The energy hierarchy — Be Lean, Be Clean, Be Green — is the backbone of almost every energy statement submitted with a UK planning application. It requires a scheme to cut carbon in a fixed order: first use less energy, then supply energy efficiently, and only then add renewables, with CO₂ savings counted at each stage.
The sequence matters because councils do not just want a low headline number — they want to see that fabric and systems were improved before solar panels were used to buy back the shortfall. This guide explains each stage and how the numbers are assembled.
Where the Be Lean, Be Clean, Be Green hierarchy comes from
The hierarchy is set out in the London Plan 2021 (Policy SI2, "Minimising greenhouse gas emissions"), supported by the GLA's Energy Assessment Guidance, and has been adopted in spirit by many councils outside London. An energy statement following the hierarchy starts from a Part L 2021 baseline — the "notional building" for the same dwelling calculated under SAP 10.2 — and reports the percentage CO₂ improvement achieved at the end of each stage.
Be Lean: use less energy
The first stage is demand reduction through fabric and passive design:
- Better U-values for walls, roofs, floors and glazing than the notional building assumes;
- Improved airtightness with appropriate ventilation, and reduced thermal bridging;
- Passive measures — orientation, glazing proportion, solar shading and form factor.
Because Be Lean savings are locked into the building for its whole life, planners weight this stage heavily, and London expects a meaningful share of the total saving to be delivered here before any technology is added.
Be Clean: supply energy efficiently
The second stage addresses how heat and power are supplied. In London this historically meant connecting to (or future-proofing for) district heat networks; Policy SI3 still requires heat-network investigation in Heat Network Priority Areas. For most small and medium schemes outside network areas, the honest Be Clean answer is "no viable opportunity", and the statement simply evidences that conclusion — an empty Be Clean stage is normal and is not a failure.
Be Green: use renewable energy
The final stage adds low- and zero-carbon technologies: air- or ground-source heat pumps, rooftop solar photovoltaics and solar thermal. Under SAP 10.2's electricity emission factor, heat pumps deliver large carbon savings relative to gas boilers — compare the two for your own scheme with our free heat pump carbon calculator. PV arrays then extend the on-site saving, subject to roof area, orientation and overshading.
The numbers: targets, shortfalls and carbon offset payments
For major schemes in London, Policy SI2 requires a minimum 35% on-site CO₂ improvement over the Part L 2021 baseline, on the way to net zero-carbon for residential development. Any remaining regulated emissions are converted into a cash contribution to the borough's carbon offset fund — you can estimate that liability with our London carbon offset calculator. Outside London, adopted local-plan policies vary from a simple "follow the hierarchy" narrative to explicit percentage targets, so always check the validation checklist before fixing the specification.
Some London boroughs also ask referable schemes to report in-use energy under the "Be Seen" strand of the GLA guidance — worth flagging early because it affects metering design.
Common mistakes that get energy statements bounced
- Presenting one combined saving. The whole point of the hierarchy is stage-by-stage accounting. A statement that reports "42% overall" without splitting Be Lean, Be Clean and Be Green savings will be returned for reworking.
- Leaning entirely on PV. A notional-spec fabric with a large array may hit the percentage but fails the policy intent; officers look for a genuine Be Lean contribution first.
- Using the wrong baseline. The comparison is against the Part L 2021 notional building for your actual dwelling, not against an older Part L edition or an arbitrary benchmark.
- Ignoring unregulated energy. London's net zero-carbon definition works on regulated emissions for the offset calculation, but the GLA guidance still expects unregulated (appliance) energy to be reported.
- Fixing the specification after the drawings. If the energy statement is commissioned last, the glazing ratios, roof form and plant space are already frozen — and with them, most of your carbon options.
One horizon note: the Future Homes Standard approved documents were published in March 2026 and take effect for building regulations from 2027, tightening the notional baseline that Be Lean is measured against. The hierarchy itself is not changing — schemes designed fabric-first now will convert cleanly.
How Fortress Associates can help
Fortress Associates prepares planning energy statements for every UK local authority. The report is free, is generated in minutes, models a Part L 2021 baseline using the SAP 10.2 notional-building method, and works through the full Be Lean, Be Clean, Be Green hierarchy with CO₂ savings calculated at each stage — including the London Plan SI2 35% target and carbon offset position for London schemes. Contact us with any questions, or see our full range of planning reports.
Sources & further reading
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