Corporate tax filing is electronic and has been for years, and the format requirement is stricter than most companies expect: HMRC wants the computation and the accounts tagged in iXBRL, not attached as a document. That single requirement is why most companies file through software or through a firm rather than directly.
The format requirement
The CT600, the computation and the accounts must be filed online, with the computation and accounts in iXBRL, a format that tags each figure so HMRC's systems can read it rather than a human having to. HMRC provides a free online service that suits companies with straightforward affairs, and commercial software covers the rest. A company whose accounts are prepared by a firm will normally have the tagging done as part of that work without ever seeing it.
What the compliance cycle looks like
Notify HMRC when the company becomes active. Keep records adequate to support the return, which HMRC expects to be retained for six years from the end of the period. Pay at nine months and one day. File at twelve months. Respond to any enquiry HMRC opens, which it may normally do within twelve months of the filing date. Repeat annually, with the added obligation to tell HMRC when no payment is due rather than staying silent.
Where it goes wrong
The two common failures are structural rather than technical. A company that has changed its accounting date has two periods to file rather than one, because a corporation tax accounting period cannot exceed twelve months, and companies routinely file for the longer period and are rejected. And a dormant company that has not told HMRC it is dormant keeps receiving notices to file, each of which starts a penalty clock whether or not there is any tax.
Questions people ask about corporate tax filing
How do I file a corporation tax return?
Online, with the computation and accounts in iXBRL format. HMRC's free service covers straightforward companies; commercial software or a firm covers the rest.
How long should a company keep its records?
HMRC expects records adequate to support the return to be kept for six years from the end of the accounting period, and longer where an enquiry is open.
What if the accounting period is longer than twelve months?
It is split. A corporation tax accounting period cannot exceed twelve months, so a longer set of accounts produces two returns, each with its own deadline and payment reference.