If you've filed a Corporation Tax return or a set of statutory accounts in the UK recently, there's a good chance the word "iXBRL" appeared somewhere in the process — usually attached to a warning that your filing would be rejected without it. Here's what it actually is, and why it exists.

The short version

iXBRL stands for Inline eXtensible Business Reporting Language. It's a way of publishing a financial document — a set of accounts, a tax computation — so that it's readable two ways at once: as an ordinary document for a person, and as structured data for a computer. Each figure in the document, like Turnover £1,284,600^{uk-bus:Turnover}, carries an invisible tag that tells software exactly what that number represents, what currency it's in, and which reporting period it covers.

Why a computer needs to read your accounts

Before iXBRL, HMRC and Companies House received accounts as PDFs or scanned images — readable by a person, meaningless to a database. Every figure that needed to be checked, compared, or fed into risk-assessment systems had to be manually rekeyed by someone on the other end. iXBRL removes that step: the turnover figure in your accounts and the turnover figure HMRC's systems record are, by construction, the same number, tagged the same way, with no rekeying and no transcription errors.

Where it's required today

HMRC has required iXBRL for Corporation Tax return attachments since 2011 — your accounts and tax computations both need to be tagged before they can be attached to a CT600. Companies House currently accepts iXBRL alongside other filing formats; a move to mandatory software-only iXBRL filing was planned for 1 April 2027, but Companies House paused that reform in January 2026, with no new date yet set. See our full update below.

What tagging actually involves

Tagging means mapping each figure and disclosure in your accounts to a defined element in a taxonomy — a standardised vocabulary maintained by the Financial Reporting Council for UK GAAP (FRS 101, FRS 102, FRS 105) or by the IFRS Foundation for international standards. Get the wrong element, the wrong currency unit, or the wrong reporting context, and the filing can be rejected or, worse, accepted with incorrect data attached to your company record. That's the part of the process worth getting a specialist to check.

See our tagging services →

article · 7 min read · updated

Companies House software-only filing: the 2027 deadline has been paused

Published 18 June 2026 · Updated 20 July 2026

If you've seen this page before, the headline has changed for a reason worth explaining. Companies House had set a firm date — 1 April 2027 — for requiring every company on the register to file accounts through commercial software with iXBRL tagging. That date is no longer in force.

What actually changed

Companies House confirmed in January 2026 that the accounts filing reforms originally planned for April 2027 have been paused and remain under review. No new date has been set. Companies House has committed to giving businesses at least 21 months' notice before any new timeline takes effect, so even once a date is confirmed, there will be a substantial runway before it applies.

What this doesn't affect

This pause is specific to Companies House's accounts filing mandate under the Economic Crime and Corporate Transparency Act 2023. It has no bearing on HMRC's iXBRL requirement for CT600 filings, which has applied since 2011 and is unaffected. It also doesn't change the separate closure of the joint HMRC/Companies House filing service on 31 March 2026 — see our note on that below.

Why we're not telling you to relax

A paused deadline removes a specific date, not the direction of travel. Companies filing through commercial software with correct iXBRL tagging today have nothing to change. Companies still relying on paper, the Companies House web service, or accounts with minimal or no tagging now have more time to plan the transition properly — which is a better position than scrambling ahead of a fixed date, not a reason to leave it indefinitely.

What we're watching

We'll update this page the moment Companies House confirms a new timeline.
If your current process already produces valid iXBRL for Companies House, there's no urgency — the underlying capability still matters, just without a hard deadline attached right now.
If you're not sure whether your accounts are properly tagged today, that's worth checking regardless of the mandate's timing.

See our tagging services →

article · 4 min read

Joint HMRC and Companies House filing has closed — what changes for you

Published 20 July 2026

If your accounts process was built around submitting to HMRC and Companies House in a single joint filing, it needs updating — that route no longer exists.

What joint filing used to offer

The joint filing service let eligible companies prepare their accounts once and submit to both HMRC (as part of the CT600 process) and Companies House at the same time, saving a separate submission step.

What changed

The joint HMRC and Companies House filing service closed on 31 March 2026^date. Company tax returns must now be filed separately from Companies House accounts, using commercial software for each.

What to check now

If your accountant, bookkeeper, or in-house process was set up around joint filing, confirm it has been updated for separate submission before your next filing is due. This is exactly the kind of change that's easy to miss if you're following a process that hasn't been reviewed recently — and it's a common gap we find when we review a company's filing setup for the first time.

Ask us to review your filing setup →

article · 5 min read

Corporation Tax penalties doubled in April 2026 — what directors need to know

Published 20 July 2026

If the CT600 late filing penalty figures you remember are from before this year, they're now out of date. HMRC's fixed penalty rates doubled from 1 April 2026 — the first change since 1998.

The new rates

Fixed penalties are now £200^{day 1} immediately, rising by a further £200^{3 months} after three months. For companies that have filed late in three consecutive years, the fixed penalty rises further still, to £1,000, or £2,000 if the return is more than three months late.

When	Old rate (pre-2026)	New rate (from April 2026)
Immediately on late filing	£100	£200
After 3 months	£100	£200
Third consecutive late year	£500 / £1,000	£1,000 / £2,000

What didn't change

The tax-geared penalties — 10% of unpaid Corporation Tax once a return is six months overdue, and a further 10% at twelve months — are unchanged, as is the underlying CT600 deadline (12 months from period end) and the tax payment deadline (9 months and 1 day). Only the fixed penalty amounts moved.

Applies even without tax owed

The fixed penalties apply automatically to any late filing, regardless of whether tax is owed — a dormant company or one with a nil-tax return still faces the full £200 fixed penalty under the new rates.

See our CT600 & computations service →

article · 5 min read

Full tagging vs minimum tagging: which does your filing need?

Published 25 June 2026 · Updated 15 July 2026

One of the most common questions we get asked is whether a set of accounts needs "full" or "minimum" tagging — and the honest answer is that it depends entirely on where you're filing and what kind of company you are.

Minimum tagging, in practice

Companies House defines a core set of elements that must be tagged in any iXBRL accounts filed with them — company name, registration number, balance sheet date, and the key primary statement totals such as Net assets^{uk-gaap:NetAssetsLiabilities}. For a straightforward set of micro-entity or small company accounts, tagging just this core set is often enough to file successfully with Companies House. It's faster and less expensive, because far fewer individual data points need to be mapped.

Full tagging, in practice

HMRC's requirements go further. Because Corporation Tax computations need every relevant figure to be extractable — not just the headline totals — full tagging maps every note, accounting policy, and comparative figure to its taxonomy element. Larger or more complex Companies House filings, where abridged accounts aren't an option, also typically need this level of detail.

A rule of thumb

If you're filing a CT600 with HMRC, you need full tagging on both the accounts and the computations. If you're a micro-entity or small company filing straightforward accounts with Companies House only, minimum tagging is usually sufficient — though once software-only filing is eventually confirmed (see our update on the paused mandate) and abridged accounts are phased out for small companies, more filings will need the fuller disclosure set regardless of tagging depth.

If you're not sure which applies to your accounts, that's a five-minute question for us to answer, not something you need to work out from the taxonomy documentation yourself.

Compare tagging tiers & pricing →

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Have a question these guides don't answer?

Check our FAQ, or send us a message directly — we're happy to talk through your specific filing.

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Frequently asked questions

How often is this page updated?

Whenever a rule changes that affects UK iXBRL filing — like the Companies House mandate pause or the CT600 penalty change covered above — rather than on a fixed schedule.

Are these articles specific to iXBRL.UK, or general guidance?

General guidance based on current HMRC and Companies House rules, written independently of our services — see our iXBRL Guide section for the evergreen fundamentals.

Where do I go for the basics rather than news updates?

Our iXBRL Guide covers what iXBRL is, who must file, and when — this Insights page is for timely updates and specific topics.