Short answer: a consultant letter and independent assurance are not the same thing. A consultant letter usually documents that a consultant helped prepare, calculate, review, or explain the GHG data and methodology. Independent assurance is a separate engagement performed by an assurance practitioner under assurance or verification standards, with independence, evidence, procedures, and a formal conclusion. If a customer request uses words like assurance, verification, auditor, limited assurance, reasonable assurance, or independent provider, do not treat a normal consultant letter as a substitute without confirming the requirement.
This distinction matters because supplier requests often use similar-sounding language. A buyer may ask for a third-party consultant letter. Another buyer may ask for independent assurance. A portal may ask whether emissions have been verified. A procurement team may forward a template without explaining which route applies.
For a supplier trying to meet a deadline, it can be tempting to blur the terms. Do not. Overstating a consultant letter as assurance can create credibility risk for the supplier and confusion for the customer.
What a consultant letter can do
A consultant letter is usually connected to advisory support. It can explain the consultant's role in preparing the emissions calculation, reviewing activity data, applying a methodology, organizing evidence, or helping the supplier prepare a customer response.
For GHG work, a consultant letter might describe:
- the reporting period and boundary reviewed;
- the emissions scopes and categories included;
- the source data used for the calculation;
- the GHG accounting standards or guidance referenced;
- the consultant's activities, such as calculation support or methodology documentation;
- the assumptions, limitations, and exclusions; and
- whether the letter is intended for a specific customer request.
A good consultant letter is careful. It should not imply that the consultant performed an independent assurance engagement unless that is actually true. It should not guarantee customer acceptance. It should not say that a buyer has approved the consultant. It should not describe advisory support as a verification opinion.
What independent assurance means
Independent assurance is a more formal process. The International Standard on Sustainability Assurance 5000, or ISSA 5000, is designed for assurance engagements over sustainability information across topics and frameworks. IAASB states that ISSA 5000 is effective for periods beginning on or after December 15, 2026. ISO 14064-3 also provides principles and requirements for verifying and validating GHG statements.
In practical terms, independent assurance usually means an assurance or verification provider evaluates sustainability or GHG information against suitable criteria, gathers evidence, performs procedures, and issues a report or statement with a conclusion. The engagement may be limited assurance or reasonable assurance depending on the requirement and applicable standard.
That is different from an advisor helping you calculate emissions. A consultant may prepare or support the information. An independent assurance provider assesses it. In many cases, the same firm cannot both prepare the information and independently assure it without creating independence issues.
Why customer wording matters
Before promising a response path, look for the exact words in the customer request. These phrases usually point toward independent assurance or verification:
- independent assurance;
- third-party assurance provider;
- limited assurance;
- reasonable assurance;
- verification statement;
- verified GHG statement;
- auditor or assurance practitioner; and
- ISO 14064-3, ISAE 3410, ISSA 5000, or similar assurance terminology.
These phrases usually point toward consultant-letter support, but still need careful reading:
- consultant letter;
- third-party consultant letter;
- methodology letter;
- supporting documentation from consultant;
- calculation support letter; and
- letter describing GHG calculation support.
If the wording is ambiguous, ask the customer whether a consultant letter is acceptable or whether an independent assurance provider is required. The answer changes timing, cost, evidence needs, and who should do the work.
What to do if the request is urgent
First, separate calculation work from assurance work. You may still need to prepare the emissions inventory, evidence file, methodology note, and customer response before any letter or assurance route is possible.
Second, avoid claiming more than you have. If the work has not been independently assured, say that clearly. If a consultant helped prepare the calculation, describe that support accurately. If the customer requires independent assurance, scope that separately with an appropriate provider.
Third, keep the evidence file tidy. Whether you use a consultant letter or assurance provider, the same underlying evidence matters: activity data, boundaries, emission factors, assumptions, calculation files, approvals, and version control.
How Keslio supports this route
Keslio can help suppliers interpret the request, prepare the GHG calculation, organize the evidence pack, write methodology notes, and provide consultant-letter support where that is the appropriate route. Keslio does not describe consultant letters as independent assurance, does not guarantee buyer acceptance, and does not claim buyer endorsement.
If the request appears to require assurance, Keslio can help identify that early so the supplier does not waste time preparing the wrong response. If the request appears to allow a consultant letter, Keslio can help build the calculation and documentation needed for that route.
Where to start
Start with the request wording, not the label someone used in an email. Then decide whether the work is a calculation, consultant-letter, service-level accounting, or independent assurance problem. Keslio's supplier request support can help with that first interpretation. Keslio's GHG emissions calculations support can help prepare the underlying inventory and methodology. If the request involves customer-specific allocation, read Service-Level GHG Accounting for Suppliers.
Source notes
This article draws on the GHG Protocol Corporate Standard, which distinguishes accounting and reporting from verification process standards, IAASB ISSA 5000, and ISO 14064-3.
