Corporation tax deadlines

Your dates depend on your year end

The first thing to understand is that corporation tax has nothing to do with 5 April. Every company has its own accounting period, and every deadline is measured from the end of that period.

When a company is incorporated, Companies House sets the accounting reference date as the last day of the month in which the anniversary of incorporation falls. A company formed on 12 June 2025 gets a first year end of 30 June 2026. That date then repeats every year unless the company changes it.

So two identical businesses can have entirely different deadlines. One with a 31 December year end pays by 1 October. One with a 31 March year end pays by 1 January. Neither is more correct than the other, and if you run more than one company you may well be juggling several sets of dates.

Payment comes before filing

This is the part that surprises directors coming from Self Assessment, where you file and pay on the same day. With corporation tax the payment deadline falls three months before the filing deadline.

The payment is due 9 months and 1 day after the period end. The return, form CT600, is due 12 months after the period end. Take a company with a 31 March 2026 year end. Corporation tax must reach HMRC by 1 January 2027. The return does not have to be filed until 31 March 2027.

In practice this ordering is theoretical, because you cannot know what to pay without preparing the accounts and the tax computation. So the real working deadline is the payment date, and the filing date is simply the outer limit. Any sensible timetable has the accounts finished well before the nine month mark.

The odd extra day exists for historical reasons and is genuinely part of the rule. A 31 March year end gives 1 January, not 31 December. It is a single day of grace, but it is real.

A worked example

Northgate Design Limited has a year end of 30 September 2026. Its accounting period ran from 1 October 2025 to 30 September 2026.

Corporation tax is due 9 months and 1 day after 30 September 2026, which is 1 July 2027. The CT600 is due 12 months after, which is 30 September 2027. The accounts also need filing at Companies House within 9 months of the year end, so by 30 June 2027, one day before the tax payment.

Notice how those two Companies House and HMRC deadlines sit almost on top of each other. That is deliberate on the part of the authorities and useful in practice, because the same set of accounts feeds both. It means the sensible internal target is to have everything finished by, say, the end of April, giving two clear months of slack.

Long and short accounting periods

An accounting period for corporation tax cannot exceed 12 months, but a company's period of account can. This trips up almost every new company, because a first period usually runs longer than a year.

Suppose a company is incorporated on 12 June 2025 and its first accounts run to 30 June 2026. That is 12 months and 19 days. For corporation tax that single set of accounts is split into two accounting periods, the first covering 12 June 2025 to 11 June 2026 and the second covering 12 June 2026 to 30 June 2026. Two CT600 returns are required, each with its own payment date calculated from its own end date.

Short periods happen too, usually when a company changes its year end or ceases trading. The same rule applies, with the deadline measured from whenever the period actually ended.

Quarterly instalments for larger companies

Companies with taxable profits above £1.5 million do not get the nine month window. They pay in four quarterly instalments, and the first falls due in month 7 of the accounting period, before the year has even finished.

For a company with a 31 December year end, instalments fall due on 14 July, 14 October, 14 January and 14 April. That means estimating profits accurately part way through the year, and revising the estimate as the year develops.

Very large companies, those with profits above £20 million, pay even earlier, with instalments in months 3, 6, 9 and 12.

The critical point for smaller groups is that the £1.5 million threshold is divided by the number of associated companies. A person controlling five companies divides the threshold by five, so each company enters the instalment regime at £300,000 of profit rather than £1.5 million. Groups that have grown organically often cross into instalments without realising, and the first they know of it is an interest charge.

Late payment interest

Interest runs automatically from the day after the payment was due until the day it is paid. It is not a penalty and there is no appeal against it, it is simply the cost of holding money that belonged to HMRC.

Interest is charged at a rate set by reference to the Bank of England base rate and is reviewed regularly, so the exact figure changes. Interest paid on late corporation tax is deductible for corporation tax purposes, which softens the blow slightly, and interest received where you have overpaid is taxable.

Where a company pays early, HMRC pays credit interest from the date of payment until the normal due date. The rate is lower than the late payment rate, but it is better than nothing if you happen to have the cash sitting idle.

Late filing penalties

Filing the CT600 late carries its own penalties, entirely separate from any interest on late payment.

One day late brings £100. Three months late brings a further £100. At six months HMRC estimates your tax and adds 10% of the unpaid amount. At twelve months another 10% is added. If a return is late three times in a row, the initial £100 penalties increase to £500 each.

The 10% charges are the ones that hurt, because they are proportionate to the tax rather than fixed. On a company with a £40,000 corporation tax bill, being twelve months late costs £8,000 in penalties on top of the tax and the interest.

Telling HMRC you have nothing to pay

A dormant company with no corporation tax liability still has obligations. If HMRC has issued a notice to deliver a return, that return must be filed even where the tax is nil, and the late filing penalties apply in exactly the same way.

Where a company genuinely is dormant, you can tell HMRC and it will usually stop issuing notices for a period. That is worth doing, because otherwise a company sitting idle for a few years quietly accumulates penalties that nobody notices until it is reactivated or struck off.

If you have a live company with no tax to pay in a particular year, perhaps because it made a loss, you should still tell HMRC that no payment is coming. Otherwise the system expects money and may issue reminders and determinations.

How to pay and how long it takes

Corporation tax is paid using a 17 character payment reference that is specific to the accounting period. This is important. The reference changes every year, and using last year's reference sends the money to the wrong period, which means the current year sits unpaid while interest accrues on a bill you thought you had settled.

Faster Payments usually arrive the same day or the next day. Bacs takes three working days. Direct Debit takes five working days the first time you set one up, and three thereafter. If your deadline is 1 January, starting a Bacs payment on 31 December is late.

Building a timetable that works

The pattern that works for most owner managed companies is simple. Close the books within one month of the year end. Have draft accounts within three months. Approve them and know the tax figure by month five. That leaves four months to arrange the money before the payment deadline, and it means the Companies House filing is comfortably early too.

The alternative, which is extremely common, is that nothing happens until month eight, the accountant is chasing missing records in month nine, and the tax figure arrives days before the money is due. That is how companies end up borrowing to pay a tax bill they earned nine months earlier.

Set money aside as you go. A reasonable rule of thumb for a profitable small company is to move a fixed percentage of profit into a separate account each month. When the bill arrives it is a transfer rather than a crisis.

Changing your year end

Companies are allowed to change their accounting reference date, and there are good reasons to do so. Aligning the year end with a natural quiet period makes stocktaking and record gathering easier. Aligning it with 31 March keeps the company year roughly in step with the personal tax year, which simplifies planning around salary and dividends.

You can shorten a period as often as you like, but you can normally only extend it once every five years, and never beyond 18 months. Changing the date changes every downstream deadline, so it needs care. Shortening a period brings the payment date forward, which can leave very little time if the change is made late in the year.

The change is made at Companies House, and HMRC picks it up from there, though it is worth checking that HMRC records have actually updated rather than assuming.

Losses and the deadlines that follow them

A company that makes a loss has extra options, and some of them have their own time limits. A trading loss can be set against other profits of the same period, carried back against the previous 12 months, or carried forward against future profits.

A carry back claim usually has to be made within two years of the end of the loss making period. Where a claim generates a repayment of corporation tax already paid, HMRC will normally repay it once the return carrying the claim is processed, so filing early after a loss making year is worth real money in cash flow terms rather than waiting for the deadline.

Group relief, which allows a loss in one company to be surrendered against profits in another, generally has to be claimed within the normal amendment window, which is 12 months after the filing date. Missing that window means the loss stays stranded in the wrong company.

Amending a return after filing

You can amend a company tax return within 12 months of the filing deadline, which for a 31 March 2026 year end means until 31 March 2028. That is a reasonably generous window and it is the route for correcting an error or making a claim that was overlooked.

HMRC in turn has 12 months from the date you actually filed to open an enquiry, so filing early shortens the period during which the return can be questioned. Beyond that, HMRC can still raise a discovery assessment within 4 years, extending to 6 years for careless behaviour and 20 years for deliberate behaviour.

Frequently asked questions