Third-Party Assurance
CSRD mandates independent third-party assurance of your sustainability report. Limited assurance is required from day one. Reasonable assurance — equivalent to a financial audit — phases in from 2028. Here is what this means in practice.
CSRD mandates independent third-party assurance of your sustainability report. Limited assurance means the assurer concludes that nothing has come to their attention to indicate the sustainability information is materially misstated.
Limited vs reasonable assurance — the practical difference
Limited assurance means the assurer concludes that nothing has come to their attention to indicate the sustainability information is materially misstated. It is a negative assurance conclusion based on limited procedures — primarily analytical review and enquiry.
Reasonable assurance means the assurer positively concludes that the sustainability information is fairly presented in all material respects. It requires extensive testing of underlying data, controls and systems — similar to a financial audit.
Reasonable assurance requires substantially more work — expect 2–3x the effort and cost of limited assurance.
Who can provide CSRD assurance
CSRD assurance can be provided by statutory auditors (the Big Four and national audit firms) and by accredited independent assurance service providers. The assurance provider must be independent of the company.
Many companies use their existing financial auditor for sustainability assurance — this simplifies the engagement but creates capacity constraints as all large EU companies seek assurance simultaneously.
Specialist sustainability assurance providers — Bureau Veritas, SGS, LRQA, DNV — are also accredited and often have deeper ESG expertise than generalist audit firms.
How to prepare your data for assurance
Your assurer needs: complete and consistent source data for every disclosed datapoint; documented methodology for every calculation; evidence of internal review and approval; and a clear audit trail from raw data to disclosed figure.
The most common assurance finding is inadequate documentation — the number is right but the trail from source data to report is unclear. Build documentation into your data collection process, not retrospectively.
Frequently asked questions
Can our financial auditor provide CSRD assurance?
Yes, if they are accredited for sustainability assurance in your jurisdiction. Most Big Four firms have obtained the necessary accreditations. However, demand far exceeds supply — engage your provider 12+ months before your filing deadline.
What does the assurer actually check?
The assurer checks: completeness of disclosures against ESRS requirements, accuracy of quantitative data against source records, consistency of qualitative disclosures with underlying evidence, appropriateness of methodology choices, and correct XBRL tagging.
What happens if we receive a qualified assurance opinion?
A qualified opinion indicates a material misstatement or limitation. It must be disclosed and will attract regulatory attention. Investors and lenders increasingly treat qualified sustainability assurance opinions as a red flag.