Yes, ESOS is mandatory. Qualifying UK organisations must comply with the Energy Savings Opportunity Scheme Regulations 2014 (SI 2014/1643), as amended by SI 2023/1182 for Phase 4. The current Phase 4 compliance deadline is 5 December 2027, with a qualification date of 31 December 2026.

Non-compliance is enforced through civil sanctions by the Environment Agency (and the equivalent regulator in Scotland, Wales, Northern Ireland, and offshore) — not criminal prosecution. The statutory maximum penalty is £45,000 for failure to notify and £90,000 for failure to undertake an energy audit, plus publication of the breach on a public register.

Below: who's in scope, what ESOS actually requires under Phase 4, the penalty structure, the changes introduced by SI 2023/1182, and what was postponed to Phase 5.

← Part of our ESOS, SECR and UK SRS guide

Who has to comply (qualification thresholds)

ESOS applies to UK "large undertakings" — bodies corporate, partnerships, or unincorporated associations carrying on a trade or business — tested at the qualification date, which for Phase 4 is 31 December 2026. An undertaking is large if it meets either of these tests:

• 250 or more employees, OR
• Annual turnover exceeding £44 million AND balance sheet total exceeding £38 million

Note the structure. There is an OR between the two limbs, but an AND inside the financial limb: exceeding the turnover figure alone does not qualify you.

Corporate group rule: an undertaking that is part of a group containing at least one large undertaking on the qualification date is in scope, even if the individual entity is below the thresholds.

Threshold alignment with SECR — not adopted. The government announced an intention to align ESOS qualification thresholds with the Streamlined Energy and Carbon Reporting (SECR) regime to reduce duplication. That change will not proceed in Phase 4 and has been postponed to Phase 5; ESOS retains the test set out above. The ESOS test is a different and higher one than SECR's two-of-three test at £36m turnover / £18m balance sheet / 250 employees, and any page telling you the two now align is wrong. Source: GOV.UK ESOS guidance; see also Burges Salmon — Changes to Phase 4 of ESOS and Survey and Test — ESOS Phase 4 Update.

For detailed coverage see ESOS Thresholds — who qualifies for Phase 4 and ESOS, SECR and UK SRS — how the three regimes work together.

What ESOS actually requires

Each ESOS phase requires qualifying organisations to complete an energy audit covering at least 95% of total energy consumption (raised from 90% by SI 2023/1182 — the "de minimis" threshold dropped from 10% to 5%). The audit must:

• Cover energy used in buildings, industrial processes, and transport
• Identify cost-effective energy-saving recommendations
• Be reviewed and signed off by an approved Lead Assessor (unless the organisation is exempt from sign-off — see below)
• Result in an ESOS report, an ESOS action plan, and ESOS progress updates (the action plan and progress update structure was introduced as a Phase 4 requirement by SI 2023/1182, Part 6A)

Compliance is then notified to the Environment Agency through the Manage your Energy Savings Opportunity Scheme (MESOS) digital system.

Compliance routes for Phase 4

Phase 4 has narrowed the available routes to compliance compared to Phase 3:

• ESOS energy audits signed off by a Lead Assessor — the primary route
• ISO 50001 certification covering total energy use (whole-organisation route under regulation 33) — the principal alternative

Routes removed for Phase 4 (a significant change from Phase 3):

• ❌ Display Energy Certificates (DECs) are no longer accepted as a compliance route
• ❌ Green Deal Assessments (GDAs) are no longer accepted as a compliance route

This change was confirmed by the Department for Energy Security and Net Zero (DESNZ) in February 2025 and applies to all Phase 4 audits. Organisations that previously relied on DECs or GDAs for partial coverage need to plan for full ESOS energy audits or expanded ISO 50001 scope. Source: GOV.UK ESOS Guidance — latest updates, Green Business Journal — Big 2026 energy shake-up.

Lead Assessor sign-off exemption

All participants must have their ESOS assessment signed off by a Lead Assessor, with two exceptions:

• Organisations whose energy supplies are 100% covered by ISO 50001 (in which case no separate Lead Assessor is required)
• Organisations with total energy consumption below 40,000 kWh (which still need to comply with ESOS but can use a self-declared compliance route)

For guidance on finding and engaging a Lead Assessor, see Finding ESOS consultants and Lead Assessors.

ESOS legal foundation

ESOS was created by The Energy Savings Opportunity Scheme Regulations 2014 (SI 2014/1643), made under the European Communities Act 1972 to implement Article 8 of the EU Energy Efficiency Directive (2012/27/EU). After Brexit, the Regulations remained in force as retained EU law and continue to operate as a domestic UK scheme.

The most significant amendment to date is The Energy Savings Opportunity Scheme (Amendment) Regulations 2023 (SI 2023/1182), made on 7 November 2023 by DESNZ. This SI strengthens audit standards for Phase 4, inserts the new Action Plan and Progress Update structure (Part 6A), and amends approximately 28 separate provisions of the original 2014 Regulations.

The new penalty provisions described below came into effect on 8 February 2022. Source: SI 2014/1643, SI 2023/1182 — table of contents.

The Phase 4 dates

• 6 December 2023 – 5 December 2027 — the Phase 4 compliance period
• 31 December 2026 — Phase 4 qualification date
• 5 December 2027 — Phase 4 compliance (notification) deadline
• 5 December 2028 — Phase 4 Action Plan
• 5 December 2029 and 5 December 2030 — Phase 4 progress updates

The last three follow from the stated rule rather than a published table; GOV.UK tabulates explicit dates for Phase 3 only.

One Phase 3 obligation is still live alongside all of this: Progress Update 2 is due 5 December 2026.

Enforcement bodies

Enforcement powers are split between five UK regulators:

The Environment Agency is the lead administrator for the UK as a whole and applies civil sanctions per its Enforcement and Sanctions Policy, Annex 2. Each devolved regulator operates an equivalent civil sanctions framework.

ESOS penalties and fines

Civil penalties regime. ESOS uses civil sanctions under the Regulatory Enforcement and Sanctions Act 2008 framework. Penalties are statutory, capped, and proportionate to the breach — not open-ended criminal fines.

The Environment Agency's published policy sets the structure for the most common breaches:

New entrants to ESOS in the compliance period in which they first enter the scheme normally face a lower initial penalty of up to £5,000 for failure to undertake an energy audit (rather than the standard £50,000). In subsequent compliance periods the lower threshold no longer applies.

Worked example from the Annex (illustrative): a large organisation with negligent culpability that failed to undertake an energy audit might face an initial penalty range of £12,600 to £67,500 within the £90,000 statutory cap, before mitigation.

Asymmetric enforcement. Penalties attach to the compliance-date obligations — up to £50,000 plus up to £40,000 in daily penalties for failure to undertake an assessment. There is no penalty for non-submission of an action plan or a progress update. The only consequence there is publication on the public register, so any source telling you that missing the action plan attracts a fine is wrong.

Publication. In addition to the financial penalty, the Environment Agency publishes details of organisations subject to enforcement action. This is typically the most material reputational consequence of non-compliance — sustainability disclosures, customer due diligence questionnaires, and investor ESG screens increasingly check for ESOS non-compliance.

Enforcement activity to date. As of April 2025, the Environment Agency has issued: 2,751 compliance notices; 2,133 enforcement notices; and 83 civil penalties amounting to £1,217,440 across Phases 1–3. Phase 3 saw notably active enforcement. Source: Compliance365 — ESOS penalties analysis (April 2025).

What changed for Phase 4 (SI 2023/1182)

SI 2023/1182, made on 7 November 2023 by DESNZ, introduced the most substantive set of changes to ESOS since 2014. The changes affect Phase 3 (deadline extended to 5 June 2024 for some purposes) and apply in full from Phase 4 (deadline 5 December 2027).

The key Phase 4 changes that apply:

• Reduction in the de minimis threshold from 10% to 5%. Audits must now cover 95% of total energy consumption, raising the bar for thoroughness.
• New ESOS action plan requirement (Part 6A inserted into the Regulations). Organisations must produce a forward-looking action plan setting out which audit recommendations they intend to implement, with what targets and by when. The action plan must have board sign-off.
• New ESOS progress update requirement (also Part 6A). Annual progress reporting against the action plan during the compliance period. Where action plan commitments have not been met, participants must provide an explanation.
• Mandatory public disclosure of energy consumption, intensity ratios, and material recommendations from the audit. Previously this information stayed in the audit report.
• Strengthened Lead Assessor role (regulation 21 amended) with more explicit responsibilities for the quality and standardisation of the audit.
• Removal of DECs and Green Deal Assessments as compliance routes (per the gov.uk update of February 2025).

Still to come, subject to parliamentary time. GOV.UK also lists three changes intended to proceed: removal of DECs and Green Deal Assessments as compliance routes; progress against action-plan commitments to be included in the ESOS assessment; and a mandatory explanation where commitments have not been met. Legislation, guidance and system changes are planned to be in place by early 2027.

Lead assessor registers. The only substantive change to the ESOS guidance during 2026 was administrative: the Institution of Chemical Engineers was removed from the list of approved registers for ESOS lead assessors on 16 February 2026. If your lead assessor was IChemE-registered, check their standing on a currently approved register before Phase 4.

Net zero requirements — postponed to Phase 5

The previous government's response to the ESOS consultation (July 2021) signalled an intention to require Phase 4 participants to include net zero considerations within their ESOS audit. This has been postponed to Phase 5 (2027–2031).

In February 2025, DESNZ confirmed the postponement, citing delays to Phase 3 legislation and the need to give organisations sufficient time to prepare. The decision was confirmed on the gov.uk ESOS guidance page:

"Following delays to the introduction of Phase 3 legislation and guidance, and in order to give sufficient time for such a major change to ESOS, the new government has decided to postpone the introduction of net zero requirements until Phase 5."

Source: GOV.UK ESOS Guidance, ESOS Newsletter Issue 33, February 2025 (PDF).

Voluntary net zero assessment in Phase 4 — PAS 51215

Two new PAS (Publicly Available Specification) standards were published on 7 February 2025 by BSI in collaboration with DESNZ, allowing organisations to voluntarily integrate net zero considerations into their Phase 4 ESOS work:

• PAS 51215-1:2025 — Energy and decarbonisation assessment, Part 1: Process — Specification. This standard describes the process for carrying out a combined energy and decarbonisation assessment in a single workflow to produce an implementation plan for achieving net zero across the scope and boundaries of the assessment.
• PAS 51215-2:2025 — Energy and decarbonisation assessment, Part 2: Competencies of lead assessors and assessment teams — Specification. This standard updates the previous PAS 51215:2014 (which remains the competency standard for ESOS Lead Assessors for Phase 4) by strengthening existing energy competencies and adding new ones for GHG and net zero.

Organisations that voluntarily follow PAS 51215-1:2025 during Phase 4 can:

• Carry out an assessment that includes GHG emissions related to the energy currently covered by ESOS
• Assess governance and risks related to achieving net zero across those emissions
• Propose an implementation plan to achieve net zero by 2050 across those emissions
• Provide feedback to DESNZ to inform decisions on Phase 5 mandatory net zero requirements

Updates to the MESOS digital system for Phase 4 will include provision for participants to disclose voluntary use of PAS 51215-1:2025. PAS 51215:2014 continues to be used as the Lead Assessor competency standard for Phase 4. Source: Burges Salmon — Changes to Phase 4 of the Energy Savings Opportunity Scheme, ESOS Newsletter Issue 33 (February 2025).

What about UK SRS, SECR, and ESOS together?

Voluntary use of PAS 51215-1:2025 in Phase 4 may also be useful preparation for UK SRS S2 (Climate-related Disclosures), which will require in-scope listed issuers to disclose Scope 1, 2, and 3 emissions, transition plan information, and progress against climate targets from accounting periods beginning 1 January 2027 under FCA Consultation Paper CP26/5. Organisations using PAS 51215-1:2025 will be building emissions data infrastructure that supports both regimes.

For detailed coverage see ESOS, SECR and UK SRS — how the three regimes work together.

Common misconceptions

"ESOS is criminal — directors can be prosecuted."

No. ESOS is enforced through civil sanctions under the Regulatory Enforcement and Sanctions Act 2008 framework. There are no Crown Court prosecutions, no criminal records, and no "unlimited" fines. The actual statutory maximums are £45,000 for failure to notify and £90,000 for failure to undertake an audit, plus publication of non-compliance.

"If I'm under the Companies Act 'large company' threshold, I'm not in scope."

Not necessarily. ESOS uses its own test — 250 or more employees, OR turnover above £44m AND balance sheet total above £38m. The April 2025 changes to Companies Act size thresholds (which raised the general "large company" thresholds) do not affect ESOS qualification. ESOS and Companies Act size tests are independent, and so are ESOS and SECR: the announced alignment of ESOS thresholds with SECR was postponed to Phase 5.

"ESOS overlaps with SECR — I only need to do one."

Different regimes, different scope, different obligations. ESOS is a four-yearly energy audit scheme administered by the Environment Agency. SECR is annual energy and emissions reporting in the Directors' Report under the Companies Act 2006. UK SRS S2 (where in scope) is annual climate-related financial disclosure under FCA Listing Rules. Most large UK organisations are in scope of all three. The data infrastructure can be shared but the outputs are separate. See ESOS vs SECR and ESOS, SECR and UK SRS combined guide.

"ISO 50001 certification means I'm exempt from ESOS."

Not exempt — but it's an alternative compliance route under regulation 33 if the certification covers total energy use. You still need to notify the Environment Agency via MESOS. Where ISO 50001 covers only part of your energy use, you must use an ESOS energy audit for the remainder to meet the 95% coverage threshold.

"I used a Display Energy Certificate for Phase 3, so I can use one for Phase 4."

Not anymore. DECs and Green Deal Assessments have been removed as compliance routes for Phase 4. The only routes remaining are ESOS energy audits (signed off by a Lead Assessor) and ISO 50001 certification. Organisations that relied on DECs or GDAs in Phase 3 will need to plan for full Phase 4 audits or expanded ISO 50001 scope.

"Net zero is now mandatory under ESOS Phase 4."

No, it's been postponed. The previous government had signalled an intention to make net zero considerations mandatory in Phase 4 audits, but DESNZ confirmed in February 2025 that this has been postponed to Phase 5 (2027–2031). Voluntary net zero assessment under PAS 51215-1:2025 is available during Phase 4.

Practical compliance steps

Six steps to Phase 4 compliance:

1. Confirm qualification at 31 December 2026. Test the corporate-group position as well as the individual entity. If any UK undertaking in the group is a "large undertaking" on the qualification date, all other UK undertakings in the group are in scope.

2. Engage a Lead Assessor registered on a currently approved register — note that IChemE was removed from the approved registers on 16 February 2026 — or maintain ISO 50001 certification covering 100% of significant energy use. The market is competitive; lead time of 6+ months is typical for Phase 4. Lead Assessor competency is set under PAS 51215:2014 for Phase 4 and is being updated to PAS 51215-2:2025 for voluntary application.

3. Determine the energy audit scope — sites, transport, processes — covering at least 95% of total UK energy consumption. Document the methodology for calculating the 95% threshold; the EA may scrutinise this if it conducts enforcement.

4. Conduct the energy audit (or use ISO 50001 as alternative route for parts of the consumption). DECs and Green Deal Assessments are no longer compliance routes. Optionally include net zero considerations using PAS 51215-1:2025 as a voluntary route.

5. Produce the ESOS report, action plan, and progress update structure (Part 6A). The action plan is new for Phase 4 and needs board sign-off. Progress updates against the action plan are required during the compliance period.

6. Notify the Environment Agency by 5 December 2027 via the MESOS (Manage your Energy Savings Opportunity Scheme) digital system. New entrants benefit from a reduced initial penalty for failure to undertake an audit in their first compliance period — but the standard penalty applies in subsequent periods.