How to claim R&D tax relief on the CT600 form

‍What is a CT600?

The CT600 is the core form used to file your Company Tax Return and calculate your Corporation Tax position for an accounting period. It is where you report taxable profits or losses and make any tax adjustments. An R&D tax relief claim is not a separate standalone submission. It is a set of figures and disclosures that sit within the CT600 process.

For most companies, your CT600 is prepared through accounting or tax software, with the underlying numbers supported by your statutory accounts, tax computation and working papers. If the CT600 does not reflect the claim correctly, or the claim is missing required submissions, HMRC can treat the claim as invalid.

How R&D claims work under the merged scheme and ERIS

For accounting periods beginning on or after 1 April 2024, most claims are made under the merged R&D tax relief scheme. A smaller subset of loss making, R&D intensive SMEs may qualify for Enhanced R&D Intensive Support (ERIS), which changes the level of support available. The way you file the claim still runs through your Company Tax Return, but the calculations and disclosures need to match the route you are claiming under.

Under the merged R&D tax relief scheme, the benefit is an expenditure credit that is taxable, so the net benefit depends on your tax position. For ERIS, the rules are different and you must meet additional eligibility tests. In both cases, the claim must be submitted in a way that allows HMRC to understand what you are claiming, why it qualifies and how the numbers have been built.

What you need before you start your CT600 R&D claim

Before touching the CT600, make sure the building blocks of the claim are in place. You should have a clear project list and a short technical narrative that explains the advance and the uncertainties being resolved. You should also have a cost model that ties staff, subcontractors, software, consumables and any other qualifying categories back to your ledgers and payroll.

You will also need to confirm whether you must submit a claim notification form. This applies in particular for first time claimants and certain infrequent claimants, and missing it can invalidate the claim.

Step by step: how to file an R&D claim through the CT600

The exact screens and box numbers vary by software provider, but the filing logic is consistent. The goal is to ensure your CT600, computations and supplementary forms all tell the same story.

Step 1: Confirm the accounting period and your filing deadline

Start by confirming the accounting period shown in your accounts and that it matches the period being filed in the Company Tax Return. Most claim deadlines are based on the end of the accounting period, so getting this right early avoids painful rework later.

Step 2: Choose the correct route and calculate the claim

Identify whether the claim is under the merged scheme or ERIS, then finalise the calculation of qualifying expenditure and the resulting credit or relief. At this stage you should already be able to reconcile totals back to payroll and the ledger. If you cannot, the claim is not ready to file.

Step 3: Prepare the supporting submissions that sit alongside the CT600

An R&D tax relief claim typically requires more than just entries in the CT600 itself. Depending on your circumstances, you may need to submit an additional information form and possibly a claim notification form. These steps are separate from the CT600 but are part of submitting a valid claim.

Treat these as part of the same filing pack. If you split responsibility across finance, technical teams and advisers, agree who owns each submission and how you will check consistency before filing.

Step 4: Complete the R&D supplementary pages in your tax software

For claims under the merged scheme, you will usually need to complete the relevant supplementary pages so the expenditure credit is calculated and applied correctly. This is where you enter the qualifying expenditure and work through how the benefit is used, including offsetting Corporation Tax and considering restrictions that affect the payable amount.

If you qualify for ERIS, ensure your software treatment reflects the ERIS route and that the eligibility and intensity position is supported by your working papers. ERIS is not just a different rate. It is a different set of conditions and HMRC may expect you to demonstrate how you meet them.

Step 5: Check the CT600, computations and schedules agree

Before submission, check that the claim figures agree across:

• The detailed cost schedule
• The tax computation
• The CT600 and any supplementary R&D pages
• The additional information form project list and narrative summary

Inconsistencies are a common cause of enquiries and delays, especially where project names, staff apportionments, or totals differ between documents.

Step 6: Submit the return and keep a clean audit trail

Once filed, keep a single pack that includes the submitted CT600, computations, cost schedules, narratives, and confirmation of any separate submissions such as the additional information form. This makes any follow up from HMRC much easier to handle and reduces disruption.

Filing through an amendment to the CT600

You can usually make an R&D tax relief claim either in the original return or by amending a previously filed return, as long as you are within the statutory amendment window. Amended claims often arise where the accounts were finalised first and the R&D analysis was completed later, or where you are correcting or improving a previous submission.

If you amend, treat it with the same discipline as an original filing. Make sure the revised claim is consistent across all documents and keep a clear record of what changed and why.

Common filing mistakes to avoid

A lot of problems are avoidable. The most common issues include filing without the required supporting submissions, using inconsistent project names between the additional information form and the narrative, or entering figures that do not reconcile back to payroll and the ledger. Another frequent issue is leaving the filing too late, then discovering that a missing form or error cannot be corrected before the claim deadline.

If you do one thing, do this: run a final cross check across every document in the filing pack before you hit submit. Happy filing!