When does a Corporation Tax accounting period end?

Last updated: 8 October 2026

Short version. A Corporation Tax accounting period normally ends at the earliest relevant event in section 10 of the Corporation Tax Act 2009. That includes reaching 12 months, an accounting date and certain changes in trading, tax status or administration. It can end sooner than 12 months. Winding up and overriding provisions have separate rules.

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1. Events that end a period

Section 10(1) uses the first event that occurs. The conditions matter, particularly where a company carries on more than one trade:

12 months pass from the beginning of the accounting period.
The company reaches an accounting date, or the end of a period for which it does not make up accounts.
The company starts trading or stops trading altogether.
If it carries on only one trade, it comes within or leaves the Corporation Tax charge in respect of that trade.
If it carries on more than one trade, it comes within or leaves the charge in respect of all those trades.
It starts or ceases to be UK resident.
It ceases to be within the charge to Corporation Tax.
It enters or ceases to be in administration. Entering administration ends the period immediately before that day. The provision covers administration under Schedule B1 to the Insolvency Act 1986 and corresponding procedures under other legislation.

Section 10 does not apply when section 12 applies to a company being wound up. Section 10 is also subject to any overriding provision. Do not apply the ordinary date sequence without considering those exceptions. This article explains the general rules and does not determine the periods for a particular company.

Under section 9, an accounting period begins when a company comes within the Corporation Tax charge, or immediately after the previous period ends if it remains within the charge. For a UK-resident company otherwise outside the charge, starting to carry on business brings it within the charge. A chargeable gain or allowable loss arising outside an accounting period can also start one. Check the applicable conditions when establishing your dates.

2. A shorter first period

The 12-month limit is a maximum, not a minimum. Another relevant event can happen first. For example, a company already within the Corporation Tax charge can begin trading before the 12-month anniversary and end its current accounting period then. Incorporation, business activity and the start of a trade are not automatically the same date.

HMRC says the first 12 months of a longer set of accounts will normally be an accounting period. “Normally” allows for an earlier event under the legislation. Check both periods in your HMRC business tax account or CT603 notice. If those records look wrong, contact HMRC to resolve them.

3. Two returns and one set of accounts

One statutory accounts period can contain two Corporation Tax accounting periods. Each CT600 reports one of those tax periods, while the accounts cover the full statutory period.

For a fictional example, accounts run from 1 August 2024 to 30 September 2025. HMRC's two periods run to 31 March 2025 and from 1 April to 30 September 2025. They contain 243 and 183 days. Neither exceeds 12 months.

On a time basis, £42,600 of recurring income is shared as £24,300 and £18,300. Amounts shown on an allocation panel follow the allocation you confirm. A £1,000 capital allowance assigned to the second return stays £0 and £1,000 rather than following those day shares. HMRC also describes circumstances where a transactions basis gives a more accurate apportionment. The supported software workflow uses days for recurring figures and the existing allocation controls for capital and event amounts. A one-off box 205 receipt can appear in both returns. See other income in CT600 box 205.

4. Common date and figure mistakes

Copying the full accounts dates onto one CT600: a return cannot cover more than 12 months. Establish the individual tax periods.
Starting the second return a day late: where the company remains within the charge, it starts the day after the first period ends.
Splitting figures twice: if the software divides recurring figures by days, enter the specified full-period amount once. Review both computations and complete the separate capital and event allocations.
Changing the accounts dates to fit HMRC: keep the statutory accounts dates and Corporation Tax coverage separate where they differ.

For the supported filing steps, see how to match both CT600 dates to HMRC. SimpleCompanyTax offers First CT600 ends for supported long periods and uses the dates and allocations you enter.