Corporation Tax Deadlines UK: Every Date You Need to Know

Missing a corporation tax deadline costs money. HMRC and Companies House both impose penalties. This guide lists every deadline, in order, so you know exactly what's due and when.

The Three Key Deadlines

After your company's accounting period ends, you have three deadlines to meet:

Visual Timeline

For a company with an accounting period ending 31 March 2026:

31 Mar 2026    Accounting period ends
     |
     |--- 9 months --->  31 Dec 2026    Accounts due (Companies House)
     |--- 9 months + 1 day --->  1 Jan 2027    Tax payment due (HMRC)
     |--- 12 months --->  31 Mar 2027    CT600 return due (HMRC)

Deadline 1: Annual Accounts at Companies House

Due: 9 months after your accounting period end

Your statutory accounts must be filed at Companies House. For micro-entities, this means an abbreviated balance sheet (no profit and loss account required).

For a first-year company, the deadline is 21 months from the date of incorporation or 9 months from the accounting reference date — whichever is earlier.

Penalty for being late:

These penalties double if you were late the previous year.

Deadline 2: Corporation Tax Payment

Due: 9 months and 1 day after your accounting period end

This is when the actual tax must be paid to HMRC. You need to know your tax liability before this date, even though your CT600 return isn't due for another 3 months.

How to pay: HMRC accepts payment via:

• Direct bank transfer (Faster Payments, BACS, or CHAPS)
• Direct debit
• Corporate credit/debit card
• Cheque by post (allow extra time)

Penalty for late payment: HMRC charges interest on late payments from the day after the deadline. There's no separate penalty for late payment (unlike late filing), but the interest adds up quickly.

Large companies: If your taxable profits exceed £1.5 million, you must pay in quarterly instalments rather than a single payment. This threshold is divided by the number of associated companies.

Deadline 3: CT600 Tax Return

Due: 12 months after your accounting period end

Your CT600 return tells HMRC:

• Your company's taxable profit
• The corporation tax calculation
• Any reliefs or deductions claimed

The CT600 must be filed electronically in XML format, along with your statutory accounts in iXBRL format.

Penalty for being late:

These flat rates apply where the filing date is on or after 1 April 2026. Older filing dates can have different rates. HMRC can also charge tax-related penalties when a return remains outstanding for much longer.

Other Important Dates

Company Tax Registration

When you start trading (or have any taxable income), you must register for corporation tax with HMRC within 3 months. HMRC then assigns your accounting period and sends reminders.

Confirmation Statement (Companies House)

Due at least once every 12 months from the date of incorporation or the last confirmation statement date. It is separate from the accounts and CT600. Check Companies House for the current filing fee.

Payroll (RTI) Deadlines

If you pay yourself a salary, you must report it to HMRC through Real Time Information (RTI) on or before each payday.

VAT Returns

If your company is VAT-registered (mandatory if turnover exceeds £90,000), VAT returns are due quarterly — typically 1 month and 7 days after the end of each VAT quarter.

First-Year Companies: Special Rules

If your company was recently incorporated, your first accounting period has special deadline rules:

• Your first accounts at Companies House are due 21 months from incorporation or 9 months from the accounting reference date — whichever comes first
• Your first CT600 is due 12 months from the end of your first accounting period
• If your first accounting period is longer than 12 months, you may need to file two CT600 returns

Example: Company incorporated 1 July 2025, accounting reference date 31 March 2026:

• First accounts period: 1 July 2025 to 31 March 2026 (9 months)
• Companies House deadline: 31 December 2026 (9 months after period end)
• CT600 deadline: 31 March 2027 (12 months after period end)

Dormant Companies Still Have Deadlines

Even if your company is dormant (no trading, no income), you still need to:

• File dormant accounts at Companies House (same 9-month deadline)
• File a CT600 return at HMRC confirming dormant status
• File an annual confirmation statement at Companies House

The deadlines are identical to active companies. Not trading doesn't exempt you from filing.

Tips to Never Miss a Deadline

1. Set calendar reminders at 6 months, 3 months, and 1 month before each deadline
2. Prepare early so you do not wait until the month before your deadline to start
3. File and pay separately because you can file your CT600 before you pay the tax
4. Use software to prepare your CT600 and accounts in a single session
5. Check your dates because your accounting reference date determines all deadlines

Summary

The three deadlines to remember are accounts at 9 months, tax payment at 9 months and 1 day, and the CT600 at 12 months, all measured from the end of your accounting period. Missing any of them can lead to penalties.