ISSB Assurance Requirements
Unlike CSRD which mandates third-party assurance from the first reporting year, IFRS S2 does not currently mandate sustainability assurance. However, investor expectations, stock exchange requirements, and emerging national regulations are rapidly making assurance a de facto requirement for ISSB reporters.
Unlike CSRD which mandates third-party assurance from the first reporting year, IFRS S2 does not currently mandate sustainability assurance. IFRS S1 and S2 do not include a mandatory assurance requirement — the standards focus on what to disclose, not how the disclosures are verified.
The current ISSB assurance landscape
IFRS S1 and S2 do not include a mandatory assurance requirement — the standards focus on what to disclose, not how the disclosures are verified. This contrasts with CSRD, which explicitly mandates limited assurance from the first CSRD reporting year.
However, the assurance landscape for ISSB reporters is rapidly evolving through three channels:
Jurisdictional mandates: Countries adopting ISSB are adding assurance requirements through their national regulations. Australia mandates phased assurance for ASRS reporters — reasonable assurance on Scope 1 and 2 GHG emissions from the first reporting year (FY2024/25 for Group 1 entities), expanding to the full sustainability report by FY2028/29. The UK FCA has published a roadmap indicating assurance requirements will follow.
Stock exchange requirements: Several exchanges require or are developing requirements for sustainability assurance as a listing condition. The London Stock Exchange, Singapore Exchange, and Hong Kong Stock Exchange are among those with sustainability assurance expectations for listed companies.
Investor expectations: PRI signatories and institutional investors increasingly expect limited assurance of GHG emissions and key sustainability metrics in their investee companies. ESG rating agencies (MSCI, Sustainalytics) assign higher data quality scores to assured disclosures.
ISSA 5000 — the new global sustainability assurance standard
The International Auditing and Assurance Standards Board (IAASB) published ISSA 5000 (International Standard on Sustainability Assurance) in November 2024 — the first global standard specifically designed for sustainability assurance.
ISSA 5000 replaces the patchwork of existing standards (ISAE 3000, ISAE 3410, AA1000AS) with a single comprehensive framework for sustainability assurance engagements. It covers: both limited and reasonable assurance; all types of sustainability information (environmental, social, governance); and all sustainability frameworks (ISSB, ESRS, GRI).
Key ISSA 5000 requirements: independence of the assurance provider; assessment of the suitability of the sustainability information criteria; evidence gathering proportionate to assurance level; and a standardised conclusion format.
For IFRS S2 reporters: engaging an assurance provider familiar with ISSA 5000 will increasingly be the market standard. The standard is effective for assurance reports on sustainability information for periods beginning on or after 15 December 2026 — assurers are already developing ISSA 5000-based methodologies in 2025/2026.
Preparing for assurance — building assurance-ready disclosures
Whether or not assurance is currently mandated, building assurance-ready sustainability disclosures from the outset reduces future cost and risk.
For GHG emissions (the primary IFRS S2 assurance target): maintain complete source data for all emission calculations; document methodology including emission factor sources and versions; establish an internal review and approval process; and ensure year-over-year consistency with documented methodology changes.
For qualitative disclosures (scenario analysis, strategy, governance): maintain evidence supporting qualitative claims — board minutes showing climate discussion frequency; risk register entries for climate risks; scenario analysis workpapers; and documentation of how climate considerations are embedded in strategic planning.
For IFRS S2 specifically: the scenario analysis and financial effect disclosures are the most challenging to assure — they involve judgements and estimates rather than verifiable data. Document the assumptions, data sources, and methodologies underpinning scenario analysis in sufficient detail for an assurer to assess reasonableness.
For companies already subject to CSRD limited assurance: your CSRD assurance engagement covers the ESRS E1 disclosures that satisfy IFRS S2. The CSRD assurance provides credible third-party verification of your climate disclosures that can be referenced in IFRS S2 reporting contexts — reducing or eliminating the need for a separate IFRS S2 assurance engagement.
Frequently asked questions
Can we use our CSRD assurance report for IFRS S2 purposes?
Yes — CSRD limited assurance covers your ESRS E1 disclosures, which satisfy IFRS S2 requirements. In ISSB-adopting jurisdictions that accept ESRS disclosures as satisfying IFRS S2 (through interoperability provisions), your CSRD assurance report provides the relevant third-party verification. Check jurisdiction-specific requirements — some may require a separate local assurance engagement.
What is the difference between ISSA 5000 and ISAE 3410 for GHG assurance?
ISAE 3410 was the primary standard for GHG-specific assurance engagements. ISSA 5000 supersedes ISAE 3410 with a broader framework covering all sustainability information including GHG. ISSA 5000 is more comprehensive and explicitly addresses the challenges specific to sustainability assurance — judgment-based disclosures, forward-looking information, and value chain data. From 2027, ISSA 5000 will be the primary standard for sustainability assurance globally.
How much does IFRS S2 assurance cost compared to CSRD assurance?
For companies already subject to CSRD limited assurance, the incremental cost of ISSB assurance is minimal if the same provider is used — the CSRD engagement already covers the overlapping content. For companies not subject to CSRD, standalone IFRS S2 assurance (primarily GHG emissions and scenario analysis) typically costs €20,000–€80,000 for limited assurance, depending on company size and complexity.