Earlier this week XBRL International’s Rendering Working Group published the Proposed Recommendation for Inline XBRL. The standard is now on a track which will reach Recommendation by the end of March, in good time for HMRC’s Corporation Tax filing service to go live in the autumn.

Inline XBRL – or iXBRL – will be required for most of the Corporation Tax returns after April 2011. And, down the road, we expect Companies House to mandate iXBRL for filing of Accounts. What will this mean for the companies who have to file ?

I don’t think that the UK is going to see joint filing – making a single filing which will handle both Companies House and HMRC obligations in one go – for some time, if at all. Current legislation provides different filing dates for the two agencies and different filing content. Thus, for instance, companies which are allowed to file Abbreviated Accounts with Companies House will generally need to provide full income statements to HMRC.

But Inline XBRL does provide a neat way to simplify the filing.

Inline XBRL is an HTML document – a web page, if you like – which contains extra information, the extra information that you need to convert the data you see in your browser into an XBRL report. But financial reports are sometimes so large that they won’t fit into a single HTML document. So an iXBRL filing is allowed to consist of, potentially, a series of related HTML documents. This complete “document set” is used by the government agency to generate a single XBRL report.

Inline XBRL doesn’t mind much what goes into each part of the document set, so long as all the information is there. Equally, it doesn’t stop individual components of the document set from being complete document sets in their own right. In simple terms, iXBRL document “A” might be an income statement, and iXBRL document “B” might be a balance sheet. Together they would represent the full Accounts required by HMRC. Document “B”, on its own, would represent the Abbreviated Accounts required by Companies House.

In a world where major accounting firms routinely download clients’ Accounts from Companies House to make sure they have the correct version when preparing the tax return, the ability to use a single document set for different purposes is an important aid to clarity and accuracy.