What needs to go with a CT600?

A Company Tax Return is the CT600 form plus the company’s accounts, a tax computation and any supplementary pages that apply. Filed online, the computation must be in iXBRL, and for most companies the accounts must be too.

Last checked against GOV.UK on 1 October 2026.

A4O Accounts prepares the CT600 from your books and files it with HMRC. See the CT600 software, or get started free.

The three parts of a return

New to the form? Start with what is a CT600.

The accounts

HMRC wants the accounts for the period the return covers. GOV.UK says that for most companies and organisations, accounts forming part of an online return must be in iXBRL. There are exceptions, set out in its guidance on the format for accounts. Where accounts aren’t filed in iXBRL, they go as a PDF attachment.

iXBRL is a tagged version of the accounts that HMRC’s systems can read. You don’t tag anything by hand: filing software produces it.

These aren’t always the same accounts you put on the public record. A small company can file a shorter set with Companies House, while HMRC gets the full accounts. See how the accounts you send HMRC differ.

The tax computation

GOV.UK says you must include computations that show how the figures in your return were calculated from the figures in the company accounts. Everything needed to explain the figures goes in a single iXBRL computations file.

A computation usually shows:

• the profit or loss in the accounts
• costs added back because they aren’t allowed for tax, such as depreciation and client entertaining
• capital allowances claimed instead
• losses used or carried forward
• the taxable profit and the Corporation Tax on it

Supplementary pages

Supplementary pages are extra parts of the CT600 for particular situations. Most small companies need none. The ones that come up most often are CT600A, for loans to directors and shareholders that are still outstanding, and CT600L, for research and development claims.

See CT600 supplementary pages explained for the full list.

If you’re not sending accounts or computations

Occasionally a return goes in without them. GOV.UK’s guide to the form says that if you’re not submitting accounts and computations, you use box 90 to explain your reasons.

What doesn’t go with it

• Payment. Filing the return doesn’t pay the tax. Corporation Tax is paid separately, usually 9 months and 1 day after the end of the accounting period.
• Your Companies House accounts. Those are a separate filing, with their own deadline. See Corporation Tax deadlines.

A checklist before you file

1. The return period matches the accounting period HMRC has for the company. See accounting periods for Corporation Tax.
2. The accounts are for the same period, and approved by the directors.
3. The computation starts from the profit in those accounts and ends at the figures on the CT600.
4. Any supplementary page you need is attached, and none you don’t.
5. You have the company’s UTR and the Government Gateway user ID and password for Corporation Tax.

Sending it all from software

A4O Accounts builds the return’s accounts and tax computation from your books, tags both in iXBRL and sends them to HMRC with the CT600, including CT600A or CT600L where you’ve added them. Before you submit, you can preview the iXBRL accounts and computation and the full XML HMRC will receive. See CT600 software and iXBRL accounts software.

More in this series: all the CT600 guides.