The CT600 company tax return
The CT600 is the company tax return, filed with HMRC within 12 months of the accounting period end, with the statutory accounts and a tax computation attached in iXBRL format. It is separate from filing accounts at Companies House, and the corporation tax itself is due three months before the return.
Three documents go to HMRC as one filing: the return, the accounts, and the computation that connects them. The computation is the one nobody sees and the one the tax actually depends on.
What is actually in it
Three things, filed together:
- The CT600 form — the return itself: the company's details, the period, the taxable profit, the tax due, and any claims or reliefs.
- The statutory accounts for the period.
- The tax computation — the working that gets from accounting profit to taxable profit.
That third document is where the tax is decided. Client entertaining is added back because it is not deductible; depreciation comes out and capital allowances go in; a director's loan may trigger its own charge. Two accountants given the same accounts can produce different computations, and the difference is real money.
Why it cannot just be a PDF
HMRC requires accounts and computations in iXBRL — readable as a document but tagged so each figure is machine-readable. It is why filing generally happens through software or an agent rather than by attaching a spreadsheet.
The penalties
| How late | Penalty |
|---|---|
| 1 day | £200 |
| 3 months | another £200 |
| 6 months | 10% of the unpaid tax, on HMRC’s own estimate of the bill |
| 12 months | another 10% of any unpaid tax |
Late three times in a row and the £200 penalties become £1,000 each.
Note what happens at six months: HMRC stops waiting, estimates the corporation tax itself and charges 10% of that estimate. The estimate is not a negotiation, and it is not generous. These are also entirely separate from the Companies House penalties for late accounts, which run on their own scale — a company that is late everywhere pays twice.
Have it prepared and filed properly
Accounts, computation and CT600 as one job, filed to both HMRC and Companies House, with the tax figure given to you in time to pay it. Fixed fee agreed in writing after a free assessment.
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Free assessment first. If there is work to do, you get a fixed quote in writing before anything starts.
The order things happen in
- Year end passes. The clock starts on every deadline at once.
- Accounts prepared from the year's records.
- Computation prepared — accounting profit adjusted to taxable profit.
- Tax paid, 9 months and 1 day after the end of the accounting period. See how to pay it.
- Accounts filed at Companies House, and the CT600 filed at HMRC.
Payment sits in the middle of that list, not at the end. Any process that leaves the accounts until the filing deadline has already missed it.
Questions people ask
What is a CT600?
The company tax return. It reports the company's taxable profit and the corporation tax due, and is filed with HMRC together with the statutory accounts and a tax computation.
Is it the same as filing accounts at Companies House?
No — different recipient, different deadline, different purpose. Companies House gets accounts for the public register; HMRC gets the CT600 with accounts and computation attached. Both have to happen.
What is iXBRL and why does it matter?
The format HMRC requires: accounts and computations tagged so the figures are machine-readable, not just a PDF. It is why company accounts are not simply emailed over, and why most companies file through software or an agent.
What if the company was dormant or made no profit?
You still file if HMRC has issued a notice to deliver a return. A loss-making year still needs a return — and it is the return that records the loss, which is what lets you use it against another year.
Can I file it myself?
Yes, and HMRC's own service covers straightforward small companies. It gets harder where there are capital allowances, losses, a director's loan, or anything needing judgement about what is deductible — which is where the fee usually pays for itself.
How long do I keep the records?
Six years from the end of the accounting period, longer where a return is filed late or an enquiry is opened. Keep the computation, not just the accounts: it is the document that explains how you got from one to the other.
Figures are for the tax year shown and change every April. They are general information, not advice — we check the current position for your circumstances before anything is filed.
Reviewed 19 October 2026.
Part of our work on company filings and deadlines — if you want someone to take it off your hands, start with our confirmation statement filing page.