iXBRL is the step at the end of the job that nobody in the practice wants to own, usually because nobody is quite sure what it is doing.

The accounts are finished and signed off, and then they have to be turned into something HMRC’s and Companies House’s systems can read. That conversion is iXBRL. For most firms it happens inside the accounts software at the press of a button, right up until a filing is rejected and someone has to work out what actually went wrong.

This post explains what iXBRL is, why HMRC insists on it, how it differs from XBRL, and what has to be tagged.

What is iXBRL?

iXBRL, or Inline XBRL, is a format that embeds machine-readable data tags inside a human-readable accounts document. The accounts still look normal to a person, but each key figure also carries a hidden tag from a standard taxonomy, so HMRC and Companies House systems can read the numbers automatically.

The tags are the whole point. Think of them as standardised labels attached to each figure, drawn from an agreed list called a taxonomy, so that “turnover” in one company’s accounts is recognised as the same concept as “turnover” in anothers. That is what lets the figures be compared and risk-assessed by software rather than by a person reading the PDF.

iXBRL vs XBRL

XBRL and iXBRL are closely related, and the difference is easy to state.

XBRL is pure machine-readable data. It is a file of tagged figures that software can read but a person cannot easily use. iXBRL, or Inline XBRL, takes those same tags and embeds them inside a normal, human-readable document, so one file works for both the reader and the software. UK filing uses iXBRL for exactly that reason: the accounts stay readable while carrying the data underneath.

Why does HMRC require iXBRL?

HMRC has required iXBRL for a long time, and the requirement is not optional.

Since legislation that came into force on 1 January 2010, companies must file their Company Tax Return online, with both the accounts and the tax computations in iXBRL. Tagging the figures lets HMRC read and risk-assess returns automatically, which is why the format is mandatory rather than a preference. For most companies this is the main reason iXBRL exists in their workflow at all.

What iXBRL tagging actually is

Tagging is the act of mapping each figure in the accounts to the right concept in a taxonomy.

In practice the accounts software does most of this automatically, matching the numbers to the relevant taxonomy. The UK FRS taxonomy covers accounts prepared under FRS 101, FRS 102 and FRS 105, there is a UK-IFRS taxonomy for companies reporting under IFRS, and a separate UKSEF taxonomy for listed companies’ annual financial reports. The preparer’s job is usually to check the automatic tagging rather than to tag from scratch, and that review is where most errors are caught.

What has to be tagged, and for whom

The requirement depends on who you are filing to.

• HMRC: accounts and computations filed with the Company Tax Return must be in iXBRL. This covers most companies in the corporation tax regime.
• Companies House: digital filing is widely used already, with around 2.2 million companies filing in XBRL, and the direction of travel is toward software-only filing from April 2028, after which paper and manual web filing of accounts are withdrawn.
• Listed companies: those with securities on a UK regulated market file their annual financial report in a tagged XHTML format under the UKSEF taxonomy.

The practical takeaway is that iXBRL is the standard way accounts reach both filing destinations.

What happens when iXBRL tagging goes wrong

Most of the time iXBRL is invisible. When it fails, it tends to fail at the worst moment, which is a filing rejected close to a deadline.

Rejections usually come from a handful of causes: a figure tagged to the wrong taxonomy concept, a mandatory item left untagged, or accounts prepared on a taxonomy version that does not match what the filing expects. Because the tagging is automatic, these are easy to miss unless someone reviews the tags rather than just the face of the accounts.

The practical defence is a review step before filing, not after the rejection. Checking that the key figures carry the right tags, on the current taxonomy, is far quicker than unpicking a bounced submission three days before a deadline.

Where Active fits

iXBRL is only painful when it is a separate step bolted onto the end of the job.

When the accounts are produced in Active, the iXBRL tagged output is produced as part of preparing the accounts, ready to file with Companies House and HMRC, rather than being a conversion you run afterwards and hope passes. If you want to see how that works on your own clients, book a demo and we will walk you through it: [Book a demo URL].

Frequently asked questions

What is iXBRL tagging?

iXBRL tagging is the process of attaching standardised data tags to the figures in a set of accounts, mapping each one to a concept in an agreed taxonomy. It is usually done automatically by accounts production software, with the preparer reviewing the tags before the accounts are filed with HMRC or Companies House.

What is the difference between XBRL and iXBRL?

XBRL is a machine-readable data file that software can read but people cannot easily use. iXBRL, or Inline XBRL, embeds the same tags inside a normal human-readable document, so one file serves both the reader and the software. UK filing to HMRC and Companies House uses iXBRL.

Do I have to file iXBRL with Companies House?

Company Tax Return accounts filed with HMRC must be in iXBRL. Filing to Companies House is currently done in several ways, but digital filing is already widespread and the move to software-only filing from April 2028 will make a tagged digital format the standard route for company accounts.

What software produces iXBRL accounts?

Most UK accounts production software generates iXBRL output as part of preparing a set of statutory accounts, so the tagging is built into the file rather than added separately. The quality of the automatic tagging, and how easy it is to review before filing, is one of the things that varies between tools.

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