Sustainability assurance

Sustainability assurance is the process by which an independent third party reviews the sustainability data a company publishes and issues an opinion on its reliability. In the ESG field it is the equivalent of the audit of financial accounts: it gives investors, customers, regulators and other stakeholders confidence that the reported information is complete and well founded.

Its importance has grown with the Corporate Sustainability Reporting Directive (CSRD), which for the first time makes the verification of sustainability information a legal obligation for companies within its scope.

Levels of assurance: limited and reasonable

There are two main levels of assurance:

• Limited assurance: the verifier performs enough work to conclude that nothing has come to its attention suggesting the information is materially misstated. It is a moderate level of confidence.
• Reasonable assurance: requires deeper verification work and offers a higher level of confidence, similar to a financial audit.

The CSRD initially requires limited assurance. The European Commission was expected to be able to move towards reasonable assurance, but the regulatory review known as the Omnibus package removed that mandatory transition, so CSRD reporting stays at limited assurance and the Commission is to adopt harmonised limited assurance standards.

The standards: ISAE 3000 and ISSA 5000

The technical reference framework comes from the IAASB (International Auditing and Assurance Standards Board). Until now, most sustainability assurance engagements have been carried out under ISAE 3000 (Revised), designed for assurance engagements other than audits of historical financial information.

The IAASB has also approved a dedicated standard, ISSA 5000, General Requirements for Sustainability Assurance Engagements, a stand-alone standard that can be used for any reporting framework (CSRD, ESRS, IFRS Sustainability Standards, GRI, etc.) and that covers both limited and reasonable assurance. ISSA 5000 is effective for engagements on sustainability information for periods beginning on or after 15 December 2026.

Who can verify

The CSRD allows verification to be carried out by the company's own statutory auditor and, depending on each Member State's choice, also by a different auditor or by an accredited independent assurance services provider. In Spain, verification can be done by audit firms or by entities accredited by the National Accreditation Body (ENAC), as well as by individuals registered as verifiers. These verifiers are subject to requirements equivalent to those for statutory audit regarding independence, ethics, training and quality control, under the supervision of the ICAC.

How it fits into the reporting cycle

Assurance is the final piece of a good sustainability report: it closes the loop on report verification and reinforces the credibility of the data identified as material in the double materiality assessment. The better the traceability and quality of the underlying data, the smoother the verification.

Prepare your information for assurance with Manglai

To pass an independent verification you need complete, traceable and well-documented data. At Manglai we help you structure and support your environmental and sustainability information so it is ready for assurance. Discover how Manglai can help you.