Filing Your First CT600 After Dormancy

If your company was dormant and has now started trading, your first CT600 will cover a different period than you might expect. This guide explains how CT600 periods work when transitioning from dormant to trading.

CT600 vs Companies House Accounts: Different Rules

Companies House accounts and HMRC CT600 returns follow different rules for period dates:

	Companies House Accounts	HMRC CT600
Period start	Day after previous accounts ended	Can start from when trading began
Period end	Always your ARD (year-end)	Always your ARD (year-end)
Dormant periods	Must file accounts every year	May not need CT600 if no CT603 issued

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Your First Trading CT600: The Most Common Scenario

Situation: Your company was dormant, then started trading mid-year. HMRC has sent you a Notice to Deliver (CT603).

Example:

• Company ARD: 31 December
• Was dormant until April 2024
• Started trading: 1 May 2024
• CT603 received showing period: 1 May 2024 to 31 December 2024

What to file:

Period	Length	Type
1 May 2024 - 31 Dec 2024	8 months	Trading CT600

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Check Your HMRC Notice to Deliver (CT603)

HMRC sends a Notice to Deliver a Company Tax Return (form CT603) when they expect you to file a CT600. This letter is your definitive guide - it specifies:

• The exact period start date
• The exact period end date
• The filing deadline

If you haven't received a CT603:

• HMRC may not require a CT600 for the dormant period
• When you notify HMRC you've started trading, they'll issue a CT603 with the correct dates
• You can notify HMRC via your Government Gateway account or by calling the Corporation Tax helpline

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What If I Want a Different Year-End?

If you want your accounting year to run from when you started trading (e.g., May to April instead of January to December), you need to change your Accounting Reference Date (ARD) with Companies House.

How to Change Your ARD

1. Go to Companies House WebFiling
2. File form AA01 (Change of Accounting Reference Date)
3. Specify your new accounting reference date

Example: Changing from December to April Year-End

Before ARD change:

• ARD: 31 December
• Started trading: 1 May 2024
• CT600 period would be: 1 May 2024 to 31 December 2024 (8 months)

After changing ARD to 30 April:

• New ARD: 30 April
• You may need a short bridging period: 1 Jan 2024 to 30 April 2024 (dormant)
• First trading CT600: 1 May 2024 to 30 April 2025 (12 months)

Rules for Changing Your ARD

Situation	Rule
Shortening a period	Can be done at any time
Extending a period	Cannot extend beyond 18 months from the start
Extending more than once	Can only extend once every 5 years (with exceptions)

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How to Enter Custom Dates in TinyTax

If your CT600 period doesn't match the default dates shown:

1. Go to the CT600 form for your company
2. Look for the Edit button next to the "Accounting Period" dropdown
3. Click Edit - a warning will appear:

[[ui:alert:warning:Your accounting year end is [your ARD]. Entering different dates is not advised unless you are an accountant. It may create a filing for an incorrect period or cause HMRC to reject your return.]]

1. Click
   Edit anyway
   to proceed
2. Enter your Period Start Date and Period End Date (as shown on your CT603)
3. Continue with the rest of the form

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Common Questions

"Do I need to file a CT600 for the dormant period?"

Not necessarily. HMRC only requires a CT600 if they've issued a Notice to Deliver (CT603). Many dormant companies don't receive a CT603, so no CT600 is required for those periods.

Check if you've received a CT603 for the dormant period. If not, you likely only need to file from when trading started.

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"My CT603 shows different dates than I expected"

The dates on your CT603 are correct - use those. HMRC determines the period based on:

• When your company started trading (or became active for CT purposes)
• Your registered year-end (ARD)

If you believe the dates are wrong, contact HMRC before filing.

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"I started trading mid-year - is my first period shorter than 12 months?"

Yes, usually. Your first trading CT600 runs from when you started trading to your year-end.

Example: Started trading 1 May 2024, ARD is 31 December. First CT600 covers 1 May 2024 to 31 December 2024 (8 months).

This is normal and correct.

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"I want my year to run May to April - can I file 1 May 2024 to 30 April 2025?"

Only if your ARD is 30 April. If your ARD is 31 December, your period must end on 31 December.

To have a May-April year, you need to:

1. Change your ARD to 30 April with Companies House (form AA01)
2. Wait for Companies House to confirm the change
3. Then file for the May-April period

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"What about my Companies House accounts?"

Companies House accounts follow different rules - they always run from year-end to year-end, regardless of when you started trading.

If you were dormant for part of the year, your accounts will show:

• £0 activity for the dormant months
• Your actual trading figures for when you were active

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Filing Deadlines

CT600 (HMRC)

Your CT600 is due 12 months after the end of the accounting period.

Accounts (Companies House)

Your accounts are due:

• 9 months after period end (for private companies)
• First accounts may have a longer deadline - check Companies House

Tax Payment

Corporation tax is due 9 months and 1 day after the period end.

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Still Need Help?

If you're unsure which dates to use for your first trading CT600:

1. Check your CT603 letter from HMRC - it has the definitive dates
2. If you haven't received one, contact HMRC to confirm when you need to file

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Last updated: January 2026