A company tax return is the CT600 plus everything filed with it, and the everything is the part people underestimate. HMRC does not accept a form with a number on it; it expects the computation that produced the number and the statutory accounts the computation started from, in a specified electronic format, by a deadline twelve months after the accounting period ends.
What goes in the return
The CT600 itself, which reports the taxable profit, the reliefs claimed and the tax due. The corporation tax computation, showing how the accounting profit became the taxable one: depreciation added back, capital allowances deducted, disallowables removed, reliefs applied. And the statutory accounts for the period. Supplementary pages attach where relevant, including CT600A for loans to participators and the R&D pages where a claim is made.
The deadline, and the other deadline
The return is due twelve months after the end of the accounting period. The tax is due three months earlier, at nine months and one day, which is the sequence that surprises people: most companies pay before they file. Late filing starts at £200 after a single day and escalates, and three late returns in a row raise those fixed penalties to £1,000 each. Late payment attracts interest from the day after the due date, separately.
It is not the Companies House filing
The accounts filed at Companies House and the accounts filed with HMRC are the same accounts but two separate obligations with two separate deadlines and two separate penalty regimes. A small company may file abridged accounts publicly while HMRC receives the full set. Missing one does not excuse the other, and a company that has filed at Companies House has not thereby filed its tax return.
Questions people ask about company tax returns
What is a company tax return?
The CT600 together with the corporation tax computation and the statutory accounts for the period, filed with HMRC in a specified electronic format within twelve months of the end of the accounting period.
When is a company tax return due?
Twelve months after the end of the accounting period. The tax itself is due earlier, at nine months and one day, so most companies pay before they file.
Is filing at Companies House the same thing?
No. They are separate obligations with separate deadlines and penalties, even though the underlying accounts are the same.