Corporation Tax Deadlines 2026: When You Need to File and Pay

Corporation Tax Deadlines 2026: When You Need to File and Pay

Missing a corporation tax deadline means penalties — and they start from day one. Whether you're filing your CT600 for the first time or you've been running your company for years, knowing your exact deadlines is essential.

Here's everything you need to know about corporation tax deadlines in 2026.

The Two Key Deadlines

Corporation tax has two separate deadlines — and they're different:

1. Payment Deadline: 9 Months and 1 Day After Your Accounting Period Ends

You must pay your corporation tax bill within 9 months and 1 day of the end of your company's accounting period.

2. Filing Deadline: 12 Months After Your Accounting Period Ends

You must file your CT600 (company tax return) within 12 months of the end of your company's accounting period.

Important: The payment deadline comes first. You need to pay your tax approximately three months before you're required to file the return. Many directors don't realise this and assume they can wait until the filing deadline to pay — that's a costly mistake. Our comprehensive CT600 deadline guide explains both deadlines in detail, including Companies House filing.

Key Dates for Common Accounting Periods in 2026

Most companies have an accounting period that ends on 31 March or 31 December. Here are the deadlines:

If Your Accounting Period Ended 31 March 2025

If Your Accounting Period Ended 30 June 2025

If Your Accounting Period Ended 30 September 2025

If Your Accounting Period Ended 31 December 2025

If Your Accounting Period Ended 31 March 2026

How to Calculate Your Specific Deadline

Your deadlines are based on your accounting period end date, not the calendar year. To find yours:

1. Check your accounting period end date — this is on your Companies House record and any previous CT600. For most companies, it matches their annual accounts date.
2. Payment deadline: Add 9 months and 1 day to that date.
3. Filing deadline: Add 12 months to that date.

For example, if your accounting period ended on 15 August 2025:

• Payment due: 16 May 2026 (9 months + 1 day)
• CT600 filing due: 15 August 2026 (12 months)

Penalties for Late Filing

HMRC's penalty regime for late CT600 filing is automatic — there's no grace period and no warning:

These penalties apply even if you don't owe any tax. A dormant company that files its nil CT600 one day late still gets a £100 penalty.

If you file late three times in a row, the initial £100 penalties increase to £500 each.

Penalties and Interest for Late Payment

Paying your corporation tax late triggers:

• Interest charges from the day after the deadline, calculated at the current HMRC rate (currently 7.25% per annum — check the latest rate on GOV.UK)
• No fixed penalties for late payment (unlike late filing), but the interest accumulates daily and can become significant on larger tax bills

If you can't pay on time, contact HMRC to arrange a Time to Pay arrangement before the deadline. HMRC is generally more accommodating if you reach out proactively.

First-Year Companies: Special Rules

If your company was recently incorporated, your first accounting period may be longer or shorter than 12 months. Key points:

• Your first accounting period typically runs from incorporation to your chosen accounting reference date
• It can be up to 18 months for your first period
• If it's longer than 12 months, you may need to file two CT600s (one for the first 12 months, one for the remainder)
• Payment and filing deadlines are calculated from the end of each accounting period

Example: Company incorporated 1 June 2025 with a 31 March accounting reference date. First accounting period: 1 June 2025 to 31 March 2026 (10 months). CT600 filing deadline: 31 March 2027.

Companies House Deadlines (Don't Confuse Them)

Your annual accounts filing with Companies House has a separate deadline:

• Private companies: 9 months after accounting period end
• Public companies: 6 months after accounting period end

This is different from your CT600 deadline. Many directors confuse the two. You typically need to file your Companies House accounts before your CT600 is due.

How to Never Miss a Deadline

1. Set calendar reminders — at least one month before each deadline
2. File early — there's no advantage to waiting. File as soon as your figures are ready.
3. Pay first, file second — remember that payment is due 3 months before the filing deadline
4. Use software that tracks deadlines — tools like Taxpipe show your filing deadline clearly

What If You've Already Missed a Deadline?

If you've missed a deadline, file and pay as soon as possible. Penalties escalate the longer you wait:

• Missed by days: File immediately to limit the penalty to £100
• Missed by months: Every additional threshold (3 months, 6 months, 12 months) triggers bigger penalties
• Can't afford the penalty: You may be able to appeal the penalty if you have a reasonable excuse

The single best thing you can do is file now. Don't wait until you have a "perfect" return — file what you have and amend later if needed. A filed return stops the penalty clock.

Don't risk late filing penalties. File your CT600 with Taxpipe for £59 → — our guided process gets your return filed accurately and on time. No subscription, no accounting jargon.

Related Articles

• CT600 Penalties: What Happens If You File Late
• When Is Your CT600 Deadline?
• CT600 Common Mistakes and How to Avoid Them
• Corporation Tax Year-End Planning: 12 Tips
• Corporation Tax Deadlines Calendar 2026