CT600 Box 530: R&D Tax Credit

CT600 Box 530 is used by companies claiming a Research and Development (R&D) tax credit — a government incentive that rewards qualifying companies for investing in innovation. If your company has conducted qualifying R&D activity, a credit may be available to reduce your Corporation Tax bill or generate a cash repayment from HMRC.

This guide explains what Box 530 is, how R&D tax credits work, and what you need to complete the claim.

What Is CT600 Box 530?

Box 530 appears in the Tax Payments and Overpayments section of the CT600 form. HMRC's official description is:

> "Enter the figure you gave in box L210 of the supplementary page CT600L Research and Development."

This means Box 530 does not stand alone — it pulls directly from the CT600L, a dedicated supplementary page that captures all the detail of your R&D claim. You cannot complete Box 530 without first completing the CT600L.

If your company does not have qualifying R&D expenditure, Box 530 will be blank.

What Is an R&D Tax Credit?

The UK Government introduced R&D tax relief to encourage companies to invest in science and technology. Because R&D is inherently uncertain and risky, the incentive recognises that not all investment will succeed — and it rewards the attempt.

The credit reduces the amount of Corporation Tax you owe. If the credit exceeds your tax liability — as often happens with loss-making companies in the early stages of development — HMRC can pay the surplus directly as a cash repayment (subject to the PAYE cap, explained below).

For an overview of the R&D credit system and eligibility, see our guide to Research and Development Tax Credits.

Which R&D Scheme Applies to Your Company?

The scheme your company uses depends on when your accounting period begins.

Merged R&D Expenditure Credit (RDEC) — from 1 April 2024

For accounting periods beginning on or after 1 April 2024, almost all companies use the Merged RDEC scheme:

  • Credit rate: 20% of qualifying R&D expenditure
  • Who it applies to: Both SMEs and large companies (with limited exceptions)
  • The credit is treated as taxable income, so the net benefit after Corporation Tax is approximately 15% for a 25% taxpayer and 16.2% for a 19% taxpayer

Enhanced R&D Intensive Support (ERIS) — from 1 April 2024

A higher rate is available for loss-making SMEs that are R&D-intensive:

  • Credit rate: Up to 14.5% of the surrenderable loss
  • Who qualifies: Loss-making SMEs where R&D expenditure is at least 30% of total expenditure
  • Provides a higher effective cash repayment than the Merged RDEC for qualifying intensive companies

Legacy SME Scheme — before 1 April 2024

For accounting periods beginning before 1 April 2024, SMEs used the legacy scheme:

  • Additional deduction: 86% extra, giving a 186% total deduction of qualifying costs
  • Payable tax credit (loss-making companies): 10% of the surrenderable loss (standard) or 14.5% for R&D-intensive SMEs (where R&D expenditure is at least 40% of total expenditure)
  • PAYE cap: £20,000 plus 300% of relevant PAYE and National Insurance liabilities
Source: GOV.UK — Corporation Tax R&D Relief

What Qualifies as R&D?

HMRC defines R&D for tax purposes as work that:

  • Seeks an advance in a field of science or technology — this excludes advances in arts, humanities, or social sciences (including economics)
  • Works to resolve scientific or technological uncertainty — meaning that an expert in the field cannot say whether something is technologically possible or how to achieve it, even after consulting all available evidence
From 1 April 2023, mathematical advances can also be treated as science for these purposes.

Activities that may qualify include developing new software algorithms, creating novel materials, engineering new manufacturing processes, or researching new drug compounds. Routine work — applying existing techniques, adapting standard software, or following established procedures — does not qualify.

It is the nature of the work, not the industry, that determines eligibility. A company in manufacturing, software, construction, or any other sector can qualify if it meets the definition.

The CT600L Supplementary Form

Before you can enter a figure in Box 530, you must complete the CT600L supplementary page. This form captures:

  • Which R&D scheme you are claiming under (Merged RDEC, ERIS, or legacy schemes)
  • The total amount of qualifying R&D expenditure
  • Details of subcontracted R&D work and externally provided workers
  • Whether you have submitted an advance notification or additional information form to HMRC
  • Whether you meet the R&D intensity threshold for the enhanced ERIS rate
Box L210 of the CT600L is the output — the R&D credit figure — which flows directly into Box 530 of the main CT600.

HMRC introduced mandatory advance notification for most new R&D claimants from April 2023. If your company is claiming R&D relief for the first time (or has not claimed in the previous three years), you must notify HMRC within six months of the end of the accounting period. Missing this deadline means losing the claim entirely.

How the Credit Reduces Your Tax

Once Box 530 is completed, the R&D credit is applied against your Corporation Tax liability. This reduces what you owe to HMRC for the period.

If the credit exceeds your Corporation Tax liability, HMRC can pay the surplus as a cash repayment directly to your company. However, this repayment is capped:

  • PAYE cap (Merged RDEC and ERIS): The repayable credit cannot exceed £20,000 plus 300% of your total PAYE and National Insurance liabilities for the period. This cap targets artificial R&D structures and does not affect most genuine claimants.
For a broader view of how corporation tax is calculated before reliefs are applied, see our guide to CT600 boxes explained.

Important: R&D Claims Are a Specialist Area

HMRC has significantly increased scrutiny of R&D claims in recent years. A substantial number of claims — particularly those involving specialist advisory firms charging a percentage of the credit — have been found to be inaccurate or fraudulent. As a result:

  • HMRC is conducting more R&D compliance checks
  • All claimants must submit an Additional Information Form alongside the CT600L
  • Claims involving subcontracted R&D or externally provided workers have become subject to stricter rules
If you are claiming for the first time, or if your claim is substantial, professional advice from an accountant with R&D tax credit experience is strongly recommended. Getting the claim wrong can result in penalties, interest, and reputational damage.

Is Box 530 Supported by TinyTax?

TinyTax is designed for UK companies with straightforward Corporation Tax positions. The CT600L supplementary form and Box 530 are not currently supported, as R&D claims require specialist handling beyond the scope of basic filings.

If your company is eligible to claim R&D credits, you will need a qualified accountant or software that supports the CT600L supplementary form.

Summary

CT600 Box 530 records the R&D tax credit drawn from Box L210 of the CT600L supplementary form. The credit can reduce your Corporation Tax liability or, for loss-making companies, generate a cash repayment from HMRC. The Merged RDEC rate is 20% for most companies (accounting periods from 1 April 2024), while loss-making R&D-intensive SMEs may qualify for up to 14.5% under ERIS. R&D claims carry strict evidence requirements and HMRC scrutiny is high — professional advice is strongly recommended.

For more on how your Corporation Tax liability is calculated before reliefs are applied, read our guide to Research and Development Tax Credits.