iXBRL connects readable accounts and computations to machine-readable tax data.
The Company Tax Return package needs more than a completed CT600. Validate the tagged accounts, computations, taxonomy and human-readable output together.
What iXBRL does
Inline XBRL combines a human-readable document with machine-readable tags. HMRC requires online Company Tax Returns to include accounts and computations in the required iXBRL format in normal circumstances.
Typical return package
The exact package depends on the company, but commonly includes:
- CT600 return data
- iXBRL statutory accounts
- iXBRL Corporation Tax computations
- Relevant CT600 supplementary pages
- Supporting attachments where permitted or required
Tagging is not a cosmetic conversion
A file can look correct to a person but contain incomplete or incorrect tags. Use supported taxonomies, validate the package, inspect warnings and confirm that key facts—entity, dates, monetary units and accounting concepts—map to the intended disclosures.
Pre-submission review
Review both the rendered document and the machine checks.
- Company identity and reporting period
- Accounts framework and taxonomy version
- Consistency between accounts, computation and CT600
- Negative values, units, decimals and comparative periods
- Required explanations or attachments
- Gateway validation output
Keep the exact submitted artefacts
Retain the final tagged accounts, computation, return payload, validation output and HMRC response. This protects the audit trail when a later amendment or enquiry needs to identify exactly what was filed.
Where professional judgement is needed
This guide explains a filing workflow, not the accounting treatment, tax position or legal duties of a particular company. Confirm the applicable accounting framework, reliefs, disclosures and filing obligations against current official guidance and, where needed, a qualified accountant or tax adviser.
FAQ
Is iXBRL the same as a PDF?
No. iXBRL is readable in a browser but also contains structured tags that software and authorities can process.
Does HMRC require both accounts and computations in iXBRL?
For normal online Company Tax Return filing, HMRC guidance requires the accounts and computations in iXBRL.
Can a visually correct document still fail?
Yes. Taxonomy, tagging, consistency or package-validation errors can exist even when the rendered page looks reasonable.
Why retain the exact submitted files?
They show the tagged facts and version actually transmitted, which is essential for rejections, amendments and later review.
Official sources
Checked on 2026-08-11. Official guidance takes priority if rules change.