Limited vs reasonable assurance for emissions: what each level means and when SB 253 requires them

Limited assurance and reasonable assurance are two levels of independent verification an accredited provider can give on your greenhouse gas numbers, and the difference is how hard they look. Limited assurance is a negative conclusion: after mostly analytical procedures and inquiry, the provider states that nothing came to their attention suggesting the emissions are materially misstated. Reasonable assurance is a positive opinion: after substantive testing of underlying records and controls, the provider states that the emissions are fairly presented. Reasonable assurance costs considerably more, takes longer, and requires evidence you cannot assemble retroactively.

For US companies the question stopped being academic when California put assurance on a statutory timetable. What you build into your inventory this year determines whether the step up to reasonable assurance later is a formality or a rebuild.

What each level actually involves

Limited assurance leans on analytics. The provider reviews your methodology, checks that your boundary and factors are sensible, compares this year against last, investigates variances that look odd, and samples a small number of items. They talk to the people who prepared the numbers. If the totals move in ways your business explains and the method is defensible, that is usually enough. It is a real review with real findings, but the burden of proof sits at the level of plausibility.

Reasonable assurance is an audit in everything but name. The provider tests the controls that produce the data, traces individual figures back to source documents, recalculates samples independently, and assesses whether your process reliably produces the same answer twice. They will ask who approved a classification change, where the meter reading came from, and how you know the AP export is complete. The gap between the two is less about rigor of thinking and more about volume of evidence.

Limited vs reasonable assurance for greenhouse gas emissions

	Limited assurance	Reasonable assurance
Form of conclusion	Negative: nothing came to our attention	Positive: the inventory is fairly stated
Main procedures	Analytical review, inquiry, limited sampling	Controls testing, substantive testing, recalculation, tracing to source
Evidence needed	Method documentation, factor sources, variance explanations	All of that plus a complete audit trail per figure and evidence of controls
Typical effort and cost	Lower	Substantially higher
Preparation lead time	Can be arranged in the reporting year	Needs controls running through the whole year
Common standards	ISO 14064-3, ISAE 3000 and 3410, AICPA AT-C, ISSA 5000	The same standards at the reasonable level

The SB 253 assurance timeline

California SB 253 requires covered companies, those over $1 billion in total revenue doing business in the state, to have their emissions independently assured, and it escalates the level over time. The practical schedule as CARB has set it out is straightforward to plan against.

• 2026 reporting cycle: Scope 1 and Scope 2 disclosure is due, with the first deadline of November 10, 2026. CARB's rules do not require third-party assurance for this first cycle.
• 2027 to 2029: limited assurance over Scope 1 and Scope 2 emissions. Scope 3 disclosure also begins in this window, without an assurance requirement attached to it yet.
• 2030 onward: reasonable assurance over Scope 1 and Scope 2, and limited assurance over Scope 3. CARB has indicated the detailed rules for this stage come in a later rulemaking, so confirm specifics before building a 2030 plan around them.

Read that schedule as a build order rather than a countdown. The 2026 report is your chance to get the inventory structurally right while nobody is testing it. The 2027 limited review tells you where your method is weak. Reasonable assurance in 2030 tests a control environment that has to have been running for a full year beforehand, which means the real deadline for that work is 2029, not 2030. For the wider regime, see our SB 253 reporting software page and the SB 253 vs SB 261 comparison.

What an assurance provider will actually ask you for

Every engagement, at either level, converges on the same handful of questions. Preparing answers to these is most of the work.

1. Completeness. How do you know every facility, entity and cost line that should be in the inventory is in it? This is the question most companies answer worst. Show the reconciliation from your general ledger or entity list to what you counted.
2. Boundary. Which consolidation approach did you apply, and is it applied consistently to leases, joint ventures and subsidiaries? See operational vs financial control.
3. Factors. Which emission factor library, which version, which GWP set? A provider will recalculate samples using your stated factors, so undocumented or inconsistent factors show up immediately. Our guides to EPA emission factors and CO2e and GWP values cover both.
4. Traceability. Pick any number in the report and get back to the invoice, meter reading or bill behind it. At limited assurance they will do this for a few figures; at reasonable assurance, for many.
5. Controls and review. Who prepared each figure, who reviewed it, what happens when a classification is uncertain, and is that process documented and followed? This is the difference between passing the two levels.
6. Estimates and exclusions. Where you estimated, what method and why; where you excluded, what and why. Documented estimates are acceptable. Undocumented ones are findings.

How to prepare now for reasonable assurance later

The mistake is treating assurance as something that starts when the auditor arrives. Reasonable assurance tests a system, so the system has to exist during the year being reported on. Three habits cover most of it, and none of them are expensive if you build them in from the start.

Keep the evidence attached to the number, not in a folder next to it. Every calculated figure should carry a link to the invoice, meter reading or export it came from, with the factor and version applied. Retrofitting that link across a year of data is the single largest cost in a late assurance scramble.

Write the inventory management plan and follow it. It records your boundary, methods, factor sources, estimation approaches, review steps and roles. It is the document a provider reads first, and a company that maintains one honestly usually finds the engagement goes quietly. Teams already running structured obligation and control tracking for other regulated processes can extend the same obligation and control tracking to emissions rather than inventing a separate discipline for it.

Make review a step, not an afterthought. A reviewer who is not the preparer should sign off on classifications, unusual variances and any change in method, with the sign-off recorded. That single control does more for a reasonable assurance opinion than any amount of methodological sophistication.

Is limited assurance enough for investors and customers?

Usually, for now. Limited assurance is what most voluntary disclosure programs and the majority of assured corporate climate reports currently carry, and an enterprise customer asking a supplier for carbon data rarely demands more than a credible, documented number. The direction of travel is clearly toward reasonable assurance, driven by regulation rather than by buyers, and companies that build their inventory to a traceable standard from the outset simply do not experience that transition as an event. Those that do not, pay for the same work twice.

That traceability is what our carbon accounting software is built around: the scope and category for every ledger line are drafted with a confidence level, a person reviews the uncertain tail, the reviewer and timestamp are recorded, and each figure keeps a link back to the source invoice. For the full first-inventory sequence, see the GHG inventory checklist, and for the standard underneath it all, the GHG Protocol Corporate Standard explained.