CT600 Deadline: Why the Tax Is Due Before the Return

Your corporation tax return is due twelve months after your accounting period ends.

Your corporation tax is due at nine months and one day.

Yes, in that order. You pay before you file.

The short answer

Three deadlines, three different dates, two different regulators. Most of the confusion around the CT600 comes from collapsing them into one.

A worked example. Accounting period ends 31 December 2026:

• 1 October 2027 — corporation tax payment due
• 30 September 2027 — annual accounts due at Companies House
• 31 December 2027 — CT600 due at HMRC

Why the tax comes first

It looks backwards, and there is a reason for it.

HMRC does not need your return to know you owe money. It expects you to calculate the liability yourself and pay it, then formalise the calculation in the return. The return is the record, not the trigger.

The practical consequence is uncomfortable:

• You must compute your tax position three months before you file the document that states it
• If you underpay, interest runs from the due date, not from when you filed
• If you overpay, HMRC holds the money and pays a lower rate of interest back

Treat the 9 months and 1 day as the real deadline. The CT600 at 12 months is administrative. The money date is the one with interest attached to it.

What the accounting period actually is

The deadlines hang off the accounting period, not the tax year and not your incorporation date.

• Usually 12 months, ending on your accounting reference date
• Your first period is often longer than 12 months, because it runs from incorporation
• A period cannot exceed 12 months for corporation tax purposes

This creates the first-year trap. If your first accounting period is 18 months, it is split into two corporation tax periods: the first 12 months, and the remaining 6. That means:

1. Two CT600s, not one
2. Two payment dates
3. Two sets of computations

Founders regularly discover this at the point of filing, having budgeted for one return.

The late filing penalties

The CT600 penalty ladder, in force since 1 April 2026:

Two things to notice:

• The first two are flat. They do not care how much tax you owe. A dormant company that files late still pays £200.
• The last two are proportional. At six months, the penalty stops being an administrative annoyance and starts scaling with your liability.

And separately, interest on late payment. It runs from 9 months and 1 day, independent of the filing penalties. You can file the CT600 perfectly on time and still be accruing interest for three months because you did not pay.

30 Sep 2027Annual accounts due at Companies House, 9 months after period end

1 Oct 2027Corporation tax payable to HMRC, 9 months and 1 day. Interest runs from here

31 Dec 2027CT600 due at HMRC, a full 12 months after period end

1. 30 Sep 2027Annual accounts due at Companies House, 9 months after period end
2. 1 Oct 2027Corporation tax payable to HMRC, 9 months and 1 day. Interest runs from here
3. 31 Dec 2027CT600 due at HMRC, a full 12 months after period end

Two regulators, two meters

This is the single most common misunderstanding, so it is worth stating plainly.

Filing your accounts at Companies House does not file your CT600. They are different documents, going to different bodies, on different dates. Being on time with one says nothing about the other.

Flat, whatever your bill

A dormant company pays these in full.

• 1 day late: £200
• 3 months late: a further £200

Proportional, with no ceiling

The penalty now scales with the liability.

• 6 months late: 10% of the unpaid tax
• 12 months late: a further 10%

Interest on late payment runs separately, from 9 months and 1 day, whether or not the return is filed on time.

Dormant companies still have to deal with this

A dormant company owes no corporation tax. It may still owe a return.

• If HMRC has issued a notice to deliver a company tax return, you must file the CT600 even if the figures are zeros
• Ignoring it produces the flat £200 penalty, then another £200
• The way out is to tell HMRC the company is dormant, and get the notice withdrawn, rather than simply not filing

The penalty for a nil return filed late is the same as for a profitable one, for the first six months. That is the part that surprises people.

How to not get caught

Four things, in the order they matter:

1. Diary the 9 months and 1 day, not the 12. The money deadline is the one with interest.
2. Check whether your first period splits. If it runs beyond 12 months, you have two returns coming.
3. Do not assume accounts and CT600 travel together. Two regulators, two dates, two penalty regimes.
4. If the company is dormant, close the notice. Do not just leave the return unfiled.

If you are working out what the full annual compliance load looks like before committing to a UK company, it is worth seeing the whole calendar and what each filing costs.

The summary