CT600 Deadline: When to File Your Corporation Tax Return (and When to Pay)
There are two deadlines every limited company director needs to know: when to pay corporation tax and when to file the CT600. They're different dates, and confusing them is one of the most common mistakes small company owners make.
Add in the separate Companies House deadline, and it's easy to see why directors get caught out.
Here's a clear breakdown of every deadline, what happens if you miss one, and how to stay on track.
The Two HMRC Deadlines
1. Payment deadline: 9 months and 1 day after your accounting period ends
This is when your corporation tax payment is due. Not the filing — the actual money. For the specific dates in 2026, see our corporation tax deadline 2026 calendar.
Example:
- Accounting period ends: 31 March 2026
- Payment due: 1 January 2027 (9 months + 1 day)
If you don't pay by this date, HMRC charges interest on the outstanding amount from the day after the deadline. The interest rate fluctuates but is currently around 7.25% (it tracks the Bank of England base rate + 2.5%).
2. Filing deadline: 12 months after your accounting period ends
This is when your CT600 return (and statutory accounts in iXBRL format) must be submitted to HMRC.
Example:
- Accounting period ends: 31 March 2026
- Filing due: 31 March 2027 (12 months)
Notice the gap: You must pay corporation tax 3 months before you need to file the return. Many directors assume they file first and pay later — it's actually the other way round.
The Companies House Deadline (Different Again)
Companies House has its own filing requirement: annual accounts must be filed within 9 months of the financial year-end for private limited companies.
Example:
- Financial year ends: 31 March 2026
- Companies House filing due: 31 December 2026 (9 months)
The three deadlines compared
| Obligation | Deadline | Example (31 March year-end) |
|---|---|---|
| Companies House accounts | 9 months after year-end | 31 December 2026 |
| Corporation tax payment | 9 months + 1 day after period-end | 1 January 2027 |
| CT600 filing (HMRC) | 12 months after period-end | 31 March 2027 |
For a 31 March year-end, the Companies House and payment deadlines fall almost on the same day. For other year-ends, they may be further apart.
What "Accounting Period" Means
Your CT600 covers an accounting period for corporation tax purposes. This is usually (but not always) the same as your company's financial year.
Standard: Financial year = Accounting period
Most companies have a 12-month financial year that matches their accounting period. If your year-end is 31 March, your accounting period runs 1 April to 31 March.
Exception: First accounting period
When a new company incorporates, the first accounting period often doesn't align neatly:
- Company incorporated: 15 June 2025
- First year-end: 30 June 2026
- First accounting period: 15 June 2025 to 30 June 2026 (just over 12 months)
Important: An accounting period for corporation tax purposes cannot exceed 12 months. If your first financial year is longer than 12 months, HMRC splits it into two accounting periods, and you'll need to file two CT600 returns.
Example:
- Company incorporated: 15 June 2025
- First year-end: 30 June 2026 (12 months 15 days)
- First CT600: 15 June 2025 to 14 June 2026 (12 months)
- Second CT600: 15 June 2026 to 30 June 2026 (15 days)
- Both CT600s are due by 30 June 2027 (12 months from the financial year-end)
Changing your year-end
You can change your company's year-end at Companies House, which changes your accounting period. Common reasons:
- Aligning with the tax year (31 March)
- Simplifying group accounting
- Tax planning (extending a loss-making period to absorb future profits)
If you change your year-end, your filing and payment deadlines change too. HMRC will issue a new notice to deliver reflecting the updated dates.
What Happens If You Miss the Filing Deadline
HMRC imposes automatic penalties for late CT600 filing:
| Delay | Penalty |
|---|---|
| 1 day late | £100 |
| 3 months late | Another £100 (total: £200) |
| 6 months late | HMRC estimates your tax and adds 10% of the unpaid amount |
| 12 months late | Another 10% of the unpaid amount |
Daily penalties
If your return is more than 6 months late, HMRC can also charge a daily penalty of £10 per day for up to 90 days (maximum £900). These are in addition to the fixed penalties above.
Worst-case scenario
A CT600 filed 13 months late for a company with £10,000 of tax due:
- £100 (1 day late)
- £100 (3 months late)
- £1,000 (10% at 6 months)
- £900 (daily penalties — 90 × £10)
- £1,000 (10% at 12 months)
- Total penalties: £3,100 — on top of the £10,000 tax plus interest
Even if the company has no tax to pay, the fixed penalties (£100 + £100) still apply. This includes dormant companies — filing a nil return on time is always cheaper than the penalty.
What Happens If You Miss the Payment Deadline
Late payment doesn't trigger fixed penalties like late filing does. Instead, HMRC charges interest from the day after the deadline.
Interest rate
The current late payment interest rate is 7.25% (base rate + 2.5%). This is calculated daily on the outstanding amount.
Example:
- Tax owed: £10,000
- Payment 3 months late
- Interest: £10,000 × 7.25% × (90/365) = £178.77
Surcharges (repeat offenders)
If you pay late for two or more consecutive accounting periods, HMRC may impose a surcharge on top of interest. This is rare for first-time offenders.
Key Dates for Common Year-Ends
31 March year-end (most common)
| Deadline | Date |
|---|---|
| Companies House accounts | 31 December |
| Corporation tax payment | 1 January (following year) |
| CT600 filing | 31 March (following year) |
31 December year-end
| Deadline | Date |
|---|---|
| Companies House accounts | 30 September (following year) |
| Corporation tax payment | 1 October (following year) |
| CT600 filing | 31 December (following year) |
30 September year-end
| Deadline | Date |
|---|---|
| Companies House accounts | 30 June (following year) |
| Corporation tax payment | 1 July (following year) |
| CT600 filing | 30 September (following year) |
30 June year-end
| Deadline | Date |
|---|---|
| Companies House accounts | 31 March (following year) |
| Corporation tax payment | 1 April (following year) |
| CT600 filing | 30 June (following year) |
How to Register for Corporation Tax
If your company has just started trading, you must register for Corporation Tax with HMRC within 3 months of commencing business activity. This is separate from company incorporation at Companies House.
Registration process
- Go to HMRC's online registration service
- Enter your Company Registration Number and UTR
- Provide the date you started trading
- Confirm your accounting period end date
- HMRC will set up your Corporation Tax record and send a notice to deliver
What if you don't register?
HMRC may register you automatically (they receive data from Companies House), but don't rely on this. Late registration can lead to estimated assessments and complications.
Quarterly Instalment Payments
Most small companies pay corporation tax in one lump sum. But if your company's annual profits exceed £1.5 million (divided by the number of associated companies + 1), you must pay in quarterly instalments.
Instalment dates
Instalments are due in the 6th, 9th, 12th, and 15th months of the accounting period:
For a 1 April to 31 March accounting period:
- 14 October (month 6.5)
- 14 January (month 9.5)
- 14 April (month 12.5)
- 14 July (month 15.5)
This means the first payment is due before the accounting period even ends. Large companies must estimate their profits to calculate instalments.
Most small companies don't need to worry about this — it only kicks in at £1.5 million of annual profits.
Tips for Staying on Track
1. Set calendar reminders for both deadlines
Payment at 9 months, filing at 12 months. Put them in your calendar the day your accounting period starts.
2. Prepare accounts early
Don't wait until month 11 to start preparing. If you keep your records up to date throughout the year, you can file and pay well before the deadline.
3. Pay early, even before filing
You can pay your corporation tax before filing the CT600. If you know roughly what you owe, pay it by the 9-month deadline even if your return isn't ready yet. This avoids interest.
4. Use your payment reference
Your corporation tax payment reference is your UTR followed by A and the accounting period end date in YYMMDD format.
Example: UTR 1234567890, period ending 31 March 2026 Payment reference: 1234567890A260331
5. File with plenty of margin
Don't leave it until the last day. HMRC's systems can have issues during peak times, and technical problems aren't always accepted as a reasonable excuse for late filing. If your return is rejected by HMRC, you'll need time to fix and resubmit before the deadline.
6. Use filing software
Taxpipe lets you prepare and file your CT600 in one sitting. Enter your figures, review the return, and submit directly to HMRC. You get instant confirmation of submission — no waiting, no postal delays. It costs £59 per filing.
Can You Extend the Deadline?
Filing deadline
Generally, no. HMRC does not grant extensions for CT600 filing. The 12-month deadline is fixed. If you have a reasonable excuse for late filing (serious illness, natural disaster, HMRC systems being down), you can appeal the penalties — but the deadline itself doesn't move.
Payment deadline
Also fixed. However, if you genuinely can't pay, contact HMRC to set up a Time to Pay arrangement. They may agree to let you pay in instalments over several months. Interest still accrues, but you avoid enforcement action.
First Year Filing: Watch Out
New companies face unique deadline complications. Our first-year corporation tax guide covers everything new directors need to know.
Scenario 1: Short first period
- Incorporated: 1 January 2026
- First year-end: 31 March 2026 (3 months)
- Payment due: 1 January 2027
- Filing due: 31 March 2027
A very short accounting period still gets the full 12-month filing window.
Scenario 2: Long first period (split into two)
- Incorporated: 1 July 2025
- First year-end: 31 March 2027 (21 months)
- CT600 #1: 1 July 2025 to 30 June 2026 (12 months) — payment due 1 April 2027, filing due 30 June 2027
- CT600 #2: 1 July 2026 to 31 March 2027 (9 months) — payment due 1 January 2028, filing due 31 March 2028
You must apportion income and expenses between the two periods proportionally.
Frequently Asked Questions
When is my CT600 due?
Your CT600 must be filed 12 months after the end of your accounting period. For example, if your accounting period ends 31 March 2026, the CT600 is due by 31 March 2027.
Is the payment deadline the same as the filing deadline?
No. Corporation tax must be paid within 9 months and 1 day of the accounting period end. The filing deadline is 12 months. You need to pay approximately 3 months before you need to file.
What's the penalty for filing 1 day late?
An automatic £100 penalty, with no warning or grace period. If you file more than 3 months late, it rises to £200, and further penalties apply at 6 and 12 months.
Can I file my CT600 before the deadline?
Yes — and you should. There's no minimum waiting period. You can file your CT600 the day after your accounting period ends if your accounts are ready. Filing early also starts the HMRC enquiry window earlier, meaning certainty sooner.
Do I still need to file with Companies House separately?
Yes. The CT600 is filed with HMRC. You must also file annual accounts with Companies House (different format, different deadline — 9 months for private limited companies). These are two separate obligations to two separate bodies. For more on how to prepare your accounts and file your CT600, see our DIY filing guide.
Don't let deadlines creep up on you. File your CT600 with Taxpipe — guided entry, automatic tax calculation, and direct HMRC submission. Takes about 15 minutes, costs £59.