COS Standards: What Dutch Auditing Standards Mean for Your Practice

What the NV COS actually is

The NV COS is not a separate set of auditing standards written from scratch. It is the Dutch transposition of the IAASB's International Standards on Auditing, International Standards on Review Engagements, International Standards on Assurance Engagements, and International Standards on Related Services. The NBA adopts each IAASB standard, applies it with its effective dates, and publishes it in Dutch with additional Dutch requirements where local law demands them.

Standaard 000N (the framework standard with the N suffix indicating it is Netherlands-specific) sets out how the entire NV COS structure works. Where Dutch-specific standards or paragraphs are needed, they are added to the relevant IAASB standard and printed in italics. Where an IAASB standard contains provisions that do not apply in the Netherlands, those provisions are simply omitted from the published Dutch version.

The NV COS covers standards numbered 100 through 5699. Definitions that apply across multiple standards were moved in 2022 to the HRA (Handleiding Regelgeving Accountancy) glossary, accessible at nba.nl.

For practitioners trained outside the Netherlands, the most important thing to understand is this: if you know the ISAs, you know approximately 90% of the NV COS. The remaining 10% consists of Dutch additions, Dutch-only standards, and omissions of ISA provisions that conflict with Dutch law. That 10% is where the review findings happen.

How the NV COS fits into the broader regulatory framework

The NV COS does not operate in isolation. It sits within a hierarchy of professional regulations that Dutch accountants must follow simultaneously.

At the top sits Dutch law: the Wet toezicht accountantsorganisaties (Wta) and the Besluit toezicht accountantsorganisaties (Bta) for firms performing statutory audits, and the Wet op het accountantsberoep (Wab) for all registered accountants. Below that sits the NBA's professional regulation: the VGBA (ethical conduct), the VIO (independence), the NVKS (quality systems for firms), and the NV COS (engagement-level standards).

When a practitioner is deciding what standards to apply, the answer is always the NV COS plus the VGBA and VIO. NV COS tells you how to perform the engagement. VGBA tells you how to conduct yourself. VIO tells you how to maintain independence. All three are mandatory on every engagement.

Practice notes (handreikingen, numbered from 1100 onwards) provide additional application guidance but do not carry the same mandatory weight as the standards themselves. Handreiking 1136 on compilation engagements and the revised Handreiking 1137 on corruption are among the most practically relevant for non-Big 4 firms.

How the NV COS maps to the ISAs

The mapping is deliberate and direct. Standaard 200 corresponds to ISA 200. Standaard 315 maps to ISA 315 (Revised 2019), and 700 maps to ISA 700. The numbering is identical throughout. When the IAASB revises a standard, the NBA adopts the revised version with the same effective date unless Dutch law requires a different implementation timeline.

Where the standards diverge from the ISAs, the divergence is always flagged. Dutch-specific text appears in italics in the body of the standard. Dutch-specific standards carry the N suffix. A practitioner reading a Dutch standard can therefore distinguish at a glance between text that originates from the IAASB and text that originates from the NBA. This typographic convention is not decorative. It tells you which requirements exist only in the Netherlands and which are internationally recognised.

What the December 2026 revised standards mean for the NV COS

The IAASB is issuing revised versions of ISA 240 (Fraud), ISA 500 (Audit Evidence), and ISA 570 (Going Concern), all effective for periods beginning on or after 15 December 2026. The NBA will adopt these as revised Standaarden with the same effective dates.

For Dutch practitioners, the revised standards will arrive with Dutch-specific additions that don't yet exist. The NBA must review each revised ISA, determine where Dutch law or practice requires additional requirements, draft those additions, and publish the Dutch text. Do not assume that the final IAASB text equals the final Dutch text. Wait for the NBA's published version before updating your firm's templates and work programmes.

ISA 240 Revised is particularly significant because the AFM has flagged fraud risk identification as a recurring concern in its inspection findings. The revised standard changes the auditor's default posture toward fraud risk, and the NBA may add further Dutch-specific requirements given the recent examination fraud scandals and the AFM's thematic focus.

The standard series and what each covers

The NV COS is organised into series by engagement type. Knowing which series applies to your engagement is the first step in scoping the applicable standards.

Dutch-specific additions: how to spot them in the text

Three signals tell you that a requirement is Dutch-only.

First: italicised text within a standard. Wherever the NBA has added requirements beyond the IAASB text, those additions appear in italics. These italicised paragraphs carry the same authority as the rest of the standard. A firm that argues "but the ISA doesn't require this" will find that the argument holds no weight when the Dutch standard does require it.

Second: paragraphs with a letter suffix after the number. When the NBA inserts additional application guidance into an existing IAASB standard, the new paragraph receives the next sequential paragraph number followed by a letter (for example, A51A). This preserves the original IAASB paragraph numbering while making the Dutch addition identifiable.

Third: entire standards with the N suffix. These are standards that exist only in the Netherlands. When you encounter an N-suffix standard, there is no international equivalent to reference. Standaard 000N explains the overall structure. Standaarden 4415N and 4416N add Dutch-specific compilation requirements. Standaard 3950N governs the SBR audit approach.

Standards that have no ISA equivalent

Standaard 3950N is perhaps the most practically significant Dutch-only standard. It governs how auditors approach the audit of financial statements that will be filed electronically through SBR in XBRL format. Under Standaard 3950N, the auditor must verify that the XBRL instance accurately reflects the audited financial information, that the correct taxonomy elements have been applied, and that the digital auditor's report is properly linked and signed.

Standaarden 4415N and 4416N supplement Standaard 4410 (compilation engagements) with Dutch-specific requirements. Given that compilation engagements represent the majority of work at many smaller Dutch practices, these standards directly affect day-to-day file documentation.

The VGBA (ethical conduct) and VIO (independence) sit alongside the NV COS as mandatory professional regulations. While not technically standards in the NV COS numbering, they carry equal authority and are tested during both AFM inspections and NBA quality reviews.

Worked example: Vermeer Audit Partners applies the right standards

Scenario: Vermeer Audit Partners B.V. is a five-partner firm in Den Haag with an AFM Wta licence. A new client, Mulder Vastgoed B.V., has €22M in assets, €18M in net revenue, and 38 employees. The client wants a statutory audit for the first time.

1. Determine the engagement type

Mulder Vastgoed meets two of the statutory audit criteria on two consecutive balance sheet dates. Statutory audit required. Record the size criteria assessment in the engagement acceptance file with both current and prior year figures.

2. Identify the applicable standards

Because this is a statutory audit, Standaarden 100 to 999 apply in full. Because the financial statements will be filed through SBR in XBRL format, Standaard 3950N also applies. Include the list of applicable standards in the planning memorandum.

3. Check for Dutch-specific requirements

The engagement partner reads Standaard 700 with attention to the italicised paragraphs. The Dutch auditor's report format has specific requirements for the management board report (bestuursverslag) opinion. Verify the firm's template against the current NBA model auditor's report (modelverklaring).

4. Consider the VIO and VGBA

The VIO requires a specific independence assessment for this new engagement. Because one of Vermeer's partners previously provided tax advisory services to the client's director, the engagement partner documents this relationship and assesses whether it creates a threat to independence under VIO Article 16.

Practical checklist

1. Before starting any new engagement, identify the engagement type and the corresponding NV COS series. Map the applicable standards explicitly in your planning file.
2. For statutory audits, check whether the client's financial statements will be filed through SBR. If yes, add Standaard 3950N to your list of applicable standards.
3. When reading any NV COS standard, note the italicised text. If your firm uses templates based on the international ISA text, verify that they also incorporate the Dutch additions.
4. Check the NBA model auditor's report (modelverklaring) against your firm's template at least once per audit season.
5. Verify compliance with both the VGBA and VIO on every engagement.
6. For compilation engagements, apply Standaard 4410 together with Standaarden 4415N and 4416N, and reference Handreiking 1136.

Common mistakes

• Using an ISA-based auditor's report template without Dutch-specific requirements. The AFM flags this in statutory audit inspections because the Dutch auditor's report format includes requirements (for example, on the bestuursverslag) that have no equivalent in ISA 700.
• Treating Standaard 4410 as the only applicable compilation standard. Standaarden 4415N and 4416N add Dutch-specific requirements, and Handreiking 1136 is the primary reference document the NBA's Raad voor Toezicht uses when reviewing compilation files.
• Overlooking Standaard 3950N. With the 2025 expansion of mandatory SBR filing to large entities, this standard now applies to a larger share of statutory audit engagements than in previous years.

Related tools and reading

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Frequently asked questions

Further reading and source references

• NBA HRA (Handleiding Regelgeving Accountancy): Contains the NV COS text, practice notes, and supporting guidance. Available at nba.nl.
• Standaard 000N: The framework standard explaining how the NV COS structure works and how Dutch additions are identified.
• IAASB Handbook 2024: The international source text for comparison with the Dutch transposition.
• Handreiking 1136: Practical guidance on applying NV COS 4410 for compilation engagements in Dutch practice.