Accounting periods for Corporation Tax explained

Your accounting period for Corporation Tax is the time covered by your Company Tax Return. It’s normally the same as the financial year in your accounts, but it can never be longer than 12 months. When your accounts cover more than that, you file two returns.

Last checked against GOV.UK on 1 October 2026.

A4O Accounts prepares the CT600 from your books and files it with HMRC. See the CT600 software, or get started free.

Two periods with similar names

Most years the two are the same 12 months, and you never notice the difference. It matters when the accounts run long.

How you find out your accounting period

GOV.UK says you’ll get a letter from HMRC giving the dates of your accounting period after you add Corporation Tax services to your business tax account. The notice to deliver a Company Tax Return also states the period the return should cover.

If you think the dates are wrong, tell HMRC.

When the accounts cover more than 12 months

GOV.UK’s rule: you must file 2 returns to cover the period of your accounts, because your accounting period cannot be longer than 12 months.

So if your accounts cover 18 months, the first accounting period is the first 12 months and the second is the remaining 6. Each has its own CT600 and its own tax computation, and the profit in the accounts is split between them.

This comes up most often in a company’s first year. Companies House sets a new company’s first year end to the last day of the month a year after it was set up, so the first accounts usually cover a little more than 12 months. GOV.UK’s example: a company set up and trading from 11 May 2024, with a year end of 31 May 2025, files one return for 11 May 2024 to 10 May 2025 and another for 11 to 31 May 2025.

Deadlines when there are two periods

Each accounting period has its own payment deadline, 9 months and 1 day after that period ends. In GOV.UK’s example above, the two payments are due on 11 February 2026 and 1 March 2026.

For more on the dates, see Corporation Tax deadlines.

When you lengthen your year end

If you change your company’s year end so the accounts cover more than 12 months, tell HMRC the new dates before the original filing deadline. GOV.UK’s example: your company’s year end is 31 December 2024 and you lengthen it to 31 March 2025. You would need to update your accounting period dates with HMRC before 31 December 2025, and file all returns by 31 March 2026.

If you don’t, HMRC still expects a return for the old dates, and you may get a late filing penalty.

When the accounts cover less than 12 months

If you shorten your year, or the company stops trading part-way through, the accounts cover less than 12 months. GOV.UK says the accounting period normally ends on the same day, so it will also be shorter than 12 months. You file one return for the short period. Update the dates with HMRC before you file.

Starting, stopping and restarting

The accounting period can also differ from the financial year in the year you set up the company, when the company becomes dormant, and when it starts trading again after being dormant. If your company has stopped trading, see dormant company accounts.

Getting it right on the return

The period on the CT600 has to match the period HMRC holds for the company. A return for the wrong dates can be rejected, so check the notice to deliver before you start. See how to file a CT600 and what to send with a CT600.

Accounting periods in software

In A4O Accounts you enter the period start and end when you create a CT600 return, and the return builds its accounts and computation for that period. A return can’t cover more than 12 months, so a longer period of account needs two returns. See CT600 software and preparing and filing a CT600.

More in this series: all the CT600 guides.