Why this is no longer just a CT600 number

R&D claims are high-risk workflow items because the tax computation, Company Tax Return, supplementary page, claim notification, and additional information form can all interact. HMRC guidance says the additional information form must be submitted before, or on the same day as, the CT600. If the CT600 goes first, the claim can be rejected.

That turns an R&D claim into a sequencing problem. The practice file needs to show not only the expenditure and relief calculation, but also that the right forms were submitted in the right order.

The claim sequence to control

Decide which R&D scheme applies to the accounting period.
Check whether a claim notification is required.
Prepare the additional information form for the correct accounting period.
Submit the additional information form before the CT600, or earlier on the same day.
Prepare the CT computation and relevant CT600 entries.
Complete CT600L where a payable credit or expenditure credit requires it.
Keep confirmation evidence, client approval, and the final submitted CT600 package.

Boxes 656 and 657 are review controls

HMRC's R&D claim guidance refers to CT600 boxes 656 and 657 for claim notification and additional information. Treat those boxes as sign-off controls, not clerical ticks. If box 657 is marked, the file should show the additional information form was actually submitted and that the accounting period matches the CT600.

If a payable credit is being claimed, bank details and CT600L may also become part of the filing package. That means the reviewer needs to see the payable-credit route, the computation, the supplementary page, and the form evidence together.

Period matching matters

HMRC guidance says an additional information form is needed for each accounting period being claimed. If accounts cover more than 12 months and create more than one CT600 accounting period, the R&D claim workflow needs to split the supporting information correctly.

A mismatch between the period on the additional information form and the period on the CT600 can cause the claim to fail operationally even where the underlying project looks valid.

What to keep in the evidence pack

Scheme basis: RDEC, SME, merged scheme, or ERIS, depending on the period.
Accounting-period dates used on the CT600 and additional information form.
Claim notification decision and evidence where relevant.
Additional information form confirmation and reference.
Project descriptions, qualifying cost categories, and technical narrative.
CT600L values and payable-credit cascade where applicable.
Client approval of the claim and any adviser involvement.

Common traps for practices

Preparing the CT600 first and leaving the additional information form until after submission.
Using one supporting form for a long period when HMRC expects period-specific submissions.
Failing to preserve the additional information form reference after submission.
Completing CT600L without reconciling it to the computation and payable-credit route.
Not recording who approved the technical claim narrative before filing.

How Robocount supports R&D filing control

Robocount is designed to make the R&D claim state visible before the CT600 leaves the practice. The goal is to reduce fragile spreadsheet-and-email handoffs around one of the most sensitive Corporation Tax workflows.

Connects R&D claim indicators to the CT600 review flow.
Surfaces CT600L and payable-credit checks alongside the main return.
Keeps additional-information and claim-notification status visible before submission.
Supports approval evidence for practice teams and API-driven workflows.
Helps agents build a cleaner return package for HMRC filing and later enquiry response.

FAQ

Does every R&D claim need an additional information form?

HMRC guidance says an additional information form must be submitted to support new claims for R&D tax relief, expenditure credit, or both. Check the latest HMRC steps for the accounting period before filing.

Can the additional information form be submitted after the CT600?

HMRC says it must be submitted before, or on the same day as, the CT600. If both are sent on the same day, the additional information form should be sent first.

When is CT600L needed?

CT600L is relevant for R&D expenditure credit or payable tax credit cases. The exact requirement depends on the claim route and period, so it should be checked as part of the CT600 review.

Useful HMRC references

This guide is general product and filing workflow information, not tax advice. Check the current HMRC guidance and the claim facts before filing.

CT600 R&D claims need the additional-information step in the right order.