ISSA (UK) 5000: What It Means for the Carbon Data Your SME Clients Need to Produce

The FRC published ISSA (UK) 5000 on 12 November 2025 — the UK’s first dedicated sustainability assurance standard. It applies to assurance engagements covering sustainability information for reporting periods beginning on or after 15 December 2026. If you’re an accountant advising SME clients on carbon reporting, this standard changes the quality bar for the data they produce.

Why accountants need to pay attention now

ISSA (UK) 5000 is profession-agnostic. Both accountants and non-accountant practitioners can conduct sustainability assurance engagements. The FRC is building an interim register of sustainability assurance practitioners by mid-2026 — which means a new market for assurance work is opening up, and accountants who understand the data requirements will be first in line.

The downstream effect matters more than the standard itself: listed companies reporting under UK SRS from January 2027 will need assurance on their sustainability disclosures. That assurance chain reaches into supply chain data. When your SME client supplies carbon figures to a listed customer, those figures may end up in an assured report. The data quality expectations flow down.

According to edie research, 58% of SME leaders have never heard of Scope 1, 2, and 3 definitions. Closing that gap with audit-ready data — not rough estimates — is where accountants add value.

What assurance providers will actually check

ISSA (UK) 5000 establishes a framework for both limited and reasonable assurance over sustainability information. For most SME supply chain data, limited assurance is the relevant level. Here’s what that means in practice.

Data provenance

Assurance providers will check where the numbers come from. “We estimated our emissions” is not sufficient. They want to see the trail: source data (utility bills, fuel receipts, mileage logs), the conversion factors applied, and the methodology used to calculate the final figure. If your client’s carbon report was produced by multiplying a single annual spend figure by a generic factor, that trail is thin.

Methodology consistency

The calculation methodology needs to be documented and consistently applied across reporting periods. Which emission factor set was used? DESNZ/DEFRA, or something else? Were any factors changed mid-year? Were organisational boundaries defined using the operational control or equity share approach? These choices need to be stated, not assumed.

Completeness

The assurance provider will assess whether material emission sources have been included. An SME that reports only electricity and ignores company vehicles, gas heating, or business travel has a completeness problem. The GHG Protocol requires disclosure of which sources are included and which are excluded — and justification for exclusions.

What “limited assurance” means in practice

Limited assurance is less rigorous than the reasonable assurance applied to financial statements, but it’s not a rubber stamp. The practitioner performs analytical procedures, inquiries, and limited testing to determine whether anything has come to their attention that causes them to believe the sustainability information is materially misstated.

For SME carbon data, this typically means the assurance provider will:

Review the source documentation — utility bills, fleet records, expense reports used to calculate emissions. They won’t audit every invoice, but they’ll check a sample.

Test the conversion factor application — confirm that the correct DESNZ factors were applied to the correct activity data. A common error: applying the wrong fuel type factor or using an outdated factor year.

Check the boundary definition — confirm that the organisational boundary matches what’s been reported. If your client leases three offices but only reports emissions for two, that’s a finding.

Assess management representations — the SME’s management will need to confirm the data’s completeness and accuracy. This is where having a documented methodology matters: it’s harder to make that representation confidently if the numbers were produced ad hoc.

How to prepare your SME clients

Keep activity data, not just spend data

The most important shift: move clients from spend-based estimates to activity-based data. A spend-based calculation (total £ on gas × generic factor) produces a number, but it’s hard to assure because the factor is broad and the data trail is indirect. An activity-based calculation (X kWh of gas from supplier invoices × DESNZ factor for natural gas) produces a number that traces back to verifiable source documents.

Document the methodology upfront

Before calculating anything, write down: the reporting period, the organisational boundary, the emission sources included, the emission factor set used, and any assumptions or estimates applied. This document becomes the backbone of the assurance engagement. Without it, the assurance provider has to reconstruct your client’s approach — which costs time and money.

Use the current DESNZ conversion factors

The 2025 DESNZ conversion factors are current. The 2026 edition is expected in June. Always match the factor year to the reporting period. Using 2024 factors for a 2025/26 report is a common error that an assurance provider will flag.

Retain source documents for at least two reporting periods

Utility bills, fuel receipts, mileage logs, and any correspondence with landlords about energy data should be retained. If your client can’t produce the source document behind a reported figure, that figure becomes an estimate — and estimates carry higher assurance risk.

If you’re working with a sustainability consultant, they may already use CYF — it’s designed to let consultants manage multiple client accounts from one dashboard, with full audit trails for each.

How CYF helps

CYF produces reports with the audit trail ISSA (UK) 5000 demands: every emission figure traces back to the activity data entered, the specific DESNZ conversion factor applied, and the factor’s provenance. The methodology is documented within the tool — not reconstructed after the fact. No competitor is currently positioning their output as “assurance-ready,” and for accountants managing multiple SME clients, this matters at scale.

Methodology guidance from Alice Roberts, Head of Methodology at Compare Your Footprint.

Prepare your clients’ carbon data for assurance — see how CYF works or read our guide to SECR for accountants.