31 December company year-end: 30 September, 1 October and 31 December deadlines explained

A 31 December year-end creates a deadline cluster that is easy to misread. For many established UK private limited companies, the annual accounts go to Companies House nine months after the financial year-end, Corporation Tax is usually paid nine months and one day after the Corporation Tax accounting period ends, and the Company Tax Return is filed 12 months after that accounting period ends. Those are three separate obligations, even though the same year-end records may feed all three.

That makes 30 September 2026 particularly timely. If your company has a 31 December 2025 financial year-end and these are not its first accounts, the usual Companies House filing date is today, 30 September 2026. If the Corporation Tax accounting period also ended on 31 December 2025, the usual Corporation Tax payment date is tomorrow, 1 October 2026. The Company Tax Return itself is normally not due until 31 December 2026.

The three dates for a typical 31 December year-end

If your accounts are due today, check Companies House first

Companies House says a private company normally has nine months from the end of the accounting reference period to deliver subsequent accounts. Delivery means Companies House actually receives acceptable accounts by the deadline. The deadline is not automatically extended because it falls on a weekend or bank holiday.

Late filing penalties are automatic. For a private company, Companies House currently lists a £150 penalty when accounts are no more than one month late, rising to £375, £750 and £1,500 as the delay lengthens. The penalty can be doubled when accounts are late in two successive financial years. Those penalties relate to the Companies House accounts obligation; the HMRC Company Tax Return has its own separate penalty regime.

If Corporation Tax is due on 1 October, check the payment timing

HMRC says Corporation Tax must reach it by the payment deadline and may attract late-payment interest if it does not. HMRC publishes different processing times for its payment routes, from same-day or next-day options to methods that take several working days. When the deadline is close, check the current GOV.UK guidance before choosing the payment route rather than assuming every method clears immediately.

Use the correct 17-character Corporation Tax payment reference for the accounting period. HMRC says that reference changes for each accounting period, and using the wrong one can delay allocation or cause the payment to be applied to a different tax bill. The reference is available in the company's HMRC online account or on the notice to deliver the tax return.

The Company Tax Return comes later — but the filing route changed in 2026

The Company Tax Return is normally due 12 months after the end of the accounting period. For a period ending 31 December 2025, that points to 31 December 2026. HMRC requires the return even where the company made a loss or has no Corporation Tax to pay, if HMRC has issued a notice to deliver a Company Tax Return.

There is also a process change worth remembering this year. The old joint Companies House and HMRC online filing service closed on 31 March 2026. From 1 April 2026, companies need software to file their Company Tax Return with HMRC. Companies House accounts can still be filed through the filing methods Companies House makes available for the relevant company and accounts type.

When these dates may not apply

• First accounts can have different Companies House deadlines, and a first set of accounts can also create more than one Corporation Tax accounting period.
• A shortened or lengthened accounting period can change the dates and may mean the financial year and Corporation Tax accounting period do not line up exactly.
• A company that has restarted after being dormant can have a Corporation Tax accounting period that begins when business activity restarts.
• Large companies and very large companies can be required to pay Corporation Tax by quarterly instalments rather than the standard 9-month-and-1-day timetable.
• In unusual cases the statutory Company Tax Return filing date can differ from the simple 12-month rule, so check the company's actual HMRC notice and account.

A five-minute year-end deadline check

1. Confirm the financial year-end shown for the company at Companies House and check the accounts filing deadline.
2. Check the Corporation Tax accounting period in the company's HMRC business tax account rather than assuming it is identical to the Companies House period.
3. Confirm the Corporation Tax liability and the 17-character payment reference for the correct accounting period.
4. Check HMRC's current payment processing times when the payment deadline is close.
5. Record the Company Tax Return filing date separately and make sure the company or its accountant has suitable software for the HMRC filing.

The practical bookkeeping lesson is simple: year-end is not one deadline. Keep the accounts close, tax calculation, payment evidence and return filing as separate checklist items. If any of the dates are unclear because the company is new, has changed its year-end, has restarted after dormancy or may fall within the quarterly-instalment rules, check the company's actual HMRC and Companies House records and take professional advice where needed.