iXBRL is part of the return package

HMRC's XBRL guidance says most companies must file Company Tax Returns online with financial accounts and computations in iXBRL. HMRC's accounts-format guidance also explains that a Company Tax Return comprises the CT600, any supplementary pages, accounts, computations, and supporting documentation.

For CT600 software, that means the return is not complete just because the tax boxes calculate. The accounts and computation attachments need to be present, valid, reviewable, and consistent with the CT600 figures.

What iXBRL means in plain English

iXBRL is a structured reporting format. It lets a document be readable by a person while also containing embedded tags that software can read. For Corporation Tax, iXBRL is commonly used for statutory accounts and tax computations filed with the Company Tax Return.

The point is not cosmetic formatting. The tags help HMRC process reported figures, and the human-readable document lets directors, accountants, and reviewers understand what has actually been filed.

Accounts and computations are different documents

Accounts and computations answer different questions. Accounts report the company's financial position and performance. Computations bridge from accounting profit to taxable profit, tax payable, losses, allowances, reliefs, and credits.

A CT600 workflow should therefore check both documents. A return may be risky if the accounts show one profit figure, the computation adjusts it differently, and the CT600 boxes do not reconcile to either.

Common attachment problems

The CT600 has been prepared but accounts are still draft or unsigned.
The computation uses old trial balance figures after late bookkeeping changes.
Capital allowances, losses, or R&D credits are changed in the computation but not reflected in the return review.
Supplementary pages are included without supporting schedules.
The iXBRL file is present but linked to the wrong accounting period.
A director or client approves the tax figure without seeing the final attachments.

These are workflow failures, not merely formatting issues. The file should make it clear which accounts and computations were reviewed and filed.

Why the closed HMRC service changed the buyer journey

HMRC closed the joint online service for filing company accounts and Company Tax Returns on 31 March 2026. Directors and micro-company owners who relied on that route now need to use commercial software or another filing route that can handle the CT600 package.

This is especially important for simple companies. A small company may have a straightforward tax calculation, but it still needs the right filing package when HMRC requires a Company Tax Return.

Review controls for accountancy practices

Before filing, practices should confirm:

The accounts period matches the CT600 accounting period or the period split has been handled properly.
The computation starts from the final approved profit figure.
Capital allowances, disallowables, losses, reliefs, and credits agree between computation and CT600.
Any supplementary pages have supporting schedules or working papers.
The client has approved the final CT600, accounts, computation, and filing authority.
The filed package and HMRC acknowledgement are retained for the practice file.

Developer and AI workflow implications

API and AI-led CT600 workflows need structured controls around attachments. An agent can assist with data extraction, review prompts, and filing preparation, but the workflow should keep deterministic checks around period dates, company identity, numeric reconciliation, attachment status, and human approval where required.

For developers, the useful abstraction is not "upload a PDF-like file". It is a return package with typed components: CT600 data, supplementary pages, accounts, computations, evidence, approval, submission response, and post-filing status.

How Robocount handles iXBRL attachment workflow

Robocount is built around the full Corporation Tax filing package. It connects CT600 preparation with computations, iXBRL attachments, supplementary-page review, and HMRC submission readiness.

Keeps accounts and computations visible in the CT600 review flow.
Connects tax computation figures to return-level checks.
Supports supplementary-page review alongside the main return.
Gives practices and directors a clearer approval trail before filing.
Supports API/MCP-style workflows where structured filing state matters.

FAQ

Do CT600 filings need iXBRL accounts and computations?

HMRC guidance says most companies must file Company Tax Returns online, including financial accounts and computations in iXBRL, for relevant accounting periods. Check HMRC guidance for exceptions and company-specific facts.

Is the CT600 enough on its own?

Usually no. The Company Tax Return package includes the CT600, required supplementary pages, accounts, computations, and supporting documentation.

Can directors still use HMRC's old joint filing service?

No. HMRC says the joint service for filing company accounts and Company Tax Returns closed on 31 March 2026.

What should software check before submission?

At minimum, software should keep company identity, accounting period, CT600 figures, accounts, computations, supplementary pages, approval, and submission evidence aligned.

Useful HMRC references

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

iXBRL accounts, computations, and CT600 attachment workflow.