ESG assurance explained: limited vs reasonable, and how to choose a provider
What sustainability assurance is, the difference between limited and reasonable assurance, which rules require it, how the process works, and how to choose an assurance provider.
Assurance is an independent check of your sustainability information, much like a financial audit checks your accounts. It is becoming a legal requirement: CSRD, India’s BRSR Core, California SB 253 and ISSB-based rules in several countries all require or plan to require it.
Limited vs reasonable assurance
| Limited assurance | Reasonable assurance | |
|---|---|---|
| Level of confidence | Moderate | High, similar to a financial audit |
| Conclusion | Negative: “nothing has come to our attention that causes us to believe the information is materially misstated” | Positive: “in our opinion, the information is fairly presented” |
| Work involved | Mainly enquiries and analytical review, with some testing | Detailed testing of controls and data, site visits, larger samples |
| Cost and effort | Lower | Significantly higher |
Most rules start with limited assurance. Under CSRD, limited assurance is the requirement; the planned later move to reasonable assurance was dropped in the 2025–26 simplification.
Who needs it
- EU CSRD: limited assurance of the sustainability statement for every company in scope.
- India BRSR Core: assurance or assessment of key indicators, phased in by market capitalisation, reaching all of the top 1,000 listed companies from FY 2026–27.
- California SB 253: limited assurance of Scope 1 and 2 emissions, moving to reasonable assurance later in the decade.
- ISSB-based rules: set country by country. Singapore, for example, has deferred external limited assurance of Scope 1 and 2 emissions to FY2029.
Which standards assurers use
Assurance work follows professional standards. The main ones are ISAE 3000 (Revised) for sustainability information in general, ISAE 3410 for greenhouse gas statements, AA1000AS, and ISSA 5000, the IAASB’s newer standard written specifically for sustainability assurance. Ask which standard a provider will use and whether your regulator accepts it.
How an assurance engagement works
- Scoping. Agree which information is covered (the whole report or selected indicators), the level of assurance and the standard.
- Planning. The assurer learns your processes, assesses risks and decides where to test.
- Fieldwork. Interviews, walkthroughs of data flows, analytical review and testing samples back to source evidence.
- Findings. You get a list of errors and weaknesses, and a chance to correct them.
- Report. The assurer issues its conclusion, which is published with your report.
Choosing an assurance provider
- Accreditation. Under CSRD, assurance must be done by a statutory auditor or, where the country allows it, an accredited independent assurance services provider. Check what your country permits.
- Independence. Your financial auditor can often do the work, which brings efficiencies, but check independence rules if they also advised on your report.
- Sector and topic experience. Emissions, supply chain and workforce data each need different skills.
- Readiness reviews. Many providers offer a pre-assurance review a year early. It is the cheapest way to find gaps.
Assurance providers on ESGListings
Bureau Veritas
Testing, inspection and certification group offering GHG verification and sustainability assurance.
Speeki
Singapore-based assurance, verification and certification provider with software for ESG report and management system audits.
Pangolin Associates
Australian climate consultancy covering emissions measurement, climate risk, strategy, verification and disclosure support.
TUV Rheinland
Technical services provider offering certification and auditing, including environmental and energy management system certification.
AccountAbility
Sustainability advisory firm that also develops the AA1000 standards on stakeholder engagement, assurance and accountability.
Microfinanza Rating
Italian rating agency providing ESG assessments, second party opinions and impact assessments for financial institutions.
Ordered by verified reviews and relevance. Listings are never ranked by payment. See all 35 providers
Related guides
This guide is general information, not legal, tax or investment advice. Rules change; check the official sources and talk to an adviser before you rely on it.
Related guides
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The evidence, documentation and controls an assurance provider typically expects to see when reviewing a sustainability report, and how to prepare before they arrive.
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The nine BRSR Core attributes and their KPIs, the calculation conventions in the Industry Standards Note, and the records to keep so your figures can be assessed or assured.