An auditor’s expert is a person or firm with expertise outside accounting and auditing, such as a property valuer, actuary or tax lawyer, whose work the auditor uses as audit evidence. ISA (UK) 620 keeps the auditor solely responsible for the opinion and requires the auditor to check the expert’s competence, capabilities and objectivity, understand their field, agree the terms of the work and evaluate its findings, assumptions, methods and source data. A revised version applies to periods beginning on or after 15 December 2026.

Who counts as an auditor’s expert?

ISA (UK) 620 paragraph 6(a) defines an auditor’s expert as an individual or organisation with expertise in a field other than accounting or auditing, whose work in that field the auditor uses to help obtain sufficient appropriate audit evidence. The expert may be internal (a partner or staff member, including temporary staff, of the audit firm or a network firm) or external. Paragraph A1 gives examples: valuation of complex financial instruments, property, plant and machinery, intangible assets and impaired assets; actuarial calculation of insurance and employee benefit liabilities; environmental liabilities; interpretation of contracts and laws; and complex tax compliance issues.

Paragraph A2 draws the boundary. Someone expert in accounting for deferred tax is an accounting specialist, not an auditor’s expert, whereas an expert in tax law is. The standard does not cover:

Who Which standard applies
Auditor’s internal or external expert in a non accounting field ISA (UK) 620
Engagement team member or consultant with specialist accounting or auditing expertise ISA (UK) 220 (paragraph 2(a))
Management’s expert, engaged by the client to help prepare the accounts ISA (UK) 500 (paragraph 2(b))

For a management’s expert, ISA (UK) 500 paragraph 8 requires the auditor, to the extent necessary given the significance of the work, to evaluate that expert’s competence, capabilities and objectivity, understand their work and evaluate its appropriateness as evidence.

When does an auditor need an expert?

Paragraph 7 requires the auditor to decide whether to use an expert whenever expertise outside accounting or auditing is needed to obtain sufficient appropriate evidence. Paragraph A7 notes that an auditor may gain enough understanding through experience of similar clients, training or discussion with other auditors. Paragraph A8 lists factors: whether management used its own expert, the nature, significance and complexity of the item, the risk of material misstatement, and the availability of other evidence. Paragraph A9 adds factors about any management’s expert, including whether management can control or influence that expert’s work.

In practice the question arises most on estimates: investment property and other revalued assets, defined benefit pension obligations, complex financial instruments and legal claims, which are also covered in auditing accounting estimates.

How does the auditor assess competence, capabilities and objectivity?

Paragraph 9 requires the auditor to evaluate whether the expert has the necessary competence, capabilities and objectivity. For an external expert, this must include inquiry about interests and relationships that may threaten objectivity. Paragraph A14 explains the terms: competence is the nature and level of expertise, capability is the ability to apply it on this engagement (location, time and resources), and objectivity concerns bias, conflicts of interest and the influence of others.

Paragraph A17 warns that specialism within a field counts: an actuary specialising in property and casualty insurance may know little about pensions. Paragraph A19 notes that where a proposed expert played a significant role in preparing the information being audited, in effect acting as management’s expert, safeguards may not reduce the threat to an acceptable level. For audits of public interest entities, paragraph 9-1 requires a confirmation of independence from any external expert used, and ISA (UK) 260 paragraph 16-2(c) requires the auditor to say so in the additional report to the audit committee.

What must be agreed with the expert?

Paragraph 11 requires agreement, in writing when appropriate, on the nature, scope and objectives of the work; the respective roles and responsibilities; the nature, timing and extent of communication, including the form of any report; and the need to keep information confidential. Paragraph A24 points towards a written agreement where the expert will see sensitive information, the matter is highly complex, or the auditor has not used the expert before, and paragraph A25 notes that with an external expert this is often an engagement letter.

Paragraph A29 deals with working papers. An internal expert’s papers form part of the audit documentation, but an external expert’s papers are their own unless agreed otherwise. Paragraph 15-1, a UK addition, requires the auditor to document any request for advice from an expert and the advice received.

How does the auditor evaluate the expert’s work?

Paragraph 12 requires evaluation of the relevance and reasonableness of the findings and their consistency with other evidence; the relevance and reasonableness of significant assumptions and methods; and the relevance, completeness and accuracy of significant source data. Paragraph A33 lists procedures: inquiry of the expert, reviewing working papers and reports, observing the work, checking published data, confirming with third parties, analytical procedures, reperforming calculations and, where findings conflict with other evidence, consulting another expert.

Paragraph A37 asks whether assumptions and methods are generally accepted in the field, consistent with the accounting framework, dependent on specialised models, and consistent with management’s, and if not why. Paragraph A38 suggests testing source data by verifying its origin and reviewing it for completeness and internal consistency. If the work is not adequate, paragraph 13 requires the auditor to agree further work with the expert or perform additional procedures. Paragraph A40 says that if the issue cannot be resolved, a modified opinion under ISA (UK) 705 may be necessary, as explained in audit opinions.

Illustration: auditing an investment property valuation

Illustration only. A property company’s directors value its portfolio at £12 million using a valuer they appointed. The audit team assesses this as a significant risk and engages its own external valuer.

  1. The auditor checks the valuer’s qualifications and experience with the relevant property type and asks about any relationship with the client or its valuer.
  2. An engagement letter sets out the properties to be reviewed, the basis of value, the deadline and a confidentiality clause.
  3. The audit team agrees the rent roll given to the valuer to signed leases, because that source data drives the valuation.
  4. The valuer reports a range of £11.2 million to £11.8 million. The auditor reviews the yields used, discusses the gap with the client’s valuer and management, and treats the difference between £12 million and the top of the range as a potential misstatement.

The audit report does not mention the valuer. Paragraph 14 prohibits referring to an expert in an unmodified report unless law or regulation requires it, and paragraph 15 allows a reference only where it helps explain a modification, stating that it does not reduce the auditor’s responsibility.

What changes for periods beginning on or after 15 December 2026?

The FRC published ISA (UK) 620 (Revised September 2026) on 23 September 2026, effective for audits of periods commencing on or after 15 December 2026. A year starting 1 January 2027 falls under the new version, while a year that began on 1 January 2026 is still audited under the November 2019 version. The core requirements are unchanged. The additions are:

  • Paragraph 8(f). When deciding how much work to do, the auditor must consider whether relevant ethical requirements contain provisions on using an expert. Paragraph A13A says that in the UK these come from the auditor’s professional body.
  • Paragraph 9A. If the expert lacks the necessary competence or capabilities, or threats to objectivity cannot be eliminated or reduced to an acceptable level, the auditor must not use that expert’s work.
  • Paragraph A18A. New guidance suggests asking the client about any relationships it has with the auditor’s external expert, discussing safeguards with the expert, and in some cases obtaining written representations about interests and relationships.

These sit alongside the auditor’s own duties under the FRC Ethical Standard.

What are the common mistakes?

  • Treating the client’s valuer or actuary as if they were the auditor’s expert.
  • Reading only the summary of the expert’s report without testing the data it relied on.
  • Failing to ask an external expert about links with the client.
  • Agreeing the scope verbally on a complex engagement where paragraph A24 points to written terms.
  • Mentioning the expert in an unmodified audit report.

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Reviewed by Christine Anderson, Broker, Solar Protect. Last reviewed 19 October 2026. Part of Accountapedia, the Solar Protect guide to UK tax and accounting. This page is general information, not personalised advice.