Extended Accounting Periods

Why do I have two CT600s? If your company's accounting period is longer than 12 months, you'll need to file two CT600 returns with HMRC. Clear Books Tax handles this automatically — here's how it works.

Why Does This Happen?

HMRC has a strict rule: no corporation tax return can cover more than 12 months. This is set out in the Corporation Tax Act 2009.

Extended periods typically occur when:

• Your company was incorporated mid-year and chooses a different year-end
• You've changed your accounting reference date
• It's your company's first accounting period

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How Clear Books Tax Detects Extended Periods

When you select an accounting period longer than 12 months, Clear Books Tax shows this banner:

The banner includes:

• The total number of days in your period
• The dates for each CT600 return
• How the split works

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How the Split Works

Clear Books Tax automatically splits your extended period into two:

Return	Coverage	Typical Example
CT600 #1	First 12 months	1 Jun 2023 to 31 May 2024 (366 days)
CT600 #2	Remaining period	1 Jun 2024 to 31 Mar 2025 (304 days)

Your profits and allowances are apportioned automatically based on the number of days in each period.

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What You Need to Enter

Here's the good news: you only enter your figures once.

Enter the totals for the entire extended period:

• Total turnover for the whole period
• Total expenses for the whole period
• Balance sheet at the period end

Clear Books Tax calculates the split for each CT600 automatically.

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How Profits Are Apportioned

For a period of 670 days split into 366 + 304 days:

Item	Total	Period 1 (366 days, 55%)	Period 2 (304 days, 45%)
Trading profit	£50,000	£27,313	£22,687
Interest income	£1,000	£546	£454
Tax @ 19%	£9,690	£5,289	£4,401

The apportionment is done on a day count basis - the most accurate method accepted by HMRC.

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What About Accounts?

Unlike CT600 returns, your accounts submission to Companies House covers the entire extended period - no splitting required.

Filing	Period Covered
CT600 #1	First 12 months
CT600 #2	Remaining days
Companies House Accounts	Full extended period

Clear Books Tax generates all three documents automatically.

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The Submission Process

When you reach the preview page, you'll see both CT600 returns ready:

1. Review Period 1 - Check the apportioned figures for the first 12 months
2. Review Period 2 - Check the apportioned figures for the remaining period
3. Submit CT600 #1 - This goes to HMRC first
4. Submit CT600 #2 - This follows immediately after
5. Submit Accounts - To Companies House (full period)

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Tax Rates Across Periods

If tax rates change during your extended period, Clear Books Tax applies the correct rate to each portion:

Period	Tax Rate	Applies To
Pre-1 April 2023	19%	Profits in this portion
Post-1 April 2023	19-25%	Depends on profit level

The marginal relief calculations are done separately for each CT600, using the apportioned profits and the rates in force during that period.

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Common Questions

"Can I avoid splitting by choosing different dates?"

No. If your accounting period is longer than 12 months, it must be split for corporation tax purposes. This is a legal requirement, not a Clear Books Tax limitation.

However, you can:

• Shorten your accounting period by changing your accounting reference date (Companies House form AA01)
• Choose dates that create a standard 12-month period going forward

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"Why are my two CT600 returns showing different tax amounts?"

This is normal. The apportionment is based on days, so:

• Period 1 (12 months) typically has more profit apportioned
• Period 2 (remaining days) has less profit
• Tax is calculated separately on each portion

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"What if I have losses?"

Losses are also apportioned between periods. If you have a loss in the overall period:

• Each CT600 shows the apportioned loss
• Loss relief is calculated separately for each period
• Losses can be carried forward from Period 1 to Period 2

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"Do I pay tax twice?"

No. You're paying tax on the same profits, just split across two returns. The total tax across both returns equals what you'd pay if HMRC allowed a single return.

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"What about capital allowances?"

Capital allowances like the Annual Investment Allowance (AIA) are pro-rated:

Period Length	AIA Available
12 months	£1,000,000
6 months	£500,000
18 months (split)	Period 1: £1,000,000 + Period 2: £500,000

Clear Books Tax calculates the correct allowance for each period automatically.

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"My first period is very long - is that a problem?"

Common for new companies. A first period of 18 months means:

• CT600 #1: First 12 months
• CT600 #2: Remaining 6 months

This is completely normal and Clear Books Tax handles it correctly.

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Extended Period Deadlines

Both CT600 returns have the same filing deadline - 12 months after the end of the full accounting period.

Extended Period	Deadline
1 Jun 2023 to 31 Mar 2025	31 Mar 2026

Payment deadlines may differ - corporation tax is due 9 months and 1 day after the end of each CT600 period:

• Period 1 payment: 9 months after Period 1 end
• Period 2 payment: 9 months after Period 2 end (which is the full period end)

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Troubleshooting

"The split dates don't look right"

Clear Books Tax always makes the first period exactly 12 months (or 12 calendar months ending on the day before the anniversary of the start date). This is the correct method per HMRC rules.

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"My tax calculation seems too high/low"

Check that you've entered the full period figures, not just one portion. Clear Books Tax needs the complete picture to calculate the split correctly.

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"I've already filed one return elsewhere"

If you've already submitted CT600 #1 through another service:

1. You can still use Clear Books Tax for CT600 #2
2. Select the shorter period (Period 2 dates) as your accounting period
3. Enter just the figures for that period

"How do I set up the second CT600 filing after the first?"

When Clear Books Tax automatically detects an extended period, both CT600s are generated within a single filing session — you do not need to create a second separate filing. At the preview step, you will see two CT600 returns ready to submit one after the other.

If, however, you need to file the second period (the remaining months) as a standalone filing — for example, because you submitted Period 1 elsewhere, or because you want to set a different associated companies count (Box 326) for each period — follow these steps:

1. Go to your Dashboard and open the company you are filing for.
2. Click Start Filing ▾ and select CT600 only (or CT600 and Accounts if accounts are also needed).
3. On the filing form, find the accounting period row and click "Need to file for different dates?"
4. In the dialogue that appears, select Other reason and click Continue.
5. Enter the start and end dates for the second period only (e.g. the remaining weeks or months after the first 12-month return).
6. Tick the confirmation box and proceed.
7. Enter the apportioned figures for that shorter period.
8. Set Box 326 (associated companies) to the correct value for this period.
9. Submit to HMRC.

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Still Have Questions?

Extended periods can be confusing. If you're unsure about your specific situation, we're here to help.

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Last updated: February 2026