Definition
An accounting period is the period covered by a company’s Company Tax Return (CT600) and used to calculate its Corporation Tax. It can never be longer than 12 months and usually matches the financial year of the company’s annual accounts. The Corporation Tax payment deadline runs from the end of the accounting period, and the CT600 filing deadline normally does too.
When it differs from the accounts
A new company’s first accounts run from incorporation to its accounting reference date, which often means more than 12 months. Because an accounting period cannot exceed 12 months, those accounts are split into two accounting periods, each with its own CT600 and payment date. The periods can also differ when a company starts trading after incorporation or changes its year end.
Example
Northgate Joinery Ltd is incorporated and starts trading on 15 September 2025. Its first accounts run to 30 September 2026. For Corporation Tax, that becomes:
- 15 September 2025 to 14 September 2026: tax due 15 June 2027, CT600 due 30 September 2027
- 15 September 2026 to 30 September 2026: tax due 1 July 2027, CT600 due 30 September 2027
Both CT600s are due 12 months after the accounts end; each payment date runs from its own period end.
Companies that were dormant before they started trading follow slightly different steps, covered in how the first two accounting periods work.
Related questions
- What if my company's first accounting period is longer than 12 months? It is split into two accounting periods, with two CT600s.
- How do I change my company's year end? File form AA01 with Companies House before the accounts deadline for the period you are changing, then update the dates with HMRC.
Related terms
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Accounting reference date
The accounting reference date (ARD) is the date a company's financial year ends each year, often called its year end.
Read definition -
CT600
The CT600 is the Company Tax Return a UK company files with HM Revenue and Customs (HMRC) for each accounting period.
Read definition -
Corporation Tax
Corporation Tax is the tax UK limited companies pay to HM Revenue and Customs (HMRC) on their taxable profits.
Read definition -
Small profits rate
The small profits rate is the 19% rate of Corporation Tax for companies with profits of £50,000 or less in a 12-month accounting period.
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