Definition
A period of account is the financial year for which the company prepares its accounts, as filed at Companies House. Most companies have a 12-month period of account, typically ending on 31 March, 31 December, or another chosen date. The accounting period is the period on which HMRC bases the corporation tax assessment. It cannot exceed 12 months. Where a company's period of account is exactly 12 months, the accounting period and the period of account coincide. Where a period of account is longer, HMRC splits it into two accounting periods.
A company's first period of account after incorporation is often longer than 12 months, for example 15 or 18 months. In that case, HMRC treats it as two accounting periods: the first 12 months and the remaining months. The CNF deadline, however, runs from the end of the period of account, not the end of either accounting period.
Why the CNF deadline uses the period of account
HMRC's guidance at gov.uk specifies that the CNF must be submitted within 6 months of the end of the period of account in which the R&D expenditure was incurred. This is the same period referenced in the company's accounts, not the shorter accounting period used for the CT assessment. The practical implication is that where an accounting period falls within a longer period of account, the clock for the CNF runs to the longer period of account end, which can give additional time.
The CNF requirement has been mandatory for R&D claims in accounting periods beginning on or after 1 April 2023, as set out in the HMRC guidance linked above.
Example
A company is incorporated on 1 October 2023 and draws up its first accounts to 31 December 2024, a period of 15 months. HMRC creates two accounting periods for CT purposes: 1 October 2023 to 30 September 2024 (12 months), and 1 October 2024 to 31 December 2024 (3 months). The period of account ends on 31 December 2024. The CNF for R&D activity within the period of account is due by 30 June 2025 (6 months after 31 December 2024). The company should not calculate the deadline from the end of either accounting period.
Common mistakes
The most common mistake is using the accounting period end date rather than the period-of-account end date to calculate the CNF deadline. For standard 12-month periods the difference does not matter. For first-year or change-of-year periods it can make a material difference to when the clock expires. A second mistake is confusing the CNF deadline (6 months from period-of-account end) with the overall claim time limit (2 years from accounting period end). Both apply to an R&D claim but are measured differently and from different dates.
For a clear check of whether your CNF deadline has passed, the free assessment covers this as a standard step.