What Is iXBRL? A Comprehensive Guide for UK Businesses

iXBRL is a digital format that combines human-readable accounts with machine-readable financial data. 

UK companies use iXBRL when submitting certain financial information, including Corporation Tax computations and accounts, to tax authorities. 

This guide explains what iXBRL is, how it works, and why it matters for your company.

Key points

• iXBRL is used for electronic company filings 💻
  You have to file company accounts and Corporation Tax computations in this format so tax authorities like HMRC can validate the information automatically.
• You don't need to create iXBRL files yourself ⚙️
  Accounting software, tax software, or an accountant can handle the tagging and generate the required files for you.
• Incorrect tagging can cause filing problems ⚠️
  Outdated taxonomies, missing tags, mismatched totals, or the wrong accounting standard can cause an iXBRL file to fail validation.
• ANNA handles the technical side of your tax filing 🚀
  ANNA brings you automated bookkeeping, prepares your Corporation Tax return and Annual Accounts, and even files them for you, so you never have to worry about iXBRL files.

What is iXBRL? 

iXBRL stands for Inline eXtensible Business Reporting Language. It's a variant of a broader standard called XBRL, used by tax authorities and regulators worldwide to structure financial data. 

The ‘inline’ part is the main difference. XBRL is built for computers only, whereas iXBRL embeds the same machine-readable tags inside a document that looks like a normal, readable set of accounts when you open it.

So you get something that looks exactly like a standard profit and loss statement and balance sheet, but every number carries an invisible tag identifying what it represents. 

Why a PDF isn't good enough

Before iXBRL became compulsory in 2011, HMRC staff checked figures manually, which was slow and left plenty of room for error.

iXBRL solves that by making the data machine-readable. HMRC's systems can automatically validate your figures, cross-check them against your CT600, and flag anomalies without anyone opening the file. 

A plain PDF gets rejected if you try to attach it to a Corporation Tax return, as it doesn't contain the structured data HMRC's systems need.

What needs to be in iXBRL format

Two documents generally need to be included in iXBRL when dealing with HMRC and Companies House:

• Your statutory accounts, including the profit and loss statement and balance sheet, filed with Companies House and often attached to your CT600
• Your Corporation Tax computation, the document showing how you got from your accounting profit to your taxable profit and the tax you owe, filed alongside your CT600

Until recently, you could file to both HMRC and Companies House through one joint service, but it’s no longer available. You now need to file separately: your Company Tax Return with HMRC using commercial software, and your accounts with Companies House.

There's no size exemption. All companies have to use iXBRL, including micro-entities filing simplified accounts under FRS 105, small companies filing full or abridged accounts, and dormant companies. The only aspect that changes based on size is the taxonomy that applies.

🧠 Good to know: 

iXBRL uses different taxonomies depending on what you're filing. Company accounts generally use the UK GAAP taxonomy, while tax computations attached to your CT600 use a separate CT taxonomy. Your software handles this mapping for you, so you don't need to select the taxonomy yourself.

How your iXBRL file gets created

Luckily, you don’t have to learn to use a code editor or tag figures by hand. Here’s how iXBRL files can be created:

Accounting software with built-in tagging

Most modern accounting software, including packages built for small companies, generates the iXBRL file automatically once you have finalised your accounts. 

The software already knows which figures map to which tags, so you fill in your numbers as normal and the tagging happens in the background.

Your accountant's practice software

If an accountant prepares your accounts and Corporation Tax return, they will almost certainly use professional practice software that handles iXBRL conversion. This is the most hands-off option as far as you’re concerned.

Standalone tagging services

If you have accounts in a spreadsheet or a format your software can’t convert, you can use a standalone tagging service to turn a set of figures into a valid iXBRL file. 

These services are a less common option, but useful if your setup is unusual; for example, if your first set of accounts was prepared outside any accounting package.

Whichever of the three routes you use, you’ll know it worked when HMRC or Companies House confirms that your submission was accepted.

Common reasons an iXBRL filing gets rejected

A rejected filing is more common than people expect, and it usually means that the structure of the file doesn’t match HMRC or Companies House standards. 

Typical problems include:

• Wrong taxonomy version: Taxonomies get updated periodically, and software that hasn’t been updated can tag a file against an old version.
• Mismatched totals: If a subtotal in your accounts doesn’t match the figures beneath it, even by a rounding difference, the file can fail validation.
• Missing mandatory tags: Certain elements, such as the company's registered number or the accounting period dates, require a corresponding tag. If your software hasn’t picked these up correctly from your company details, the file can be incomplete.
• Incorrect accounting standard applied: Filing under FRS 105 when your accounts were prepared under FRS 102, or vice versa, causes the wrong taxonomy to apply, and figures to fail their checks.

If a filing bounces back, the rejection message will usually specify the tag or section causing the issue, which your software or accountant can then fix and resubmit.

How ANNA helps with iXBRL and Corporation Tax filing

If you do your own accounts, creating iXBRL files is one more technical step to deal with. ANNA handles it for you, keeping your data in one place and filing your accounts and Corporation Tax return.

Here’s what you get with ANNA:

• Automatic iXBRL preparation: Your company accounts and Corporation Tax information are prepared in the format required for electronic filing
• Corporation Tax filing: ANNA prepares and files your CT600 directly with HMRC, so you don't have to worry about submission
• Annual Accounts filing: Your Annual Accounts are prepared alongside your Corporation Tax return to keep the two filings consistent
• Automated bookkeeping: ANNA’s Auto Accountant categorises your transactions automatically and keeps your financial records up to date throughout the year
• Live Corporation Tax estimate: Get an ongoing estimate of your Corporation Tax liability rather than waiting until year-end to find out what you owe
• Built-in business account: Your business account, bookkeeping, and tax information are connected within the same platform
• Automatic transaction imports: Transactions from your ANNA business account feed directly into your bookkeeping to reduce manual data entry
• Receipt Scanner: Capture receipts and keep supporting records connected to the relevant transactions
• VAT support: Manage VAT alongside your other business tax obligations, with MTD-compatible VAT filing built in
• Payroll: Run payroll in the same place as your business account and bookkeeping, instead of a standalone payroll tool
• 24/7 support: Get help within the app if you need assistance with your bookkeeping or tax filing

Register today and let ANNA handle your Corporation Tax filing, iXBRL and all.