London's carbon offset system has a simple, expensive logic: your energy statement works through Be Lean, Be Clean, Be Green — and whatever annual CO2 shortfall remains against the 35% on-site target is multiplied by 30 years and the borough's price per tonne, then secured as a section 106 payment. At the GLA's recommended £95/tonne, each annual tonne of shortfall costs £2,850.
Calculate your offset payment
The calculator needs JavaScript. The method is set out in the GLA's energy assessment guidance.
The prices boroughs actually charge
Boroughs are expected to use the GLA-recommended £95/tonne or set their own from local evidence — and the spread is dramatic: Islington charges £920/tonne (nearly ten times the GLA figure) while Lewisham uses £104/tonne. The same one-tonne annual shortfall is a £2,850 liability in most of London and a £27,600 one in Islington. Check your borough's figure before you fix the energy strategy — the calculator lets you set any price.
How to shrink the bill
- Every on-site tonne counts thirty-fold — a heat pump that saves 1.5 tCO2/yr against a gas boiler removes £4,275 of offset liability at £95/tonne (run your own numbers in the heat pump carbon calculator).
- Fabric first still pays — Be Lean savings compound through every later stage of the hierarchy.
- PV fills the gap — rooftop solar is usually the cheapest final push toward the 35% line, especially now electrified heating dominates the Be Green stage.
How Fortress Associates can help
Our automated energy statement is free and ready in minutes — Part L 2021 baseline via SAP 10.2, the full energy hierarchy with CO2 savings at each stage, and the London Plan 35% target with the carbon-offset calculation built in for London schemes. Pair it with the document checklist, see all tools at /free-tools, or get in touch.
Sources & further reading
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