Definition
Under the merged scheme, the research and development expenditure credit (RDEC) is treated as a taxable receipt. Before a company can apply the credit against its corporation tax liability or receive any surplus as cash, HMRC requires it to compute a notional tax charge on the credit itself. At the main rate of corporation tax (25% from April 2023), a gross credit of £100 produces a notional charge of £25, leaving a net credit of £75 available to offset tax. This is why the commonly cited net benefit on qualifying R&D spend under the merged scheme is approximately 15% rather than 20%.
How the waterfall works
The merged-scheme credit flows through a six-step waterfall set out in the legislation at Part 13 of the Corporation Tax Act 2009. The notional tax calculation sits at step 2. Step 1 brings the gross credit into the profit calculation as income. Step 2 computes the notional tax on that income at the applicable rate. Steps 3 and 4 apply the net credit first against any corporation tax payable for the period, then against any other amounts owed to HMRC. Steps 5 and 6 deal with any remaining surplus, which can be surrendered to a group company or paid out as a cash repayment. The CIRD manual covers the waterfall at CIRD90100 onwards.
For a loss-making company the cash payment at step 6 is subject to the PAYE cap, which limits the cash repayment to £20,000 plus three times the company's PAYE and NIC liability for the period.
Example
A profitable company has £500,000 of qualifying R&D expenditure under the merged scheme. The gross 20% credit is £100,000. The notional tax charge at 25% is £25,000. The net credit available to offset the corporation tax bill is £75,000. If the company's CT liability is £200,000, the bill reduces to £125,000. The company receives no cash payment because the credit is fully absorbed. The net economic benefit is £75,000 on £500,000 of spend, or 15%.
Why the 20% headline can mislead
HMRC's published guidance describes the merged-scheme rate as a 20% above-the-line credit. That figure is correct for the gross credit before the notional charge. Advisers who quote 20% as the cash benefit are overstating the return for profit-making companies. For a company paying corporation tax at 25%, the net benefit is 15%. For companies paying the small profits rate (19%), the arithmetic is slightly different: the notional charge is lower and the net benefit is marginally higher. The CIRD manual provides worked examples at CIRD90200.
Loss-making companies eligible for ERIS operate under a different credit rate (approximately 27% net), so the notional-tax step produces a different outcome for intensive SMEs.
Common mistakes
The most common error is treating the 20% credit as equivalent to a 20% cash rebate on spend. A second error is omitting the notional tax step entirely from the CT computation, which typically leads to an overstated credit being submitted. HMRC enquiry letters frequently identify this omission when reviewing the self-assessment return alongside the Additional Information Form. A third mistake is applying the wrong corporation tax rate to the notional charge, particularly where a company is close to the marginal relief band between the 19% and 25% rates.
If you want to check what net benefit your company might receive, the eligibility checker works through the key variables.