CSRD in Belgium: Requirements for Audit Firms

How Belgium transposed the CSRD

The Belgian Parliament approved the CSRD transposition on 28 November 2024. The Law of 2 December 2024 was published in the Belgian State Gazette (Moniteur Belge) on 20 December 2024 and entered into force on 30 December 2024. Belgium was five months late relative to the EU’s July 2024 deadline, which is worth noting because it compressed the window for practitioners to get ready.

The Belgian legislator opted for transposition without significant gold-plating. The law amends the Code des sociétés et des associations (CSA) and the Law of 7 December 2016 on professional organisation and public supervision of company auditors. Together, these amendments create the legal framework for both sustainability reporting and its assurance.

A few specific Belgian choices matter for practitioners. First, Belgium adopted a progressive approach to who can provide sustainability assurance. During the initial years, only réviseurs d’entreprises can perform the statutory assurance engagement. The market will open to independent assurance service providers (prestataires de services d’assurance indépendants) no earlier than 20 December 2027, three years after the law’s publication. That opening requires a Royal Decree (arrêté royal) to establish the framework conditions, and the IBR/IRE has been vocal about ensuring those conditions create a level playing field.

Second, Belgium explicitly addressed the value chain trickle-down problem. The law prohibits in-scope companies from requiring assurance of sustainability information from SMEs in their value chain. Voluntary assurance remains an option for those SMEs, and for voluntary engagements the market is open to any qualified professional. At firms like ours, this distinction between statutory and voluntary assurance comes up constantly when advising Belgian SME clients who are getting data requests from their larger customers.

Third, the commissaire (statutory auditor for the financial statements, or FS) has the option, not the obligation, to issue an integrated report. When the same réviseur d’entreprises is appointed for both the statutory audit and the sustainability assurance engagement, they can either produce a single integrated report or issue a separate sustainability assurance report. Most firms will choose separate reports during these first years, given the different methodological frameworks involved.

Fourth, the law defines sustainability information under a new Article 1:31/2 of the CSA. This matters because the definition determines the boundary of what the assurance engagement covers. Information published voluntarily outside this definition does not trigger the statutory assurance requirement. But information within the definition that is published in the management report does.

The Belgian Council of State (Conseil d’État) reviewed the draft bill and raised a procedural point: the law uses the term “opinion” (in the French-language text) for the sustainability assurance conclusion, even though the engagement is a limited assurance engagement that technically produces a “conclusion.” The IBR/IRE’s technical notes address this terminological inconsistency and confirm that the engagement methodology follows the limited assurance model under ISAE 3000 (Revised), regardless of the statutory label. It’s a small thing, but it trips up teams drafting their first Belgian sustainability assurance report.

The three-wave Belgian timeline

Belgium’s original CSRD timeline mirrors the EU directive’s phased approach. Each wave is defined by entity size and public-interest status.

Wave 1 (FY 2024, reports published 2025): Large Belgian public-interest entities already subject to the NFRD. This covers listed companies, credit institutions, insurance companies, and other PIEs with more than 500 employees. These entities have been reporting since January 2025.

Wave 2 (FY 2025, reports published 2026): Other large Belgian undertakings meeting at least two of the following criteria (on an individual or consolidated basis): 250+ employees, €25M+ balance sheet total, €50M+ annual turnover.

Wave 3 (FY 2026, reports published 2027): Listed Belgian SMEs (excluding micro-companies), with a two-year opt-out option pushing the latest start to FY 2028.

Non-EU companies (FY 2028, reports published 2029): Third-country groups with €150M+ EU-generated revenue and at least one large EU subsidiary or branch.

The IBR/IRE estimated that at least 2,280 Belgian companies exceeded the Wave 2 size criteria based on 2021 annual accounts filed with the National Bank of Belgium (Banque Nationale de Belgique, or BNB). Those companies produced €126B in value added, representing 46% of all value added reported through the BNB’s standard filing format. That figure excludes companies filing in non-standard formats (parts of the financial sector and hospitals, among others), so the actual number of affected entities was higher.

The timeline has shifted. The Stop-the-Clock Directive (adopted April 2025) postponed Wave 2 by two years (to FY 2027, reports in 2028) and Wave 3 by two years (to FY 2028, reports in 2029). Belgium introduced legislation to implement Stop-the-Clock, though the formal adoption process overlapped with the Omnibus I negotiations.

For audit firms, the delayed timeline creates a genuine planning paradox. You don’t need to deliver assurance reports for Wave 2 clients until 2028. But if you haven’t started building the engagement methodology and training the team by now, you won’t be ready when the deadline arrives. We’ve seen this pattern before with ISA 540 (Revised) and ISQM 1: the firms that waited for final guidance lost a full preparation cycle and spent the first engagement season firefighting.

What Omnibus I changed for Belgian companies

The Omnibus I Directive (Directive 2026/470, published 26 February 2026) restructures the CSRD scope at EU level. The new mandatory thresholds require exceeding both 1,000 employees and €450M net annual turnover. Listed SMEs are removed from scope entirely, and financial holding companies are exempted.

For Belgium, this means the vast majority of those 2,280 companies that the IBR/IRE identified as in scope under the original thresholds will fall out. A company with 600 employees and €200M turnover is no longer subject to mandatory CSRD reporting. Nobody enjoys telling a client they spent a year preparing for an obligation that just evaporated, but that is the conversation many engagement partners (EPs) are having right now.

Member states must transpose Omnibus I by 19 March 2027. Belgium has not yet adopted implementing legislation. Until transposition, the original Belgian CSRD law (30 December 2024) remains the applicable legal framework. Wave 1 entities continue reporting under the existing rules regardless.

Omnibus I also gives member states the option to exempt Wave 1 entities that fall below the new thresholds for FY 2025 and FY 2026. Whether Belgium exercises this option will depend on the transposition legislation. The IBR/IRE and the FSMA (Financial Services and Markets Authority) are monitoring this closely.

For Belgian audit firms, the Omnibus I changes create a bifurcated market. A small number of very large companies (multinationals and listed groups, plus financial institutions) will remain in scope for mandatory CSRD reporting and statutory assurance. A much larger number of companies that were preparing for Wave 2 will need to decide whether to report voluntarily. Value chain pressure from their larger customers will be the primary driver, and the VSME standard (published by the European Commission in July 2025) provides the reporting framework for voluntary adopters.

The IBR/IRE has positioned the réviseur d’entreprises as the natural choice for voluntary sustainability assurance as well. Its 2024 ESG brochure emphasises that sustainability assurance expertise, combined with existing audit relationships, gives réviseurs d’entreprises a structural advantage. But the voluntary market is unregulated. Accounting firms (ITAA members) and ESG consultancies can both compete for voluntary engagements. The réviseur d’entreprises has no statutory monopoly over voluntary work.

The Omnibus I changes also affect the timeline for opening the statutory assurance market to non-réviseur providers. The Belgian law sets the opening date at 20 December 2027 (three years after publication). But with the mandatory scope now limited to very large companies, the pool of statutory assurance engagements is much smaller than originally anticipated. The commercial case for non-réviseur providers to seek accreditation may be weaker than the IBR/IRE feared. The voluntary assurance market for the thousands of companies that fell out of scope may be where the real growth opportunity lies. In our experience, firms that pivot early to voluntary VSME-level engagements will own that market before the larger players even notice it.

Who can provide sustainability assurance in Belgium

The Belgian transposition follows the directive’s member state option to allow both the commissaire and another réviseur d’entreprises to provide sustainability assurance. During the initial period (until at least 20 December 2027), only réviseurs d’entreprises can perform the statutory assurance engagement.

This is a deliberate policy choice. The IBR/IRE advocated strongly for maintaining the réviseur d’entreprises’ exclusive access during the first years. In its January 2024 joint opinion with the ITAA (submitted to the Minister of the Economy), the IBR/IRE argued that public interest and legal certainty require equal quality standards regardless of who performs the engagement.

After the three-year exclusive period, independent assurance service providers may apply for accreditation under a framework to be established by Royal Decree. The IBR/IRE’s position is that these providers must meet identical requirements to réviseurs d’entreprises: equivalent stage exams and CPD, equivalent independence and ethical rules, equivalent firm organisation standards, and equivalent public supervision by the Collège de supervision.

The appointment process follows the same governance path as a statutory audit. The entity’s general meeting of shareholders appoints the réviseur d’entreprises for the sustainability assurance engagement. If the commissaire is also appointed for sustainability assurance, they can choose between an integrated report and a separate report.

Professional secrecy rules apply. Article L.822-2 of the Code de commerce (as amended by the CSRD law) extends professional secrecy to all facts encountered during the sustainability assurance engagement. The law also permits information sharing between the commissaire and the réviseur d’entreprises when they are performing different missions (the FS audit and the sustainability assurance engagement) for the same entity. This carve-out is necessary because, without it, the strict secrecy provisions would prevent the two professionals from coordinating on inconsistencies between the FS and the sustainability report.

The IBR/IRE professional standard and what it requires

The IBR/IRE submitted its draft professional standard for sustainability assurance engagements to the Conseil supérieur des Professions économiques (CSPE) and the Minister of the Economy on 5 July 2024. The approval process has been lengthy. The CSPE held its first hearing on 21 August 2024. Reformulations requested by the CSPE depended on the Belgian CSRD transposition law being adopted, which hadn’t happened yet. After the law was published on 20 December 2024, the IBR/IRE Council adopted the modified draft standard on 14 January 2025. A second CSPE hearing was held on 17 March 2025.

The draft standard requires the application of ISAE 3000 (Revised) as the primary engagement standard. It also references ISAE 3400 for forward-looking sustainability information (targets and transition plans). The standard adds Belgian-specific provisions in four areas: engagement letters, assurance reports, the impact on the commissaire’s statutory audit report, and the réviseur d’entreprises’ role vis-à-vis the works council (conseil d’entreprise).

The European Commission must adopt harmonised limited assurance standards by 1 July 2027 (per Omnibus I). Until then, the IBR/IRE standard governs Belgian engagements. This creates a transitional window where Belgian practitioners need to master both ISAE 3000 (Revised) and the IBR/IRE’s specific provisions simultaneously. Once the EU standard is adopted, another transition will follow. Two transitions in two years is not unusual in this profession, but it does test your methodology team’s bandwidth.

The IBR/IRE has published supporting materials through its ESG platform. Two technical notes are particularly relevant. The first (published 18 February 2025) covered which entities must publish sustainability information and the scope of the CSRD in Belgian law. The second covers the assurance engagement itself: the procedures, the report format, the interaction with the commissaire’s statutory audit report, and the réviseur d’entreprises’ role regarding the works council.

For practitioners already experienced with ISAE 3000, the Belgian framework adds procedural layers but does not fundamentally change the engagement methodology. The core tasks remain: understanding the entity’s double materiality assessment, evaluating the process for preparing sustainability information, performing analytical procedures and inquiries, and expressing a limited assurance conclusion in negative form.

What is new is the scale. A sustainability assurance engagement for a large Belgian industrial group touches environmental data (emissions calculations and energy consumption) alongside social data (workforce demographics and health and safety indicators). Governance data (board composition and anti-corruption policies) adds further scope. The réviseur d’entreprises needs subject-matter expertise that goes well beyond the traditional FS audit skill set. This is not ticking and bashing. You cannot apply the same mechanical procedures you would use on a bank reconciliation to a Scope 2 emissions calculation.

For mid-tier Belgian firms, the competence gap is the most immediate operational risk. Environmental data verification (particularly Scope 1 and Scope 2 emissions under ESRS E1) requires technical knowledge that most FS audit teams do not hold. The IBR/IRE expects practitioners to either develop this expertise internally or engage specialists under ISAE 3000’s provisions for using the work of an expert. Documenting the basis for relying on a specialist, and assessing that specialist’s competence and independence, will be a focus area during early inspections by the Collège de supervision. Firms that have not yet mapped their competence gaps against the full set of ESRS disclosure requirements are behind schedule.

The IBR/IRE’s CPD offerings have expanded to address this gap. New training modules cover ESRS content and double materiality assessment methodology. Separate modules address the practical application of ISAE 3000 (Revised) to sustainability information. The Belgian Awards for Sustainability Reports (BAS), organised by the IBR/IRE for over 25 years, have also been repositioned to recognise reporting quality under the new CSRD framework.

Worked example: assessing a Belgian Wave 2 client

This is the kind of scoping exercise most Belgian mid-tier firms are running right now. The numbers are illustrative, but the analysis mirrors what we’ve seen on about half the engagements we’ve reviewed.

Client: Janssens Bouwgroep NV, Antwerp

Annual revenue: €95M | Balance sheet total: €38M | Employees: 420 | Listed: No | Sector: Construction

Under the original Belgian CSRD law (Wave 2 scope):

1. Janssens exceeds all three criteria: 420 employees (exceeds 250), €38M balance sheet (exceeds €25M), €95M turnover (exceeds €50M). Only two are required under Article 1:24 of the CSA.

Documentation note: Record the assessment against Article 1:24 of the CSA (large company definition). Cite the specific financial data from the most recent annual accounts filed with the BNB.

1. Under the original timeline, Janssens would report for FY 2025 with the sustainability report published in 2026. Stop-the-Clock delayed this to FY 2027 (reports published 2028).

Documentation note: Document both the original and delayed timelines. Reference the Belgian Stop-the-Clock implementing legislation and the CSRD law of 2 December 2024.

1. The réviseur d’entreprises would plan a limited assurance engagement under ISAE 3000 (Revised), applying the IBR/IRE’s professional standard once approved. The engagement would cover the ESRS sustainability report and the double materiality assessment, plus the EU Taxonomy disclosures required under Article 8 of the Taxonomy Regulation.

Documentation note: Document the engagement framework. Reference the IBR/IRE draft standard and ISAE 3000 (Revised).

After Omnibus I (revised scope, effective for FY 2027 onwards):

1. Janssens has 420 employees and €95M turnover. The Omnibus I thresholds require exceeding both 1,000 employees and €450M net turnover. Janssens falls below both by a wide margin.

Documentation note: Update the scoping memorandum. Record the Omnibus I thresholds (Directive 2026/470). Note that the entity will not be subject to mandatory CSRD reporting once the Belgian transposition is in force.

1. Janssens’ position for the transitional period depends on Belgium’s transposition of both Stop-the-Clock and Omnibus I. Under Stop-the-Clock, Wave 2 was delayed to FY 2027. Under Omnibus I, Janssens falls out of scope from FY 2027 onwards. The net result: Janssens will likely never be required to file a mandatory CSRD sustainability report.

Documentation note: Document the sequential regulatory analysis. Record the interaction between the Belgian CSRD law, Stop-the-Clock, and Omnibus I. Conclude on the entity’s mandatory reporting obligation (none, pending Belgian transposition).

1. Janssens holds a framework contract with a listed Belgian infrastructure group (2,800 employees, €1.4B revenue). That group falls within the CSRD scope under Omnibus I and will request sustainability data from its supply chain. Janssens qualifies as a protected undertaking (fewer than 1,000 employees) and can limit its response to information covered by the VSME standard.

Documentation note: If the firm accepts a voluntary assurance engagement, document it as a contractual engagement under ISAE 3000 (Revised). The IBR/IRE professional standard for statutory assurance does not apply to voluntary engagements. Independence requirements follow the general provisions of the Law of 7 December 2016, not the specific CSRD provisions.

Practical checklist for Belgian audit firms

Common mistakes in Belgian CSRD engagements

• Conflating the commissaire’s role in the FS audit with the sustainability assurance engagement. These are separate mandates under Belgian law, even when performed by the same person. The independence analysis, the engagement letter, the report, and the file documentation are all distinct. The Collège de supervision will inspect sustainability assurance files separately from FS audit files.
• Treating sustainability assurance as a tick box exercise. The temptation is to repurpose your existing FS audit programme structure and bolt on sustainability procedures. That approach will not survive Collège inspection. ESRS data has fundamentally different characteristics from financial data, and the procedures need to reflect that.
• Waiting for the IBR/IRE’s professional standard to be formally approved before starting engagement planning. The standard has been in draft since July 2024. Its core methodology (ISAE 3000 Revised) is stable and widely understood. Waiting for formal approval means losing a full engagement cycle of preparation time.

Related content

• CSRD glossary entry. Covers the directive’s structure and ESRS framework in plain language.
• CSRD/ESRS Double Materiality Assessment Tool. Interactive tool for conducting and documenting the IRO assessment required by ESRS 1.
• CSRD in France: Timeline and Obligations. Companion post covering the French transposition (first in the EU) and the H2A assurance framework.

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Frequently asked questions

Further reading and source references

• Law of 2 December 2024: the Belgian CSRD transposition law, published in the Moniteur Belge on 20 December 2024.
• IBR/IRE ESG platform: technical notes, model documents, and the draft professional standard for sustainability assurance engagements.
• Directive (EU) 2026/470 (Omnibus I): the amending directive narrowing CSRD scope to 1,000+ employees and €450M+ turnover.
• ISAE 3000 (Revised): the primary engagement standard referenced by the IBR/IRE’s professional standard for Belgian CSRD assurance.