Key facts
- 9 months after period end — accounts must be filed at Companies House (private companies).
- 9 months and 1 day after period end — Corporation Tax must be paid.
- 12 months after period end — CT600 return must be filed with HMRC.
- 12 months from the filing deadline — the window to amend the CT600 closes.
- Large companies must pay by quarterly instalments starting in month 7 (or month 3 for very large companies).
Deadline Timeline
The key dates for a standard 12-month accounting period are calculated relative to the accounting period end date. Here is the full timeline:[1][2][3]
| Deadline | When | What |
|---|---|---|
| End of accounting period | Day 0 | The last day of the period covered by the accounts |
| Companies House accounts | 9 months after period end | File statutory accounts (private companies; 6 months for public companies) |
| Corporation Tax payment | 9 months and 1 day after period end | Pay the CT liability to HMRC |
| CT600 filing | 12 months after period end | File the Company Tax Return with HMRC |
| CT600 amendment window | 12 months from filing deadline | Last date to amend the return (i.e. 24 months after period end) |
| HMRC enquiry window | 12 months from actual filing date | HMRC can open an enquiry into the return |
| Claim time limit | 2 years after period end (typically) | Deadline for most loss carry-back and relief claims |
Worked Examples for Common Year-End Dates
The tables below show the exact dates for the most popular year-end choices:[1][2]
Year End: 31 March 2026
| Event | Deadline |
|---|---|
| Companies House accounts | 31 December 2026 |
| CT payment | 1 January 2027 |
| CT600 filing | 31 March 2027 |
| CT600 amendment | 31 March 2028 |
Year End: 31 December 2025
| Event | Deadline |
|---|---|
| Companies House accounts | 30 September 2026 |
| CT payment | 1 October 2026 |
| CT600 filing | 31 December 2026 |
| CT600 amendment | 31 December 2027 |
Year End: 30 June 2026
| Event | Deadline |
|---|---|
| Companies House accounts | 31 March 2027 |
| CT payment | 1 April 2027 |
| CT600 filing | 30 June 2027 |
| CT600 amendment | 30 June 2028 |
Year End: 30 September 2025
| Event | Deadline |
|---|---|
| Companies House accounts | 30 June 2026 |
| CT payment | 1 July 2026 |
| CT600 filing | 30 September 2026 |
| CT600 amendment | 30 September 2027 |
Aligning with the financial year: Many companies choose 31 March as their year end so the accounting period aligns with the government’s financial year (1 April – 31 March). This avoids straddling two different Corporation Tax rate periods. However, any year-end date is permissible.[1]
Quarterly Instalment Dates (Large Companies)
Companies with profits above £1.5 million (divided by associated companies) pay by quarterly instalments rather than in a single payment:[4]
| Company Size | Instalment 1 | Instalment 2 | Instalment 3 | Instalment 4 |
|---|---|---|---|---|
| Large (£1.5m–£20m) | 14th of month 7 | 14th of month 10 | 14th of month 13 | 14th of month 16 |
| Very large (over £20m) | 14th of month 3 | 14th of month 6 | 14th of month 9 | 14th of month 12 |
For a large company with a year end of 31 March 2026, the instalments fall on: 14 October 2025, 14 January 2026, 14 April 2026, and 14 July 2026.
Tip: Set up calendar reminders for each deadline at the start of the accounting period. Missing even one date can trigger interest charges or penalties. Software like GoFile can help track and remind you of upcoming obligations.
First Accounting Period After Incorporation
A newly-incorporated company’s first accounting period starts on the date of incorporation and can last up to 18 months for Companies House purposes. However, for Corporation Tax, the maximum accounting period is 12 months. HMRC will split a longer period into two.[1]
Each period has its own set of deadlines (payment, filing, etc.), so a company with an 18-month first period will have two payment deadlines.
Frequently Asked Questions
When is Corporation Tax due after the accounting period ends?
Corporation Tax must be paid within 9 months and 1 day after the end of the accounting period. For a 31 March 2026 year end, the payment deadline is 1 January 2027.
How long do I have to file a CT600?
The CT600 must be filed with HMRC within 12 months of the end of the accounting period. This is 3 months after the payment deadline.
When do quarterly instalment payments start for large companies?
Large companies (profits over £1.5 million) must pay by quarterly instalments starting in month 7 of the accounting period. Very large companies (profits over £20 million) start paying in month 3.
How long do I have to amend a CT600?
You can amend a CT600 within 12 months of the filing deadline, giving you up to 24 months from the end of the accounting period to submit a revised return.
Further Reading
- Payment Deadlines — the 9-months-and-1-day rule in detail
- Filing Deadlines — when the CT600 must be submitted
- Quarterly Instalment Payments — the instalment regime for large companies
- Interest & Penalties — consequences of missing deadlines
- Accounting Periods — how accounting periods work for Corporation Tax
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