iXBRL and CT600

Directors filing CT600 now need to understand iXBRL computations.

If you used to rely on HMRC's joint filing service, iXBRL can feel like the hidden part of Corporation Tax software. This guide explains what it means and how to prepare.

What iXBRL means in a Company Tax Return

iXBRL stands for Inline eXtensible Business Reporting Language. In practical terms, it is a machine-readable tagging format wrapped into human-readable accounts and computations. HMRC and Companies House use the tags so financial data can be read consistently by systems, not only by people opening a PDF.

The important point for directors is that a CT600 filing package is not just a form. It usually includes the CT600, Corporation Tax computations, company accounts, and any supplementary pages. HMRC's XBRL guide says companies must send Company Tax Returns online using iXBRL for accounts and computations, with limited exceptions.

Why directors are noticing iXBRL now

The old GOV.UK joint filing route hid some of the machinery. When a director moves to commercial CT600 software after that service closed, the filing package becomes more explicit. You need software that can prepare or accept the correct accounts and computation attachments, not only fill in the CT600 tax boxes.

GOV.UK says that from 1 April 2026 companies should use commercial software to file annual accounts and Company Tax Returns with HMRC. It also tells users to check that their software can file the CT600, Corporation Tax computation, and company accounts.

Accounts, computations, and the CT600 are different

These three pieces are connected, but they are not interchangeable.

  • The accounts show the company's financial statements for the period.
  • The Corporation Tax computation explains the tax adjustments and tax calculation.
  • The CT600 is the Company Tax Return form sent to HMRC with the tax result and relevant declarations.

If the profit in the accounts, the adjusted profit in the computation, and the CT600 boxes do not reconcile, the file is not ready. Good software should make the relationship visible before submission.

Common iXBRL mistakes in small-company CT600 work

  • Uploading the accounts but forgetting the computation.
  • Using a draft accounts file instead of the final approved version.
  • Assuming a normal PDF is enough where iXBRL is required.
  • Changing taxable profit after the iXBRL computation was generated.
  • Filing a CT600 that does not agree to the computation.
  • Missing a supplementary page that changes the main CT600 result.

Director pre-filing checklist

Before choosing software or pressing submit, directors should gather the pieces that drive the return.

  • Company name, UTR, Companies House number, and accounting period.
  • Final annual accounts for the same period.
  • Tax computation showing accounting profit, add backs, deductions, reliefs, and Corporation Tax due.
  • Capital allowances, losses, R&D, director loan, group, or special-relief details.
  • Any supplementary page decisions and supporting evidence.
  • Government Gateway access or agent filing arrangement.

What to expect from CT600 software

A useful director workflow should do more than ask for a final tax number. It should guide you through the filing package and show whether the pieces match.

  • Clear CT600 sections for the company, accounting period, profits, charge, reliefs, payments, and declarations.
  • Support for accounts and computation attachments in the required format.
  • Supplementary-page review where director loans, losses, R&D, group relief, or special regimes apply.
  • Validation before submission.
  • Submission evidence after filing.

How Robocount fits

Robocount is designed for modern CT600 work after the closure of the old joint filing service. It connects the Corporation Tax return, computation, iXBRL attachments, supplementary pages, and review trail in one browser-based workflow.

  • Helps directors understand the CT600 package before filing.
  • Keeps computations and attachments visible during review.
  • Supports accountant-led and director-led filing routes.
  • Gives AI-assisted workflows a structured review surface rather than a black-box form fill.

FAQ

Can I file a CT600 with only a PDF accounts file?

Usually no. HMRC guidance says most companies must file accounts and computations in iXBRL as part of online Company Tax Return filing, with limited exceptions. Check the current rules for your company type.

Are iXBRL accounts the same as Companies House accounts?

They are related, but the filing routes and requirements can differ. You still need to file annual accounts with Companies House, and the Company Tax Return package sent to HMRC needs the right CT600, computation, accounts, and attachments.

Do directors need to understand XBRL tags?

You do not need to become an XBRL developer. You do need enough understanding to choose software that handles the filing package and to spot when accounts, computations, and the CT600 no longer agree.

Can Robocount help if I used the old HMRC filing service?

Yes. Robocount is aimed at the post-joint-service CT600 workflow for directors, practices, developers, and AI-assisted filing. It is built around the full Corporation Tax package rather than only a single form.

Useful official references

This guide is general product and filing workflow information, not tax advice. Check current HMRC guidance and the facts of the company before filing.