Does an accounts extension extend the CT600 deadline

Q: How might it work in practice?

Illustration only. Kestrel Data Ltd is a private company with a 30 June year end. For the year to 30 June 2025 its accounts are due at Companies House by 31 March 2026, its Corporation Tax is payable by 1 April 2026 and its CT600 is due by 30 June 2026. In March 2026 the finance director falls seriously ill. Before 31 March the company applies to Companies House and is given until 30 June 2026, the most that section 442(5A) allows. The tax is still due on 1 April 2026. The company pays its best estimate of £18,000 on that date, so interest only runs on any shortfall. The CT600 is still due on 30 June 2026. Paragraph 19 gives nothing, because the registrar's new date is the same day. In early June it becomes clear the accounts will slip again. Before 30 June the company writes to HMRC asking for further time under section 118(2), explaining the illness. If it had simply filed on 15 August 2026 without agreement, it would face a £200 flat rate penalty, open to appeal on reasonable excuse grounds.

No. An extension from Companies House gives the directors more time to deliver accounts to the registrar, but the Company Tax Return is still due 12 months after the end of the accounting period and the Corporation Tax is still payable on the normal date. The only tax concession, in paragraph 19 of Schedule 18 to the Finance Act 1998, removes the flat rate late filing penalty in a narrow case that, for most UK private companies, cannot now arise.

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Does a Companies House extension change the CT600 filing date?

No. The two deadlines come from different laws. Section 442 of the Companies Act 2006 sets the period for filing accounts: nine months after the end of the accounting reference period for a private company and six months for a public company. Section 442(5) lets the Secretary of State extend that period for a special reason, on an application made before it expires.

The filing date for the Company Tax Return comes from paragraph 14 of Schedule 18 to the Finance Act 1998. HMRC’s Company Taxation Manual at CTM93030 explains that it is the last day of whichever of these periods ends last:

12 months from the end of the period the return covers;
3 months from the date the notice to deliver the return was served;
12 months from the end of the relevant period of account, if that period is not longer than 18 months, or 30 months from its start if it is longer.

Nothing in that list refers to Companies House. gov.uk says the return is due 12 months after the end of the accounting period, and there is a separate deadline to pay, usually 9 months and one day after the end of the period. See the Corporation Tax deadlines.

What does paragraph 19 actually do?

Paragraph 19 says a company is not liable to the flat rate penalty under paragraph 17 if the return period is one for which it must deliver accounts under the Companies Act 2006, and it delivers the return no later than the last day for delivering those accounts to the registrar. HMRC’s guidance at CTM94100 explains the limits:

It only helps where the registrar’s date is later than the CT600 filing date.
It is an excuse, not a new filing date. If the return arrives after the registrar’s date, the penalty is worked out as if the company had simply missed the normal filing date.
It does not touch tax related penalties under paragraph 18. HMRC does say that where an extension runs past the date those penalties arise, the reason for the extension is likely to amount to a reasonable excuse.

CTM94100 also notes that paragraph 19 strictly requires the return period and the period of account to be the same, but that in practice HMRC applies it to an accounting period that forms part of a longer period of account.

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Why does paragraph 19 rarely help a UK private company?

Because of a cap added to the Companies Act by regulations made in 2015. Section 442(5A) says an extension must not take the period for filing beyond 12 months after the end of the relevant accounting reference period. For an ordinary 12 month period, the CT600 is due 12 months after the end of the accounting period. The most generous extension Companies House can give therefore runs out on, or about, the day the tax return is due anyway, and paragraph 19 adds nothing.

Treat the worked examples at CTM94120 with care. They show a registrar extension to 13 months after the period end and another running into the following spring, which section 442(5A) no longer allows for a UK company’s own accounts. CTM94100 notes that a company with business or interests outside the UK can have a longer period under the Overseas Companies Regulations 2009, so those cases need checking on their own facts.

Plan as if the extension did not exist for HMRC purposes. Our guide to getting an extension to file accounts covers the Companies House side.

What happens to the tax bill and the later penalties?

Nothing changes. Section 59D of the Taxes Management Act 1970 makes Corporation Tax due on the day after nine months from the end of the accounting period, and an accounts extension does not move it. Unpaid tax carries late payment interest from that date.

If the return itself is late, the penalties are:

When the return is delivered	Penalty
Within 3 months after the filing date	£200 flat rate (paragraph 17)
More than 3 months after the filing date	£400 flat rate (paragraph 17)
Third successive late return	£1,000 or £2,000 instead
Not within 18 months after the period end (or by the filing date if later)	Add 10% of the tax unpaid, or 20% if not delivered within two years of the period end (paragraph 18)

The flat rate amounts were doubled by section 265 of the Finance Act 2026 for returns with a filing date on or after 1 April 2026. CTM94060 explains that the tax geared penalty is based on the tax still unpaid when that penalty first arises, so paying a sensible estimate on time reduces it. gov.uk also warns that if a return is 6 months late HMRC will issue a tax determination, which cannot be appealed; the company has to pay and file. More detail is in Corporation Tax late filing penalties.

Can you file the CT600 before the accounts are finished?

Only in a limited way. The accounts are part of the return. CTM93180 explains that paragraph 11 of Schedule 18 requires a UK resident company to deliver the accounts it must prepare under the Companies Act, meaning the full accounts prepared for members, not filleted accounts filed at Companies House.

Where final figures genuinely cannot be produced despite the company’s best efforts, CTM93280 says the CT600 asks for details of estimated entries. The company must explain why it used an estimate and when it expects to replace it, and should replace it with an accurate figure as soon as possible. An estimate that turns out not to be a reasonable best estimate can lead to an inaccuracy penalty under Schedule 24 to the Finance Act 2007. Words such as “information to follow” instead of a figure do not meet the filing obligation at all.

CTM93290 reproduces HMRC’s long standing position, agreed with the ICAEW Tax Faculty: a return completed with reasonable, disclosed estimates and filed on time is treated as filed on time, but a return that is clearly incomplete because entries are missing attracts late filing penalties. Once the final figures are ready, amend the CT600.

Can HMRC give the company more time?

Yes, but it is HMRC’s decision, not Companies House’s. CTM93060 explains that although the filing dates are fixed by statute, section 118(2) of the Taxes Management Act 1970 allows HMRC to give further time for something to be done or to accept a reasonable excuse. It works on an all or nothing basis: it either removes the penalty or does not affect it at all.

CTM94130 says a company that does not expect to meet its filing date may ask for further time, but it must ask before the filing date. HMRC applies its reasonable excuse criteria and, if it agrees, allows time up to the date it expects the excuse to end. CTM93290 adds that a company should not assume beforehand that further time will be granted.

What if a penalty has already been charged?

gov.uk says you can appeal against a late filing penalty online if you have a reasonable excuse, but you must file the Company Tax Return before appealing. You will need the company’s Unique Taxpayer Reference, the date and amount on the penalty notice, the end date of the accounting period and an explanation of why the return was late. The facts behind a Companies House extension may support the appeal, but HMRC applies its own reasonable excuse test.

How might it work in practice?

Illustration only. Kestrel Data Ltd is a private company with a 30 June year end. For the year to 30 June 2025 its accounts are due at Companies House by 31 March 2026, its Corporation Tax is payable by 1 April 2026 and its CT600 is due by 30 June 2026.

In March 2026 the finance director falls seriously ill. Before 31 March the company applies to Companies House and is given until 30 June 2026, the most that section 442(5A) allows.
The tax is still due on 1 April 2026. The company pays its best estimate of £18,000 on that date, so interest only runs on any shortfall.
The CT600 is still due on 30 June 2026. Paragraph 19 gives nothing, because the registrar’s new date is the same day.
In early June it becomes clear the accounts will slip again. Before 30 June the company writes to HMRC asking for further time under section 118(2), explaining the illness.
If it had simply filed on 15 August 2026 without agreement, it would face a £200 flat rate penalty, open to appeal on reasonable excuse grounds.

What mistakes should you avoid?

Treating the Companies House extension letter as if it covered HMRC.
Delaying the tax payment until the accounts are finished.
Filing a CT600 with blank boxes or “to follow” notes and assuming that stops the clock.
Asking HMRC for more time after the filing date has already passed.
Delivering filleted accounts to HMRC, when the return needs the full accounts prepared for members.

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Sources

Reviewed by Christine Anderson, Broker, Solar Protect. Last reviewed 24 October 2026. Part of Accountapedia, the Solar Protect guide to UK tax and accounting. This page is general information, not personalised advice.