Company accounts are no longer documents as far as the authorities are concerned. They are data. Every set of accounts and computation filed with a corporation tax return must be tagged in iXBRL — inline eXtensible Business Reporting Language — so that machines can read the numbers directly, and listed companies face a parallel requirement for their annual financial reports.
What iXBRL is
iXBRL embeds machine-readable tags inside an ordinary HTML document. A human sees a normal set of accounts; software reading the same file sees each number identified by what it is — turnover, profit before tax, the date of the period, the reporting currency — with the tags drawn from a published taxonomy.
The taxonomy is the dictionary. It defines thousands of elements, their meanings and their relationships, aligned to the applicable accounting framework — FRS 101, FRS 102, FRS 105 or IFRS — and to the disclosures those frameworks require. Tagging is therefore an accounting judgement rather than a formatting exercise: choosing the right element for an item is deciding what that item is.
Where it is required
Three regimes matter to UK companies. Corporation tax filings have required iXBRL accounts and computations since 2011; a PDF is not a valid filing. Companies House accepts and increasingly requires structured filing, and the reforms under the Economic Crime and Corporate Transparency Act require all accounts filings made on or after 1 April 2028 to be made by commercial software in iXBRL, closing the web and paper routes, alongside the removal of the abridged and filleted options. Separate filing from publication when reading about this: small and micro companies will have to file a profit and loss account, but are to be given an option to keep it off the public register, and the mechanism for that has not yet been announced. Listed companies' electronic reporting to the FCA is a third regime again, with its own scope and its own tagging requirements — the three should not be described as one universal obligation.
For listed companies, annual financial reports must be prepared in structured electronic format with the primary statements tagged, and consolidated IFRS accounts also require block tagging of the notes. Auditors are asked to consider the tagging as part of their work on the report.
What goes wrong
Tagging errors do not usually stop a filing being accepted. They produce data that says something the accounts do not, which is worse, because it feeds directly into the risk analysis that decides whether a return is examined:
- Wrong element selected — the classic being a figure tagged as turnover that is actually gross income, or a line tagged to a similarly named element with a different definition;
- Sign conventions reversed. The taxonomy has rules about whether a value is entered positive or negative, and a reversed sign turns a profit into a loss in the data even though the document reads correctly;
- Inconsistency between periods, so comparatives do not line up and apparent movements are artefacts of tagging;
- Over-reliance on automatic tagging. Software maps standard line items well and unusual ones badly, and the unusual ones are exactly where the risk is;
- Under-tagging the notes where the requirement extends to them, which is the most common finding in listed company reviews;
- Entity identifiers and contextual data — company number, period dates, currency, scale — entered incorrectly, which can misstate every number in the file by a factor of a thousand.
Why the data quality matters more than it used to
The reason to care is not the filing; it is what happens to the data afterwards. HMRC compares tagged figures across returns and against other filings automatically, and inconsistencies — turnover in the accounts differing from the VAT returns, wages differing from payroll submissions — are among the most common triggers for an enquiry.
Companies House data flows outward too: credit reference agencies, procurement assessors and data aggregators all consume it. A tagging error that understates net assets propagates into credit scores and supplier assessments, and correcting the source does not immediately correct everything downstream.
Running it properly
The practical approach depends on scale. Smaller companies are served well by accounts production software that generates the tagging as a by-product of preparing the accounts — provided somebody reviews the unusual items rather than accepting every automatic mapping. Larger and listed entities need a defined process: a tagging review as a distinct step, a record of judgements made on non-standard items, consistency checks against the prior period, and validation before submission.
Three controls catch most errors. Run the validation tools and clear every warning rather than only the errors. Review the tagged output as data — render the tags and read them — rather than reviewing the document, because the document looks right either way. And keep a tagging decisions log, so next year's preparer makes the same choices for the same items instead of introducing a movement that never happened.
Acumon prepares and tags accounts and computations through iXBRL services, statutory accounts and tax compliance work, including review of tagging prepared elsewhere. If your accounts are tagged automatically and nobody has ever looked at the output, that is a half-day worth spending before the next filing.