iXBRL Accounts: What Are They and Why You Need Them for Your CT600
When you file your CT600 Corporation Tax return, HMRC requires you to attach your company accounts in iXBRL format. If you've never heard of iXBRL, don't worry — most company directors haven't. This guide explains what it is and how to handle it.
What Is iXBRL?
iXBRL stands for Inline eXtensible Business Reporting Language. It's a way of tagging financial data in your accounts so that HMRC's computers can read and process the numbers automatically.
Think of it as a translation layer:
- You see: A normal-looking set of accounts (profit & loss, balance sheet)
- HMRC's system sees: Tagged data it can automatically validate and process
An iXBRL file is essentially an HTML document with invisible tags embedded in it. When you open it in a browser, it looks like a normal PDF-style document. But behind the scenes, every financial number is tagged with a standard identifier.
Why Does HMRC Require iXBRL?
Since April 2011, HMRC has required all Corporation Tax returns to include accounts in iXBRL format. The reasons:
- Automated processing — HMRC can check millions of returns without human intervention
- Cross-referencing — Numbers in your accounts can be automatically compared with your CT600 boxes
- Risk assessment — HMRC's algorithms use the tagged data to flag anomalies
- International standards — XBRL is used by tax authorities worldwide
What Needs to Be in iXBRL Format?
When filing your CT600, you need to attach:
- Company accounts — Your statutory accounts (profit & loss account and balance sheet) in iXBRL format
- Tax computation — The calculation showing how you arrived at your corporation tax figure, also in iXBRL format
Both documents must use the correct HMRC taxonomy — a standardised set of tags that HMRC recognises.
Which Taxonomy Do You Use?
The taxonomy depends on your company size:
| Company Type | Taxonomy | Standard |
|---|---|---|
| Micro-entity | FRS 105 | UK-GAAP Micro-entities |
| Small company | FRS 102 Section 1A | UK-GAAP Small Entities |
| Medium/large company | FRS 102 | UK-GAAP Full |
| Listed company | IFRS | International Financial Reporting Standards |
Most small limited companies use either FRS 105 (micro-entity) or FRS 102 Section 1A (small company).
Micro-Entity Thresholds (FRS 105)
You qualify as a micro-entity if you meet two of three criteria:
For financial years beginning on or after 6 April 2025 (Companies (Accounts and Reports) (Amendment etc.) Regulations 2024):
- Turnover not more than £1,000,000
- Balance sheet total not more than £500,000
- Not more than 10 employees
For earlier financial years:
- Turnover not more than £632,000
- Balance sheet total not more than £316,000
- Not more than 10 employees
FRS 105 vs FRS 102 comparison →
How to Generate iXBRL Accounts
Option 1: Accountant (£300-£800/year)
Most accountants use professional software (Sage, Xero, TaxCalc) that generates iXBRL automatically. Your accounts preparation fee typically includes iXBRL generation.
Option 2: Software Like Taxpipe (£59)
Taxpipe handles the full CT600 filing process, including iXBRL generation, for £59 per filing. Here's how it works depending on your accounts type:
- Micro-entity (FRS 105): Taxpipe generates the iXBRL accounts automatically from your answers — no separate accounts software needed.
- Small company (FRS 102 Section 1A): You prepare your accounts in your own software (or with your accountant), then upload the iXBRL file to Taxpipe, which attaches it to your CT600 submission.
- iXBRL tax computation: Generated automatically for all filing types.
- Direct HMRC submission via your own Government Gateway credentials.
All for £59 per filing.
Option 3: Manual Tagging (Not Recommended)
Technically, you could create iXBRL files manually using an XML editor. In practice, this requires deep technical knowledge of the XBRL taxonomy and is not practical for most users.
Common iXBRL Errors
HMRC's systems validate your iXBRL before accepting your filing. Common rejection reasons:
Error 9113: No iXBRL Accounts Attached
Your CT600 submission is missing the accounts attachment. Every return must include iXBRL accounts unless you've provided a valid reason for not doing so.
Error 9965: No Tax Computation Attached
Similar to 9113, but for the tax computation document. You must include a tax computation showing how you calculated the corporation tax figure.
Taxonomy Mismatch
Using the wrong taxonomy version or tagging standard. Make sure your iXBRL uses the current HMRC-accepted taxonomy version.
Validation Errors
Numbers in your iXBRL accounts don't match the CT600 boxes. For example, if Box 145 (total turnover) says £100,000 but your iXBRL accounts show turnover of £90,000, HMRC will flag an inconsistency.
What Does an iXBRL File Look Like?
Here's a simplified example of what's happening inside an iXBRL accounts file:
<p>Turnover: <span data-xbrl-tag="uk-gaap:Turnover">£100,000</span></p>
<p>Cost of sales: <span data-xbrl-tag="uk-gaap:CostOfSales">£60,000</span></p>
<p>Gross profit: <span data-xbrl-tag="uk-gaap:GrossProfit">£40,000</span></p>
Each number has an invisible tag that tells HMRC exactly what it represents. The tags follow the UK GAAP taxonomy — a standardised dictionary of financial terms.
Do I Need to Understand iXBRL?
No. If you use software like Taxpipe or hire an accountant, iXBRL is generated automatically. You never need to see or edit the iXBRL code.
The only reason to understand iXBRL is if you're troubleshooting a filing rejection — and even then, the error messages from HMRC usually point to the specific issue.
iXBRL and Companies House
Companies House also accepts accounts in iXBRL format (and has since 2010). If your accounts are already in iXBRL for your CT600, you can often use the same file for your Companies House filing.
However, Companies House has slightly different requirements:
- Different accepted taxonomies
- Different validation rules
- Separate submission portal
File Your CT600 with Automatic iXBRL
Stop worrying about iXBRL. Taxpipe handles everything — accounts, tax computation, CT600 return, and HMRC submission — for £59.