS is being claimed could prevent irrelevant SME surrender validations from being triggered.
The policy intent may be clear, but the compliance pathway needs to be workable. If valid claims cannot be filed because of circular validation logic, the problem is administrative rather than technical tax analysis.
For advisers and claimants, this is a reminder to keep screenshots of validation errors, document the filing attempts, and raise the issue early with software providers and HMRC.
