Why file in iXBRL?
Not a formatting preference — a legal requirement, with automatic penalties for missing it. Here's the actual legal basis and the reasoning behind it.
It's a legal filing requirement, not a best practice
HMRC has required iXBRL-tagged accounts and computations for Company Tax Return attachments since 1 April 2011. A plain PDF is rejected outright — the return does not count as filed, regardless of whether the tax calculation itself is correct. See our CT600 & computations service.
Why regulators mandate it instead of accepting PDFs
Tagged data can be validated automatically the moment it's submitted and compared across thousands of filings systematically — something scanned or plain-text accounts were never built for. That's the real driver: making corporate and tax data usable at scale, not simply digitising paperwork for its own sake.
What happens if you don't
A CT600 submitted without valid iXBRL tagging isn't received as a late or imperfect filing — it isn't received at all. That triggers the same fixed late-filing penalties as never submitting anything: £200 immediately, rising by a further £200 after three months, as of the rates in force from April 2026. See our late filing penalties update.
Companies House is a different story — for now
Unlike HMRC, Companies House currently accepts several accounts formats, including non-iXBRL routes. A move to mandatory software-only iXBRL filing was planned for April 2027 but was paused by Companies House in January 2026, with no new date set. See our full update.
Frequently asked questions
Is filing in iXBRL optional if I file on paper?
No — HMRC only accepts paper CT600 filings in narrow circumstances (such as a reasonable religious objection or insolvency). Almost every company must file online with iXBRL-tagged attachments.
Can I use a normal PDF instead of iXBRL for my CT600?
No — HMRC explicitly rejects plain PDF submissions for CT600 attachments. The return does not count as filed, which triggers the same penalties as not filing at all.
Does Companies House require iXBRL the same way HMRC does?
Not currently. Companies House accepts several accounts formats today; a move to mandatory software-only iXBRL filing was paused in January 2026 with no new date set. See our update.
Don't want to risk a rejected filing?
We tag and validate before you submit — send your accounts over for a free, fixed quote.
Get a free quote