CT600 filing deadline: what UK directors need to know
Founders often file annual accounts at Companies House and assume tax is done. It is not. Your CT600 (company tax return) goes to HMRC on a different timeline, with separate payment rules.
Miss the corporation tax payment date and HMRC charges interest. Miss the CT600 filing deadline and penalties apply.
What is a CT600?
A CT600 is HMRC's company tax return form. It reports taxable profits, corporation tax due, and claims such as R&D tax relief for a UK limited company's accounting period.
It is filed online with HMRC. Companies House annual accounts are a separate filing, though both draw on the same underlying numbers.
Corporation tax and CT600 timeline
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Key deadlines after year end
For a standard UK company with a 12-month accounting period:
Obligation | Typical deadline |
Corporation tax payment | 9 months and 1 day after year end |
CT600 filing | 12 months after year end |
Companies House accounts | 9 months after year end (private Ltd) |
Example: year end 31 March 2025:
Corporation tax payment due: 1 January 2026
CT600 due: 31 March 2026
Companies House accounts due: 31 December 2025
Founders confuse the three constantly. Put all three in your compliance calendar on day one.
CT600 vs annual accounts: what is the difference?
Companies House accounts | CT600 | |
Recipient | Companies House (public record) | HMRC (tax authority) |
Purpose | Statutory disclosure | Calculate and report tax |
Format | iXBRL accounts | CT600 return + computations |
Typical deadline | 9 months (private) | 12 months |
Your accountant usually prepares both from the same trial balance, but they are separate filings with different deadlines. Founders often look up "CT600 vs annual accounts" when they realise the two are not interchangeable.
What goes on the CT600?
Common elements for startups:
Taxable profit per accounts, adjusted for tax rules
Corporation tax rate and liability
R&D tax relief (if claimed, enhanced expenditure on CT600)
Losses brought forward or surrendered
Associated companies and group considerations (if applicable)
R&D claims often need a supporting report to HMRC alongside the CT600. Do not bolt this on the night before the deadline.
What happens if you miss the CT600 deadline?
HMRC can charge late filing penalties for company tax returns. Interest accrues on unpaid corporation tax from the payment due date.
For startups:
HMRC enquiries delay R&D refunds
Investors flag tax compliance in diligence
Carry-forward losses may still be usable, but messy filing creates noise
If you are late, file and pay as soon as possible. Do not wait for the next year end.
How founders should prepare
Close books promptly using a month-end close checklist
Brief your accountant early on R&D, share schemes, or unusual transactions
Calendar payment and filing dates separately from Companies House
Reconcile tax provision in management accounts to avoid surprise cash calls
In practice
AccountUp aligns operational reporting with year-end readiness so CT600 preparation is an extension of monthly discipline, not a fire drill.
FAQs
When is the CT600 filing deadline?
The CT600 is usually due 12 months after the end of your company's accounting period.
When is corporation tax due?
Corporation tax is normally due 9 months and 1 day after the end of the accounting period.
Is CT600 the same as filing accounts at Companies House?
No. Companies House accounts are a separate filing with an earlier deadline for private companies (typically 9 months). The CT600 goes to HMRC.
Can I file CT600 before paying corporation tax?
Payment and filing deadlines differ. You may need to pay before or around filing depending on your liability. Confirm amounts with your accountant before the payment date.
Where do R&D tax credits appear?
R&D claims are included in the CT600 company tax return, with supporting documentation for HMRC.
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