Limited assurance vs reasonable assurance

Reasonable assurance, the level given by an audit, means the practitioner has reduced the risk of a wrong conclusion to an acceptably low level and can state a positive opinion such as “in our opinion, the statement is fairly stated”. Limited assurance accepts a higher risk, involves less extensive work and produces a negatively framed conclusion such as “nothing has come to our attention that causes us to believe the statement is materially misstated”. ISAE (UK) 3000 requires a limited assurance report to say plainly that the assurance obtained is substantially lower.

How do the standards define the two levels?

The FRC’s ISAE (UK) 3000 (July 2020), the UK standard for assurance engagements other than audits or reviews of historical financial information, defines both in paragraph 12:

• Reasonable assurance engagement: the practitioner reduces engagement risk to an acceptably low level as the basis for a conclusion expressed as an opinion on the subject matter against the criteria.
• Limited assurance engagement: the practitioner reduces engagement risk to a level acceptable in the circumstances but higher than for reasonable assurance, and expresses a conclusion on whether anything has come to its attention suggesting the information is materially misstated. The work is limited compared with a reasonable assurance engagement but must still give assurance that is “meaningful”, which the standard explains as likely to enhance users’ confidence to a degree that is clearly more than inconsequential.

Neither level is a guarantee. ISA (UK) 700 requires every audit report to say that reasonable assurance is a high level of assurance but not a guarantee that an audit will always detect a material misstatement. The ICAEW assurance glossary adds that absolute assurance implies a confidence no amount of work could justify.

The July 2020 version applies to assurance reports dated on or after 15 September 2020. The FRC has published ISAE (UK) 3000 (Revised September 2026), effective for assurance engagements where the report is dated on or after 15 December 2026.

How does the work differ?

ISAE (UK) 3000 sets out parallel requirements for each level, labelled L and R:

Stage	Limited assurance	Reasonable assurance
Understanding the subject (47L, 47R)	Consider the process used to prepare the information	Understand internal control over its preparation, evaluating the design of relevant controls and whether they have been implemented, using more than inquiry
Risk (48L, 48R)	Identify areas where a material misstatement is likely to arise and design procedures for those areas	Identify and assess risks of material misstatement and design responses
Controls testing (48R)	Not required	Required where the risk assessment expects controls to work, or where other procedures alone cannot give enough evidence
When something looks wrong (49L, 49R)	Perform additional procedures until the matter can be concluded on	Revise the risk assessment and planned procedures

For a review of historical accounts, ISRE 2400 (Revised) applies instead. ICAEW’s TECH 09/13AAF describes the work as enquiries of management and others plus analytical procedures, covering all material items, and says further substantive testing is not required unless the accountant has reason to be concerned. See review engagements under ISRE 2400.

Cost follows effort. The ICAEW glossary describes limited assurance as less intensive and less costly, and reasonable assurance as more time consuming and costly because it often needs significant substantive work.

How are the conclusions worded?

Paragraph 69(l) of ISAE (UK) 3000 requires a reasonable assurance conclusion to be expressed in a positive form, and a limited assurance conclusion in a form that conveys whether anything has come to the practitioner’s attention to cause it to believe the information is materially misstated. Application paragraphs A179 and A181 give examples:

Reasonable assurance	Limited assurance
“In our opinion, the entity has complied, in all material respects, with XYZ law.”	“Based on the procedures performed and evidence obtained, nothing has come to our attention that causes us to believe that [the entity] has not complied, in all material respects, with XYZ law.”
“In our opinion, the [appropriate party’s] statement that the key performance indicators are presented in accordance with XYZ criteria is, in all material respects, fairly stated.”	“Based on the procedures performed and evidence obtained, we are not aware of any material amendments that need to be made to the assessment of key performance indicators for them to be in accordance with XYZ criteria.”

The same split appears in financial statements. An audit opinion under ISA (UK) 700 states that the accounts give a true and fair view. The ICAEW illustrative review report under ISRE 2400 says: “Based on our review, nothing has come to our attention that causes us to believe that the financial statements have not been prepared” to give a true and fair view, in accordance with the applicable standards and the Companies Act 2006.

What must a limited assurance report disclose that an opinion does not?

Because a negative conclusion only means something if the reader knows what was done, paragraph 69(k) requires an informative summary of the work. For limited assurance it must state that the procedures vary in nature and timing from, and are less in extent than, a reasonable assurance engagement, and that the assurance obtained is consequently substantially lower. The ICAEW glossary makes the same point: a report with a negative conclusion should outline the scope of the work.

Both kinds of report share other elements under paragraph 69, including identification of the criteria, a statement on independence and ethics, and, where the criteria are designed for a specific purpose, a warning that the information may not be suitable for another purpose.

Where do you see each level in practice?

Engagement	Level	Source of the requirement
Statutory audit of company accounts	Reasonable	ISAs (UK); ISA (UK) 700 objective to obtain reasonable assurance
Review of an audit exempt company’s accounts	Limited	ISRE 2400 (Revised), described by ICAEW as providing limited assurance
FCA client assets report where the firm holds client money or custody assets	Reasonable	FCA SUP 3.10.4AR(1)
FCA client assets report where the firm claims not to hold client money or custody assets	Limited	FCA SUP 3.10.4R(2) and 3.10.4AR(2)
Assurance on KPIs, sustainability information or compliance with a law	Either, as agreed	ISAE (UK) 3000, paragraph A8 examples

The client assets example shows how a regulator can fix the level. Under SUP 3.10.4R(2), where a firm claims not to hold client money or custody assets, the auditor states whether anything has come to its attention that causes it to believe the firm did hold them, which is a limited assurance conclusion. See CASS audits for FCA firms. For sustainability information, see assurance on sustainability reporting.

How might the choice look on a real engagement?

Illustration only. A facilities company bidding for a public contract reports a customer complaints KPI of 1.2 complaints per 1,000 jobs. The buyer wants independent assurance.

• Limited assurance: the accountant asks how complaints are logged, compares the KPI with prior months and with call centre volumes, recalculates the figure and traces a small sample of complaints to the log. If nothing looks wrong, the conclusion says nothing has come to its attention suggesting the KPI is materially misstated, with a summary of that work.
• Reasonable assurance: the accountant also evaluates and tests the controls that ensure every complaint is logged, tests completeness from other sources such as emails and call recordings, and uses larger samples. The conclusion is a positive opinion that the KPI is fairly stated in all material respects.

If the buyer only wants the figures checked and reported as facts, neither applies: an agreed upon procedures report gives factual findings with no conclusion at all.

What mistakes come up most often?

• Reading “nothing has come to our attention” as almost an audit. ISAE (UK) 3000 requires the report to say the assurance is substantially lower.
• Ignoring the summary of work. In a limited assurance report it is essential to understanding the conclusion.
• Treating agreed upon procedures as assurance. ICAEW’s glossary says only factual findings are reported, with no conclusion or opinion.
• Agreeing the level without checking who will rely on the report. Where a regulator, lender or contract sets the level, that requirement prevails.
• Overlooking independence. See independence in non audit assurance work.

To see how these levels map onto choices for company accounts, see audit, review or compilation.

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Related Accountapedia entries

• ICAEW practice assurance reviews
• EIS and SEIS advance assurance
• Independence in non audit assurance work
• Limited partnerships
• What Counts as a Reasonable Excuse for HMRC?

Sources

• Financial Reporting Council: IASE UK 3000 Jul 2020 UAONHEZ
• Financial Reporting Council: ISAE UK 3000 Revised September 2026 zlSt1f6F
• Financial Reporting Council: Assurance standards
• Financial Reporting Council: ISA UK 700 Revised November 2019 Updated September 2025
• ICAEW: Assurance glossary
• ICAEW: Tech 09 13 aaf assurance review engagements on historical financial st
• FCA Handbook: handbook.fca.org.uk

Reviewed by Christine Anderson, Broker, Solar Protect. Last reviewed 24 October 2026. Part of Accountapedia, the Solar Protect guide to UK tax and accounting. This page is general information, not personalised advice.