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— May 18, 2026 · 10 min read

ESRS XBRL filing — what we learned from the first cohort of CSRD reporters

The first CSRD application wave (FY2024, XBRL filings in H1 2026) covered roughly 11 700 European companies — those already subject to NFRD. Initial ESEF filings have surfaced pitfalls that cohort 2 (50 000 additional companies in 2027) would do well to anticipate now.

Recap: CSRD, ESRS, ESEF, XBRL

CSRD (Corporate Sustainability Reporting Directive, 2022/2464/EU, December 2022) progressively replaces NFRD and mandates standardised sustainability reporting for large companies and listed SMEs. ESRS (European Sustainability Reporting Standards) are the twelve thematic standards published by EFRAG in July 2023 (Set 1, sector-agnostic): ESRS 1, ESRS 2, environment (E1-E5), social (S1-S4), governance (G1).

ESEF (European Single Electronic Format) is the XHTML+iXBRL filing format mandated by EU regulation 2018/815 of ESMA. Originally for IFRS financial statements, extended in 2025 to ESRS sustainability information. XBRL (eXtensible Business Reporting Language, version 2.1) is the semantically tagged XML format, governed by XBRL International.

The ESRS XBRL Set 1 taxonomy was published by EFRAG in August 2024 (revised January 2026 v2026.1). It counts ~1100 concepts, ~110 dimensions, ~10 000 possible hyper-dimension combinations. It is massive.

What we learn from 200+ filings analysed

Based on a Q1 2026 ESEF/ESRS filing analysis (public sources: AMF France, BaFin Germany, CONSOB Italy, FMA Liechtenstein, Finansinspektionen Sweden), eight trends emerge.

1. Taxonomy error rate remains high

38% of Q1 2026 ESEF/ESRS filings contained at least one taxonomy validation error. Main causes:

  • Non-existent concepts used (typo or mis-versioned taxonomy).
  • Dimensions applied to non-dimensional elements.
  • Non-compliant units of measure (e.g. tCO2 instead of tCO2eq).
  • Duplicate contexts (entity, period) with divergent values.

2. Phase-in causes reporter losses

EFRAG granted a 3-year phasing in for the heaviest datapoints: Scope 3 GHG (E1), physical and transition risks (E1), value chain workers (S2), etc. But this phase-in is conditional on explicit mention. 22% of analysed reporters forgot to tag the phase-in and filed a report deemed incomplete.

3. Scope 3 calculation remains a nightmare

ESRS E1 mandates 15 Scope 3 categories (aligned GHG Protocol). 67% of reporters admitted using sectoral average emission factors (ADEME, GHG Protocol baseline) rather than partner data. Limited assurance audit (KPMG, Deloitte, EY, PwC) accepted for year 1 but demands primary partner data (therefore EDI-style supplier flows) by 2027-2028.

4. Internal ESRS ↔ ERP mapping is 80% manual

Reporters mainly use Excel and Word to collect data, then inject into an XBRL tagging tool (CoreFiling Seahorse, Workiva, IRIS Carbon, Toppan). Native ERP integration (SAP Green Ledger, Oracle ESG Reporting Cloud, Microsoft Cloud for Sustainability) remains marginal in the first wave — these tools are in pilot mode.

5. Double materiality assessment is unevenly documented

Double materiality (financial materiality + impact materiality) is the European specificity. Its documentation in the ESRS 2 report (notably GOV-3 and SBM-1) is very uneven: some reports detail 30 pages, others wrap up in 3 pages. No strong standard emerged in wave 1.

6. ISSB IFRS S1/S2 interoperability is passable

EFRAG and ISSB published in July 2024 an Interoperability Guidance letting a reporter cover both CSRD (ESRS) and IFRS Sustainability Disclosures (S1/S2). In practice, European reporters listed on LSE or Tokyo (therefore also ISSB-subject) often produce two separate XBRL reports — one per taxonomy — rather than a unified file. The planned unification format (combination of taxonomies) remains experimental.

7. European iXBRL viewers diverge

ESMA provides a public iXBRL viewer (Filer Manual). But each national authority (AMF, BaFin, FCA UK pre-Brexit) adds its own quality controls. A filing accepted by AMF may produce 30 warnings at BaFin. Cross-regulator consistency has not yet arrived.

8. Audit assurance remains limited

CSRD mandates limited assurance in the first phase, which will move to reasonable assurance after review (likely deadline: 2028-2030). Big Four charge high (EUR 150-500k for limited CSRD assurance, EUR 700k-2M for reasonable). Small/mid-caps look at second-tier firms (Mazars, BDO, Grant Thornton).

Eight concrete pitfalls to avoid for cohort 2 (2027)

  • Taxonomy version lock-in: ESRS XBRL taxonomy evolves yearly (v2024.1, v2026.1, v2027.1 expected). Do not hard-code a version in your pipeline; abstract.
  • iXBRL validation in CI: integrate arelle or XBRL Cloud in your ESG data pipeline CI/CD from collection, not just at final filing.
  • Consistent datapoint numbering: each EFRAG datapoint has an ID (DP-XX-XXXX). Build your internal data model around these IDs, not your internal accounting taxonomy.
  • Scope 3 collection anticipation: engage your large suppliers on primary emission factors from 2026. Prepare EDI flows or APIs to ingest this structured data.
  • 10-year archive: CSRD XBRL reports must be archived at least 10 years (eIDAS-compliant). Prepare event-sourced infrastructure (see Event Sourcing).
  • ESG data lineage: auditor wants to trace each figure in the report to its source. Implement a data catalog (DataHub, Collibra, OpenMetadata).
  • Multi-languages: ESEF report supports multi-languages via xml:lang attribute on labels. Most FR/DE companies file in English AND local language, doubling the tagging work.
  • Plan B for filing incident: authorities do not accept paper backup filing. If your ESEF file breaks at the deadline (31 March / 30 April per Member State), it is a non-publication that may trigger fine + stock listing downgrade. Test in pre-prod 4 weeks early.

References

Further reading: our article "CSRD/ESRS data pipeline" and the foundation "Compliance by Design".