Standardising sustainability assurance

The publication of the International Standard on Sustainability Assurance 5000 (ISSA 5000) and implementation guidance is a welcome first step in creating consistency in sustainability assurance – irrespective of whether assurance is conducted by the accountancy or another independent assurance provider, or the sustainability reporting framework applied. 

Additionally, the requirements to comply with the International Standard on Quality Management 1 (ISQM 1) and the International Ethics Standards Board for Accountants’ (IESBA) International Code of Ethics for Professional Accountants help to enhance trust in assured sustainability reports.

ISSA 5000, General Requirements for Sustainability Assurance Engagements, published in November 2024 under an accelerated process due to stakeholder feedback, builds on existing International Auditing and Assurance Standards Board (IAASB) standards that provide assurance on sustainability information, such as ISAE 3000 (Revised), Assurance Engagements Other than Audits or Reviews of Historical Financial Information, and ISAE 3410, Assurance Engagements on Greenhouse Gas Statements.

It also incorporates concepts from other International Standards on Auditing (ISAs). ISAE 3410 will be withdrawn once ISSA 5000 comes into effect – for periods beginning on or after 15 December 2026, with early application permitted.

Framework-neutral

Designed as a framework-neutral standard, ISSA 5000 can be applied to sustainability information reported under any framework – including IFRS Sustainability Disclosure Standards, European Sustainability Reporting Standards (ESRS) and Global Reporting Initiative (GRI) Standards – using any reporting mechanism.

It enables assurance on a range of sustainability topics, such as environmental, social, economic and cultural matters, as well as aspects of topics such as related risks and opportunities, governance, processes, strategy, scenario analysis and key performance indicators (KPIs).

The standard is also profession-agnostic, meaning it can be used by both professional accountants (PAs) and non-PA assurance practitioners with relevant expertise. However, two key premises must be met for its application:

• The assurance practitioner must be a member of a firm subject to ISQM 1 or other professional, legal or regulatory requirements that are at least as demanding as ISQM 1.
• Members of the engagement team and, where applicable, the engagement quality reviewer, must adhere to the provisions of the IESBA’s International Code of Ethics for Professional Accountants (including International Independence Standards), or other professional, legal or regulatory requirements that are at least as demanding.

ISSA 5000 supports both limited and reasonable assurance engagements, as well as combined engagements where different levels of assurance apply to different elements. The standard distinguishes requirements using the letter L for limited assurance and R for reasonable assurance.

To facilitate implementation, the International Auditing and Assurance Standards Board (IAASB) has developed a variety of resources, including detailed guidance, offering practical insights on applying the standard’s requirements, along with illustrative examples to aid practitioners in its adoption.

Ethics

On the global ethics front, the IESBA has published International Ethics Standards for Sustainability Assurance (including International Independence Standards (IESSA)), along with revisions to its Code of Ethics, to address the use of external experts in sustainability assurance.

With these updates, the IESBA aims to establish a global baseline for ethics and independence in sustainability assurance engagements. Both the IESSA and the revisions related to Using the Work of an External Expert were published by the IESBA in January 2025.

The IESSA introduces a newly added Part 5 to the IESBA Code of Ethics, which consists of Ethics and Independence Standards for Sustainability Assurance. Similar to ISSA 5000, IESSA is profession-agnostic, meaning it applies to all sustainability assurance practitioners, whether PAs or non-PAs.

Additionally, it is framework-neutral, making it compatible with any reporting or assurance framework, including ISSA 5000. The IESSA applies to both limited and reasonable assurance engagements on sustainability information, and includes standards and provisions equivalent to those used in audit engagements.

As part of the IESSA project, IESBA also revised its Code of Ethics to incorporate specific provisions for the preparers of sustainability disclosures, strengthening existing requirements to combat greenwashing. These provisions are framework-neutral, applying regardless of the reporting framework used, such as IFRS Sustainability Disclosure Standards, ESRS, GRI Standards and others. While designed for PAs, they can also be applied by non-PAs.

Skills assessment

The revisions to the Code of Ethics concerning Using the Work of an External Expert recognise the growing need for specialised expertise in areas such as sustainability, technology and tax.

The updated standards help sustainability assurance practitioners assess the competence, capabilities and objectivity of external experts whose work may be relied upon in preparing information, conducting audits or other assurance engagements, and providing advisory or other services.

Additionally, the standards ensure that sustainability assurance practitioners maintain ethical conduct when using the work of an external expert.

Given the comprehensive scope of changes introduced in Part 5, the effective date for the IESSA is phased in stages to allow sustainability assurance practitioners and preparers of sustainability disclosures adequate time to implement the new ethical and independence requirements starting from 15 December 2026.

ACCA recommends referring to the standard for a comprehensive overview of the effective dates and transition provisions. The effective date for the revisions to the code with respect to Using the Work of an External Expert is 15 December 2026.