The usual deadlines
- Pay 9 months and 1 day after the end of the accounting period. When the period ends on the last day of a month, that’s the first day of the tenth month: for a year to 31 March 2027, pay by 1 January 2028.
- File the company tax return (CT600) 12 months after the end of the period: for a year to 31 March 2027, by 31 March 2028.
The payment deadline comes first, so work out the tax before you finish the return. Interest runs on tax paid late.
Accounts longer than 12 months
An accounting period for corporation tax can never be longer than 12 months (CTM01800). If the company’s accounts cover, say, 15 months, it has two accounting periods for tax: the first 12 months, then the remaining 3. Each has its own tax, worked out separately, its own payment date 9 months and 1 day after it ends, and its own return. Both returns are due 12 months after the end of the accounts.
Profit is usually split between the two periods by days, unless some of it clearly belongs to one period, such as a gain on selling an asset. The second, shorter period has its limits cut for the number of days.
Large companies: quarterly instalments
A company is large if its augmented profits for the period are over £1.5 million (CTM92520). The £1.5 million is divided by one plus the number of associated companies, and cut in proportion for a period shorter than 12 months.
A large company pays its tax in four instalments, based on its estimate of the year’s tax. For a 12-month period they fall 6 months and 13 days after the period starts, then every 3 months. For a year starting 1 April 2026, that’s 14 October 2026, 14 January 2027, 14 April 2027 and 14 July 2027.
Very large companies
A company with augmented profits over £20 million is very large. It pays even earlier: 2 months and 13 days after the period starts, then every 3 months, so all four instalments fall within the period itself. The £20 million is shared between associated companies and cut for a short period in the same way.
When instalments don’t apply
A company that would be large doesn’t pay by instalments in two cases (CTM92530):
- The tax is £10,000 or less. For a period of less than 12 months the £10,000 is reduced in proportion. It isn’t shared with associated companies.
- It’s the first large year, as long as the company wasn’t large in the 12 months before the period and its profits are £10 million or less. The £10 million is cut for a short period and divided by the number of associated companies, counting the company itself.
So a growing company usually gets one year’s grace. If it is large again the next year, instalments start then. If your period is shorter than 12 months and you are large, the number of instalments depends on its length; GOV.UK’s instalments guidance explains how.