iXBRL Tagging for Company Accounts and CT600: What You Need to Know

Every company filing a CT600 online must submit its accounts and tax computation in iXBRL format (Inline eXtensible Business Reporting Language). This requirement has been in place since 2011, and there's no opt-out — if you're filing Corporation Tax, you need iXBRL.

But what actually is iXBRL tagging? What gets tagged? And how do you generate compliant files without paying hundreds of pounds to an accountant or buying expensive software?

What Is iXBRL Tagging?

iXBRL is a way of embedding machine-readable data tags within an HTML document. When you look at an iXBRL file in a browser, it looks like a normal set of accounts. But behind the scenes, each financial figure is wrapped in an XML tag that tells HMRC's computer exactly what that number represents.

For example, the number "£50,000" in your profit and loss account might be tagged as:

<ix:nonFraction name="uk-gaap:TurnoverRevenue" ...>50,000</ix:nonFraction>

This tells HMRC's system: "This number is turnover/revenue, reported under UK GAAP."

Why Does HMRC Require iXBRL?

HMRC uses iXBRL tags to:

Automatically process millions of Corporation Tax returns
Cross-reference figures between accounts and CT600 boxes
Run risk assessments to flag returns for enquiry
Compile national statistics from company financial data

Without iXBRL, HMRC staff would need to manually read and interpret every set of accounts — an impossible task given the volume of filings.

What Needs to Be Tagged?

When you file a CT600, you must submit two iXBRL documents:

1. Company Accounts (iXBRL)

Your statutory accounts — profit and loss (or income statement), balance sheet, and notes — must be tagged using the appropriate taxonomy:

Accounting Framework	Taxonomy
FRS 102 Section 1A (small companies)	UK-GAAP (FRS 102) taxonomy
FRS 105 (micro-entities)	UK-GAAP (FRS 105) taxonomy
FRS 102 (full)	UK-GAAP (FRS 102) taxonomy
IFRS	EU IFRS taxonomy

For most small companies filing with Taxpipe, you'll use either FRS 105 (micro-entities) or FRS 102 Section 1A (small companies).

What gets tagged in the accounts:

Company name, registration number, accounting period dates
Every line item in the profit and loss account (turnover, cost of sales, administrative expenses, etc.)
Every line item on the balance sheet (fixed assets, debtors, creditors, share capital, reserves)
Director names and key notes
Accounting policies

2. Tax Computation (iXBRL)

Your Corporation Tax computation — showing how you get from accounting profit to taxable profit — must also be in iXBRL format. This includes:

Profit per accounts
Disallowable expenses (add-backs)
Non-taxable income (deductions)
Capital allowances
Trading losses brought forward or carried back
Final taxable profit and tax calculation

The tax computation uses the HMRC Corporation Tax computation taxonomy (a different taxonomy from the accounts).

Common iXBRL Tagging Errors

If your iXBRL tagging is wrong, HMRC will reject your filing. Here are the most common problems:

1. Missing Mandatory Tags

Certain tags are mandatory. If they're missing, HMRC's system rejects the submission outright:

Company registration number — must be tagged and match Companies House records
Accounting period start and end dates — must match the CT600 period
Balance sheet date — must match the period end
Director approval date — accounts must show when directors approved them

2. Inconsistent Periods

The dates in your iXBRL accounts must exactly match the period on your CT600. If your CT600 says the period is 1 April 2024 to 31 March 2025, but your accounts are tagged with different dates, the filing will be rejected.

3. Wrong Taxonomy Version

HMRC periodically updates the taxonomies. Using an outdated taxonomy version can cause rejection. As of 2025, you should be using:

UK GAAP: 2024-01-01 version or later
CT computation: 2024-01-01 version or later

4. Sign Errors

In iXBRL, most values are tagged as positive numbers, with the tag itself indicating whether it's a debit or credit. A common error is double-negating — entering a negative number for a tag that HMRC already interprets as negative (like a loss). This makes the figure appear positive to HMRC's system.

5. Decimal Precision

Financial figures should be tagged in whole pounds (no pence) for most items. Tagging "50000.00" instead of "50000" can cause validation warnings, though most systems handle this automatically.

How to Generate iXBRL Files

You have several options:

Option 1: Commercial Accounting Software

Packages like Xero, QuickBooks, and Sage can generate accounts, but most don't produce iXBRL output directly. You typically need to export data and use a separate filing tool.

Option 2: Specialist iXBRL Tagging Software

Tools designed specifically for iXBRL tagging include products from IRIS, Thomson Reuters, and others. These are powerful but expensive — typically £500–£2,000+ per year — and designed for accountancy practices rather than individual companies.

Option 3: CT600 Filing Software With Built-In iXBRL

This is the most practical option for small companies filing their own returns. Software like Taxpipe generates both the CT600 and the iXBRL accounts/computation in a single workflow. You enter your financial data through a guided wizard, and the software handles all the tagging automatically.

Option 4: HMRC's Own Service (Now Ended)

HMRC previously offered a free CT600 filing service that handled iXBRL generation. This service has been withdrawn — since April 2026, all companies must use commercial software. See our guide to HMRC free filing alternatives.

Validating Your iXBRL Before Filing

Before submitting to HMRC, you should validate your iXBRL files. Common validation checks include:

Business Validation Rules (BVR)

HMRC applies over 100 Business Validation Rules to every CT600 submission. These check for logical consistency:

Does the tax computation match the CT600 boxes?
Do the accounting period dates match across all documents?
Are mandatory fields present?
Do the numbers add up (e.g., does total assets equal total liabilities plus equity)?

If any BVR check fails, HMRC returns an error and the filing is rejected.

XML Schema Validation

The iXBRL files must conform to the XML schema for the relevant taxonomy. Malformed XML, missing closing tags, or invalid attribute values will cause immediate rejection.

HMRC's Test Service

HMRC provides a test gateway where you can submit filings to check for errors before making a live submission. Professional software tools typically run these checks automatically before allowing you to submit.

The Filing Process

Here's how iXBRL fits into the overall CT600 filing process:

Prepare accounts — profit and loss, balance sheet, notes
Prepare tax computation — adjustments from accounting profit to taxable profit
Complete CT600 form — enter the summary figures
Generate iXBRL — tag the accounts and tax computation
Attach to CT600 — the iXBRL files are sent as attachments with the CT600
Submit via HMRC Gateway — using your company's Government Gateway credentials
Receive confirmation — HMRC sends an acknowledgement (or rejection with error details)

Do Micro-Entities Need Full Tagging?

Yes, but less of it. Micro-entity accounts (FRS 105) are simpler than small company accounts (FRS 102 Section 1A), so there are fewer items to tag. A typical micro-entity balance sheet might have only 10–15 tagged items, compared to 30–50 for a small company.

The tagging requirement itself doesn't change — it's still mandatory — but the volume of tags is lower because the accounts themselves are simpler.

What Happens If iXBRL Is Wrong?

If your iXBRL files fail validation:

HMRC rejects the submission and sends an error report
Your CT600 is not filed — the clock keeps ticking on any deadline
You need to fix the errors and resubmit
If the resubmission is after the deadline, you could face late filing penalties (£100 for 1 day late, £200 for 3 months late, and escalating from there)

This is why using software that validates before submission is so important — it catches errors before they become penalties.

Skip the iXBRL Headache — Use Taxpipe

Generating compliant iXBRL files is technically complex and error-prone if you're doing it manually. Taxpipe generates fully tagged iXBRL accounts and tax computations automatically as part of the CT600 filing process. You answer questions in plain English, and we handle all the taxonomy mapping, validation, and submission. Just £59 + VAT for a complete, HMRC-ready filing. Start filing now →

iXBRL Tagging for Company Accounts and CT600: What You Need to Know