Definition
Corporation Tax Self Assessment, often abbreviated CTSA, is the regime governing how UK companies calculate and pay corporation tax. Under CTSA, a company files a CT600 return with HMRC within 12 months of the end of its accounting period, self-assessing its tax liability. R&D tax relief is claimed within the CT600 and, since 8 August 2023, must be supported by an Additional Information Form. HMRC has nine months from the date a claim is received to open an enquiry, and longer where information is outstanding.
How HMRC defines it
The CTSA framework is set out in Schedule 18 to Finance Act 1998. HMRC's Company Taxation Manual (CTM) from CTM90000 onwards is the main reference. Interaction with R&D claims is addressed at CIRD81800 and in paragraph 83 of Schedule 18. The enquiry window for R&D claims is extended in the usual discovery circumstances.
Practical example
A company with a 31 December 2024 year-end files its CT600 by 31 December 2025 including an R&D tax relief claim supported by an AIF submitted earlier in the year. HMRC has until 31 December 2026 to open an enquiry into the return, subject to ordinary discovery rules that may apply for longer in cases of insufficient disclosure or careless error.