CT600L explained: the R&D supplementary page

If your company claims research and development (R&D) tax relief, the claim goes on supplementary page CT600L, sent with your Company Tax Return. Before the return, HMRC needs an additional information form, and some companies must also send a claim notification. Miss either and the claim fails.

Last checked against GOV.UK on 1 October 2026.

A4O Accounts prepares the CT600 from your books and files it with HMRC. See the CT600 software, or get started free.

Who has to complete CT600L

GOV.UK’s guidance says you complete CT600L if your company is claiming R&D expenditure credit (RDEC), a payable R&D tax credit, or both. It’s one of the CT600 supplementary pages, so it goes to HMRC as part of the return, not on its own.

Whether your work counts as R&D for tax, and which costs qualify, are separate questions that GOV.UK covers in its R&D guidance. This guide is about the form.

Which scheme applies

Under the merged scheme, the company gets a taxable expenditure credit of 20% of its qualifying R&D spending.

ERIS is for loss-making, R&D intensive small and medium-sized companies: those whose relevant R&D expenditure is at least 30% of their total expenditure. It gives an extra 86% deduction on qualifying costs, and a payable tax credit of up to 14.5% of the loss surrendered.

Two forms HMRC needs before the return

The additional information form. This sets out the detail behind the claim. GOV.UK says it must be submitted before, or on the same day as, the Company Tax Return, and if it’s the same day it must go first. If the return arrives first, the claim is rejected and HMRC removes the R&D claim from the return.

The claim notification form. For accounting periods beginning on or after 1 April 2023, you must tell HMRC you plan to claim if you’re claiming for the first time, or your last claim was more than 3 years ago. The window opens on the first day of the period of account and closes 6 months after it ends. Miss it and the claim is invalid.

What the page works out

For an expenditure credit, CT600L takes the credit through seven steps, each deciding how much is used where:

1. Set against the company’s Corporation Tax for the period.
2. A notional tax restriction on the amount that can be paid out.
3. A cap based on the company’s PAYE and National Insurance for its R&D workers.
4. Set against Corporation Tax owed for other periods.
5. Surrendered to other companies in a group.
6. Set against other amounts the company owes HMRC.
7. What’s left is payable to the company.

The page also records the qualifying expenditure and, for ERIS and older SME claims, the payable tax credit.

How it feeds the CT600

The results are carried to boxes on the main return. Among them: the payable expenditure credit goes to box 880, a payable SME tax credit to box 875, and the total R&D credit set against liabilities to box 530. Software carries these across for you.

If the claim changes

The claim is part of the return, so changing it means amending the return. R&D claims have their own time limits, which GOV.UK’s R&D guidance sets out.

CT600L in software

In A4O Accounts you add a CT600L to the return from its tax computation. The R&D deduction counts in the computation once the page is attached, and the page goes to HMRC with the rest of the return. The additional information form and the claim notification are HMRC’s own online forms, so you send those to HMRC yourself, before you file. See CT600 software and Corporation Tax software.

More in this series: all the CT600 guides.