Long period of account: time apportionment or actual figures for Corporation Tax?
Short version. Income and expenses are normally apportioned by time when one accounts period covers two Corporation Tax periods. HMRC also describes a transactions basis where it is more accurate. That choice needs to fit your circumstances.
On this page
1. Choose an appropriate basis
A Corporation Tax accounting period cannot exceed 12 months. An 18-month period of account commonly therefore needs two CT600s. Section 52 provides for time apportionment when accounts do not coincide with the tax periods. HMRC CTM01405 describes a transactions basis where it produces a more accurate result, particularly where a few identifiable transactions form the trade.
The manual refers to Marshall Hus v Bolton and gives property development as an example of a trade with uneven profit. Actual figures are not a general option to move profit into the return with a lower tax bill. Consider the transactions and the appropriate tax treatment before choosing your figures.
A fictional example
Oak Example Ltd has accounts from 1 April 2024 to 30 September 2025. Its records attribute £20,000 profit to the first 12 months and £80,000 to the final six months. A daily split of £100,000 would give a different result. If a transactions basis is appropriate and more accurate, the prepared figures for those two periods can be entered separately. Tax adjustments may change both figures.
2. Avoid common mistakes
- Do not assume every uneven expense justifies a transactions basis.
- Do not enter a period-specific receipt again through a whole-period allocation panel.
- The rate calculation within each CT600 still divides by days where the return crosses financial years.
- Keep the statutory accounts P&L for the full accounts period.
The default entry route divides recurring amounts by days and uses customer allocations for the specified claims and events. For supported separate entry, follow Enter separate Companies House and CT600 figures. You choose the appropriate basis and figures. The service does not verify that choice against your records.