Are R&D tax credits taxable?

No, SME R&D tax credits are not taxable for UK limited companies.

R&D tax credits are a corporation tax relief and therefore, the tax credit rebate is recognised in the corporation tax line for accounting purposes - this means that it does not fall within profits chargeable to tax within a company's profit & loss account / income statement.

The Research & Development Expenditure Credit (aimed at Large Companies) is treated differently in that the credit is recognised within taxable income and therefore is subject to corporation tax.

You can read more detail about the accounting treatment of R&D tax credits here.