iXBRL accounts for a CT600: what to check beyond the PDF

In short: For returns delivered on or after 1 April 2011, most companies must send the accounts and tax computations with an online Company Tax Return in iXBRL, not as a PDF (GOV.UK: Businesses XBRL guide). Beyond the readable pages, check that the tagged files show the same company, period and figures as the accounts you approved, and that the software's taxonomy covers your accounting period (HMRC: accepted taxonomies).

iXBRL combines readable financial information with computer-readable tags. The figures may look familiar while the file carries additional meaning for HMRC's systems.

You do not need to become a tagging specialist to review your filing. You do need to know which documents you are approving and whether they describe the right company and period.

What goes in a CT600 filing package?

A CT600 filing package has three parts: the CT600 return with any required supplementary pages, the accounts, and the tax computations. For most companies the accounts and computations go in iXBRL rather than as a PDF, and every part should come from the same final version you approved.

A folder labelled "accounts" can contain a draft PDF, an approved PDF and an older tagged file. Their names alone do not establish that they contain the same version.

Before filing, give the final package a clear version reference. Compare the readable output from the actual submission files with the accounts you approved, rather than checking an unrelated download.

Can a file that opens correctly still be wrong?

Yes: a readable file can still carry outdated figures, so compare the actual submission files with the accounts you approved. Consider this hypothetical mismatch: the approved accounts show turnover of £84,000, but the generated filing preview still shows £81,000 because it was created before the final bookkeeping adjustment.

The £3,000 difference must be explained before submission. It is not solved by the document opening successfully or by a validation result that does not test this comparison.

Use a short cross-document checklist:

• Confirm company name and identifying details.
• Match the accounts period to the approved statements.
• Check how any different Corporation Tax period is explained.
• Reconcile turnover, profit, assets, liabilities and equity to the final accounts.
• Trace the important tax adjustments into the computation.
• Confirm the final package replaced any earlier version after corrections.

Where one accounts period covers more than one Corporation Tax period, do not force all dates to match. Ask the preparer to explain the relationship and the documents required for each return.

Does "supports iXBRL" mean the software supports your accounting period?

Not necessarily: HMRC publishes accepted taxonomies with period conditions, so a provider saying it "supports iXBRL" has not by itself confirmed support for every accounts framework or accounting period. A taxonomy is the dictionary of tags used for a reporting purpose. HMRC: accepted taxonomies.

Ask the provider to confirm:

• Your exact accounting period start and end dates.
• The accounts framework your company uses.
• Any supplementary pages your return requires.
• Whether externally prepared documents can be used.

If the period is unsupported, changing a date or reusing an older file is not a legitimate workaround.

Does passing validation mean the accounts are right?

No: a file can meet a format rule while containing a mistaken figure or an inappropriate accounting judgement, because technical checks and financial review answer different questions. HMRC's own manual makes clear that acceptance as a valid return does not prevent later scrutiny through a compliance review. HMRC: accounts forming part of a return.

Resolve unexpected warnings, retain the final documents and save the submission evidence. Keep a note of who reviewed the financial content and what they actually checked; do not describe an automated validation message as an accountant's approval.

How do you confirm Taxley supports your filing?

Ask Taxley about your exact period and document requirements before paying. A question about importing existing iXBRL is different from a question about generating new accounts from figures. This guide does not assert that every import, framework or period is supported. For a general first view, take the 30-second check to see whether Taxley fits your company.

Frequently asked questions

Can I rename a PDF to make it iXBRL?

No. Renaming a file does not add the required structure or tags. iXBRL combines readable financial information with computer-readable tags used by HMRC's systems, so the accounts and computations have to be generated as tagged files (HMRC: businesses XBRL guide).

Does filing with HMRC also file the Companies House accounts?

Do not assume so. The Company Tax Return goes to HMRC, and filing accounts at Companies House is a separate obligation even where one product offers both routes. Confirm the destination of each submission and retain separate evidence for each required filing.

What is a taxonomy in iXBRL?

A taxonomy is the dictionary of tags used for a reporting purpose. HMRC publishes the taxonomies it accepts, with conditions about the periods each one covers, so the right taxonomy depends on your accounts framework and accounting period (HMRC: accepted taxonomies).

Does HMRC accepting the return mean the accounts are correct?

No. HMRC's manual makes clear that acceptance as a valid return does not prevent later scrutiny through a compliance review. A file can meet a format rule while containing a mistaken figure, so note who reviewed the financial content and what they checked (HMRC: accounts forming part of a return).

General information for UK company directors. Review financial statements and tax computations carefully yourself; a format check is not tax advice.