ESOS Phase 4 Deadline 2027: What You Need to Do in 2026

The formal ESOS Phase 4 compliance deadline is 5 December 2027. That sounds comfortably distant from where we are in April 2026. It isn't — and for organisations that were in scope for Phase 3, there is a separate and much closer deadline sitting in December 2026 that is quietly catching people out.

This page sets out the Phase 4 timeline as it actually operates in 2026, what you should be doing this year to stay in control, and where the genuine pressure points are between now and final submission.

The dates that matter

Four dates define the Phase 4 compliance journey:

6 December 2023 — Phase 4 compliance period begins. Your 12-month ESOS reference period must fall inside this window.

5 December 2026 — Progress Update 2 deadline. Organisations that submitted Phase 3 notifications of compliance must report their progress against the Action Plan commitments they made. This deadline is already live, and it applies during Phase 4 rather than at the end of it.

31 December 2026 — Phase 4 qualification date. Your organisation's size is assessed at this point. If you meet the thresholds on this date, you are in scope for Phase 4.

5 December 2027 — Phase 4 compliance deadline. Notification of compliance must be submitted through the Environment Agency's MESOS portal by this date.

The pattern should be obvious: Phase 4 is not a single end-of-period reporting exercise. It is a rolling compliance regime with two significant 2026 milestones and a final submission window that opens on 1 January 2027.

The 5 December 2026 Progress Update: the deadline most organisations are missing

Organisations that submitted ESOS Phase 3 notifications are required to submit two Progress Updates against the Action Plan they filed. Progress Update 1 has already passed. Progress Update 2 is due by 5 December 2026.

The content of the Progress Update is straightforward: what energy-saving measures from your Phase 3 Action Plan have you actually implemented, what savings have been achieved, and — if commitments have not been met — what is the explanation?

Three things are worth knowing about this requirement:

1. A Lead Assessor is not required for Progress Update submissions. Organisations can submit directly through MESOS. But many choose to use their Phase 3 Lead Assessor because the data reconciliation benefits from their familiarity.

2. There are no financial penalties for late Progress Update submission. But non-submission means your organisation appears on a public register as not reporting progress — a reputational signal that matters increasingly in ESG-indexed financing, public procurement, and supply chain due diligence.

3. Phase 4 reporting picks up from here. Under the Phase 4 amendment regulations, progress against your Action Plan commitments must be included in your Phase 4 ESOS assessment, with explanations where commitments have not been met. Organisations that sail past 5 December 2026 without preparing this data are storing up extra work for their Phase 4 submission.

If you were in scope for Phase 3, this is the most immediate ESOS action on your 2026 calendar.

Qualification at 31 December 2026: who is in scope for Phase 4

You qualify for ESOS Phase 4 if, at 31 December 2026, your organisation meets either of the following criteria:

• Employs 250 or more people
• Has an annual turnover exceeding £44 million and an annual balance sheet total exceeding £38 million

Qualification is assessed against the financial year ending on or immediately before 31 December 2026. The group aggregation rules apply: if any entity in your corporate group meets the qualification thresholds, the whole UK operation of the group falls into scope, and the ultimate UK parent becomes the responsible undertaking for notification purposes.

Three practical implications:

You may be newly in scope. Organisations that grew through 2024–2026, acquired UK subsidiaries, or crossed thresholds after Phase 3 qualification will be new ESOS participants for Phase 4. Existing in-house energy management routines may not exist — and starting from scratch takes longer than people expect.

You may be newly out of scope. Organisations that have restructured, divested or shrunk may be below thresholds at 31 December 2026. If so, Phase 4 does not apply — but confirming that position formally is worth doing, because Phase 3 data may have created an assumption in the Environment Agency's records that needs to be corrected.

Parent changes matter. If your group structure has changed materially since Phase 3, the responsible undertaking may also have changed. Mis-identifying the responsible undertaking is one of the most persistent ESOS errors, and it is one that the regulator focuses on.

The reference period window

For Phase 4, the 12-month ESOS reference period must include the qualification date of 31 December 2026 and end before the compliance deadline of 5 December 2027. The earliest the reference period can start is 1 January 2026, and the latest it can end is 4 December 2027. Within that 23-month window, you pick any consecutive 12 months.

For most organisations, the simplest choice is calendar year 2026 (1 January 2026 to 31 December 2026). It aligns with most financial reporting, it closes out at the qualification date, and it means data collection can start in earnest from the beginning of 2027.

There is one consequence of this that organisations often miss: your 2026 energy data is your ESOS Phase 4 reference data. If 2026 is the year your metering fails, your invoices get miscategorised, or your fleet fuel card is mis-coded, those data quality problems become embedded in your submission. Checking that your data collection routines are fit for purpose now, in Q2 2026, is worth a disproportionate amount of the effort you will spend on the whole Phase 4 engagement.

What to do in each quarter of 2026

A realistic Phase 4 workplan for an organisation that was in scope for Phase 3:

Q1–Q2 2026 (now)

• Review your Phase 3 Action Plan and collect implementation evidence
• Audit your energy and transport data collection: meter readings, supplier invoices, fleet fuel cards
• Begin Lead Assessor procurement — contact two or three approved-register consultants for scoping conversations
• Review group structure and confirm responsible undertaking remains correct
• Check whether any sites have been added or divested since Phase 3

Q3 2026

• Draft Progress Update 2 content with your Lead Assessor or internal team
• Start preliminary site data collection if using calendar year 2026 as the reference period
• Confirm whether ISO 50001 certification (if held) covers 100 percent of energy use at the qualification date — if not, you cannot use it as a Phase 4 compliance route without adjustment
• Engage with finance on Phase 4 budget allocation for 2027

Q4 2026

• Submit Progress Update 2 by 5 December 2026
• Confirm qualification at 31 December 2026 based on finalised financial data
• Lock in Lead Assessor engagement with signed proposal

Q1 2027

• Close out 2026 reference period data collection
• Begin site visits and detailed audits
• Lead Assessor begins review and analysis

Q2–Q3 2027

• Audit fieldwork completed
• Draft ESOS Phase 4 report and Action Plan
• Internal review and sign-off cycle

Q4 2027

• Notification of compliance submitted through MESOS
• Action Plan submission
• Records retained as required

Organisations that follow this rhythm routinely submit in October or November 2027. Those who start in mid-2027 almost invariably submit in the final two weeks of November or early December, under pressure, with higher fees and less choice of Lead Assessor.

What has changed in Phase 4

The Energy Savings Opportunity Scheme (Amendment) Regulations 2023 introduced three material changes that affect how you approach Phase 4:

Display Energy Certificates and Green Deal Assessments are no longer accepted compliance routes. Organisations that previously relied on DECs for public-sector or partially-occupied estates must now use an alternative route — energy audit, ISO 50001 certification, or a hybrid approach.

Action Plan progress reporting is embedded in the scheme. This is not a one-off Phase 3 exercise. Progress updates are a continuing feature of how ESOS operates.

PAS 51215 alignment for Lead Assessors. The competence framework for ESOS Lead Assessors has been formalised against the BSI PAS 51215 standard, parts of which were published through 2025.

The net effect is a scheme that is more rigorous, more rolling and more focused on delivery of energy savings rather than the production of audit reports that sit on shelves.

The cost of waiting

The biggest risk to a clean Phase 4 submission is not regulatory — it is operational. Every phase to date has seen the same pattern:

• Lead Assessor capacity tightens from approximately June of the final year
• Fees rise in Q3–Q4 of the final year as demand outstrips supply
• Quality of engagement falls as assessors take on too much work in a compressed window
• Data collection scrambles in the final weeks create errors that carry into submission

For Phase 4, that compression point falls in the second half of 2027. Organisations that commission work in Q1 2026 or Q1 2027 can still choose their Lead Assessor. Organisations that start in Q3 2027 are choosing from whoever has capacity left.

The government's consultation response on Phase 4 also confirmed that a net-zero element is planned for Phase 5, which will begin on 6 December 2027 with a qualification date of 31 December 2030. The implications for Phase 4 Action Plans are that they should increasingly be set up to support net-zero alignment rather than purely energy efficiency — an additional reason to engage a Lead Assessor who understands the direction of travel.

For organisations navigating ESOS alongside wider climate disclosure, the UK SRS deadline arriving 11 months before ESOS Phase 4 makes the 2026–2027 period particularly demanding on sustainability functions.

Next steps

• Book a Phase 4 readiness call to confirm your Progress Update 2 status
• Request a qualification review against 31 December 2026 thresholds
• Download our Phase 4 data collection template for 2026 reference periods

Sources and References

• GOV.UK — Energy Savings Opportunity Scheme (ESOS) — Official scheme guidance, Phase 4 dates, qualification criteria
• Energy Savings Opportunity Scheme (Amendment) Regulations 2023 — SI 2023/1182, the statutory instrument introducing Phase 4 changes
• Energy Savings Opportunity Scheme Regulations 2014 — The primary ESOS statutory instrument
• UK Government ESOS Post-Implementation Review 2025 — Review of scheme effectiveness and Phase 4 transitional arrangements
• GOV.UK — Comply with ESOS: Appendix D — Regulator contact points for England, Wales, Scotland and Northern Ireland

Related pages

• ESOS Consultant UK — How to hire a certified Lead Assessor
• ESOS Assessment Cost UK 2026 — Typical Phase 4 fee ranges
• UK SRS Deadline — The 2027 climate disclosure deadline that sits alongside ESOS
• SECR and UK SRS — How SECR reporting interacts with the wider compliance landscape