When the base ESEF taxonomy doesn't include concepts you need to report, you must create an extension taxonomy. This guide covers the technical requirements, best practices, and common pitfalls when extending the ESEF taxonomy.
Key Takeaways
- Minimize extensions - use base elements when possible
- Every extension must anchor to a base concept
- Follow ESMA naming conventions
- Include all required linkbases
- Test extension against validation rules
When to Extend
Before creating extensions, verify the concept doesn't exist in:
• ESEF base taxonomy
• IFRS taxonomy extensions
• Common practice elements
Extensions should be used only for truly company-specific concepts that have no semantic equivalent in the standard taxonomy. Over-extension reduces comparability and increases maintenance burden.
Extension Schema Structure
Your extension XSD must:
• Import ESEF base taxonomy
• Declare namespace correctly
• Define element types properly
• Follow XBRL 2.1 syntax
Example namespace pattern:
http://www.company.com/esef/2025
Anchoring Requirements
ESEF requires all extension elements to anchor to base taxonomy concepts:
• Use arcrole 'wider-narrower'
• Select the closest semantic match
• Anchor to non-deprecated elements
• One anchor minimum, multiple allowed
Anchoring enables comparability by linking custom concepts to standardized meanings.
Linkbase Files
Required linkbases: Label (mandatory, at least English), Presentation (mandatory for used elements), Definition (mandatory for dimensional elements), Calculation (if numeric with relationships). Missing linkbases cause validation errors.
Validation Testing
Before filing, validate your extension:
1. XSD syntax validation
2. XBRL 2.1 conformance
3. ESEF-specific rules (anchoring, etc.)
4. Linkbase consistency
5. Integration with base taxonomy
Use multiple validation tools to catch different types of errors.