Carbon accounting for UK SRS S2 — Scope 1, 2 and 3 practitioner guide
The practitioner guide to carbon accounting under UK SRS S2. GHG Protocol Corporate Standard methodology, DESNZ conversion factors, Scope 1 + 2 + 3 calculation and the value-chain assessment UK SRS S2 demands from 2027. Plus the audit-ready evidence trail ISSA (UK) 5000 will expect.
The GHG Protocol Corporate Standard
The global methodology used by UK SRS S2, IFRS S2, ESRS E1, SECR, TCFD and most national reporting regimes. Three scopes, two consolidation approaches, seven greenhouse gases.
Scope 1, Scope 2, Scope 3 explained
The architecture every UK carbon accounting exercise uses. Scope 1 (direct), Scope 2 (purchased energy), Scope 3 (value chain across 15 categories).
- Scope 1 — Direct emissions Owned or controlled sources
- Direct GHG emissions from sources owned or controlled by the company: fuel combustion in company vehicles, on-site combustion (boilers, furnaces, generators), process emissions, fugitive emissions (refrigerants, methane leaks). Calculated by multiplying activity data (litres of fuel, kg of refrigerant) by DESNZ emission factors. Mandatory under UK SRS S2, SECR and SI 2022/31.
- Scope 2 — Purchased energy Indirect from purchased electricity, heat, steam, cooling
- Two methods both required: location-based (using UK grid emission factor from DESNZ) and market-based (using residual mix factor where no contractual instruments, or supplier-specific factor where verified). UK SRS S2 follows GHG Protocol Scope 2 Guidance — both methods must be disclosed.
- Scope 3 — Value chain emissions 15 GHG Protocol categories
- Upstream and downstream emissions across 15 categories. Upstream: Cat 1 purchased goods and services; Cat 2 capital goods; Cat 3 fuel and energy related; Cat 4 upstream transport; Cat 5 waste; Cat 6 business travel; Cat 7 employee commuting; Cat 8 upstream leased assets. Downstream: Cat 9 downstream transport; Cat 10 processing of sold products; Cat 11 use of sold products; Cat 12 end-of-life treatment; Cat 13 downstream leased assets; Cat 14 franchises; Cat 15 investments.
- Operational vs financial control Boundary choice
- GHG Protocol allows two consolidation approaches. Operational control: include emissions from operations the company has authority to operate. Financial control: include emissions from operations consolidated in financial statements. Most UK preparers use operational control for emissions and financial control for connectivity to financial statements. Document the choice and apply consistently.
- Seven Kyoto Protocol GHGs Coverage
- CO2, CH4, N2O, HFCs, PFCs, SF6, NF3 — converted to tonnes of CO2-equivalent (tCO2e) using IPCC Global Warming Potentials (typically AR5 100-year GWPs unless stated). UK SRS S2 follows GHG Protocol on coverage.
UK SRS S2 vs SECR — what changes
SECR has required Scope 1 + 2 since 2019. UK SRS S2 adds Scope 3 and significantly tightens every other dimension of carbon accounting.
Six-step UK SRS S2 carbon accounting process
From boundary definition to disclosure. Typically 6–12 months end-to-end for the first full Scope 1 + 2 + 3 exercise.
Typical first-time UK SRS S2 carbon accounting cycle
Companies starting from a SECR-only baseline typically need 12-18 months to reach UK SRS S2 audit-readiness.
The longest pole is Scope 3 — supplier engagement for Categories 1 and 11, business-travel data centralisation, and use-of-sold-products methodology development.
Companies that start in 2025 are well-placed for FY 2027 reporting; companies starting in 2026 face significant catch-up.
What ISSA (UK) 5000 will demand
The FRC’s sustainability assurance standard under development codifies what auditors will expect. Designing carbon accounting for audit-readiness is the cheapest path to credible disclosure.
- Source documentation Foundation
- Every reported data point traceable to source: utility bill PDFs, fleet management exports, travel-management reports, supplier-confirmed emissions, payroll records (for commuting). Sample-tested by assurer. Missing source documentation is the most common assurance failure.
- Methodology log Calculation transparency
- Per-calculation log: emission factor source (DESNZ table, IEA, supplier), factor version and date, calculation formula, assumptions, estimation method (activity-based vs spend-based for Scope 3). Auditable trail from source data to disclosed metric.
- Boundary documentation Scope
- Operational vs financial control choice documented. Consolidated entities listed. Joint ventures and associates treated consistently. Boundary changes vs prior year explained. Subsidiaries acquired/divested disclosed.
- Restatement policy Comparability
- Documented restatement policy. Material restatements (typically >5% of prior reported figure) trigger restatement with explanation. Improvements in data quality, methodology changes, acquisition impact and boundary changes are common triggers.
- Internal controls + sign-off chain Governance
- Role-based approval: data owner enters; reviewer validates; controller signs off; CFO/CSO authorises final disclosure. Documented in the company's internal control framework. Aligned with forthcoming Audit Reform expectations.
- Materiality assessment for Scope 3 Scope justification
- Documented materiality screen for the 15 Scope 3 categories. Categories assessed against size of company spend, relevance to business model, stakeholder concern, and emissions intensity. Categories included AND excluded both explained.
Carbon accounting for UK SRS — frequently asked
What carbon accounting is, UK SRS S2 vs SECR, conversion factors, verification, and time to complete.
What is carbon accounting?
Carbon accounting is the systematic measurement, calculation and reporting of greenhouse gas (GHG) emissions associated with a company's activities.
Under the GHG Protocol Corporate Standard — the global methodology used by UK SRS S2, IFRS S2, ESRS E1, TCFD, SECR and most regulatory regimes — emissions are categorised across three scopes: Scope 1 (direct), Scope 2 (purchased energy) and Scope 3 (value chain across 15 categories).
How does UK SRS S2 differ from SECR on carbon accounting?
Three differences. (1) Scope coverage: SECR is Scope 1 and 2 only (with optional Scope 3 disclosure); UK SRS S2 requires Scope 1, 2 and 3. (2) Scope 3 depth: UK SRS S2 requires material categories across all 15 GHG Protocol categories with comply-or-explain in year one; SECR has no Scope 3 requirement. (3) Connectivity: UK SRS S2 requires explicit connectivity between emissions and financial statements; SECR sits in the directors' report largely separate from financials.
What conversion factors should UK companies use?
DESNZ (formerly BEIS) Greenhouse Gas Conversion Factors, updated annually each summer for use in reports.
The current versions are also referenced by UK SRS S2 and SECR.
For Scope 2 location-based calculations: UK grid emission factor from DESNZ; for market-based: residual mix factor or supplier-specific contractual instruments.
International operations use IEA country-specific factors or supplier-specific factors where verified.
Do I need to verify carbon accounting data?
Increasingly yes.
UK SRS S2 disclosures from FY 2027 will be subject to the FRC's ISSA (UK) 5000 sustainability assurance standard under development — limited assurance initially, with expectation of progression to reasonable assurance over time.
ISAE 3410 — the GHG-specific assurance standard from IAASB — applies in the interim.
Practical implication: design data systems for audit-readiness from day one.
How long does a first carbon accounting exercise take?
Scope 1 + 2: typically 3-6 weeks for a mid-market company with reasonably-organised utility data.
Scope 3: 4-12 months depending on value-chain complexity and supplier engagement maturity.
UK SRS S2 preparation usually requires software (Watershed, Persefoni, Sweep, Workiva, Climatise) above £100m turnover.
See our /carbon-reporting-software guide.
Continue across the related guides
From carbon accounting, continue to broader carbon accounting concept, GHG Protocol detail, software and UK SRS readiness.
Carbon accounting — concept guide
Definitional guide to carbon accounting methodology and history.
MethodGHG Protocol
The methodology underpinning carbon accounting under UK SRS S2.
StandardUK SRS S1 and S2
The UK standards that require carbon accounting from 2027.
Scope 3UK SRS Scope 3 reporting
Scope 3 specific guidance under UK SRS S2.
SoftwareCarbon reporting software
Independent reviews of UK carbon platforms.
AdjacentSECR reporting guide
The current UK carbon-reporting regime UK SRS S2 builds on.
Related guides & references
Primary references
GHG Protocol, DESNZ, IFRS Foundation, DBT and IAASB cited throughout this page.