Carbon offset payments and how councils calculate them
Residual regulated emissions multiplied by an assumed period and a locally set price per tonne. Only the first of those three numbers is within a developer's control.
A carbon offset payment is the cash a developer pays where a local plan sets a carbon reduction target that the scheme does not meet on site. The calculation is nearly always the same shape: take the residual annual regulated carbon emissions after on-site measures, multiply by an assumed number of years, and multiply again by a price per tonne set in the adopted policy. Three numbers, one of which you can influence and two of which you cannot.
London has run this mechanism longest and most of the country's policies are modelled on it. If you are budgeting a scheme, the number to establish first is the price per tonne in the authority you are dealing with, because it varies far more than people expect.
Where the payment comes from
The London Plan requires major development to be net zero-carbon, with a minimum on-site reduction beyond the Building Regulations target and any shortfall met through a cash contribution to the relevant borough's carbon offset fund. The energy strategy follows the be lean, be clean, be green and be seen hierarchy, and the offset applies to what is left after the first three stages.
Outside London an increasing number of local plans carry equivalent policies, usually expressed as a percentage improvement over the Building Regulations target emission rate with a payment for the residual. The mechanism is the same. The numbers are not.
Two things to check in the adopted policy before doing any arithmetic. Whether the policy applies to residential, non-residential or both, and at what threshold. And whether it applies to regulated emissions only.
Thresholds vary. Some policies apply to major development alone, some to all new dwellings, and some set a floorspace trigger for commercial schemes. A scheme sitting just below a threshold is worth checking twice, because the difference between nine and ten units can be the difference between a payment and none.
The formula
The standard structure is:
residual regulated emissions in tonnes of CO2 per year, multiplied by the assumed period in years, multiplied by the price per tonne.
The assumed period is commonly 30 years, representing an assumed building life for the purposes of the calculation. The price per tonne is set by the authority, and the figures adopted across London boroughs have diverged considerably as councils have reviewed them upwards. Do not carry a figure from one borough to another, and do not use a figure from an older version of a policy.
The output can be large. On a major residential scheme, a shortfall of a few tonnes a year turns into a six-figure obligation once it is multiplied by 30 and by the price. That is why running the energy model early matters more here than almost anywhere else.
What counts and what does not
Regulated emissions cover the fixed building services: space heating, hot water, ventilation, fixed cooling and lighting. Those are what the Building Regulations calculation deals with, and those are what the offset is normally based on.
Unregulated emissions from cooking, appliances and equipment sit outside the calculation in most policies, though some authorities have started to address them, and a small number ask for whole life carbon reductions separately.
Embodied carbon is a different question again. Where a policy requires a Whole Life Carbon Assessment, that assessment reports embodied and operational carbon over the building's life and is used to inform design decisions. It is not usually the basis for the offset payment, so treat the two as separate exercises with separate outputs, and say clearly in the Energy Statement which figures feed the offset calculation.
How the payment is secured
Through a section 106 obligation, not through the Community Infrastructure Levy. The levy funds infrastructure under its own regulations and cannot be used for this purpose, so the two are cumulative rather than alternatives.
Four points of drafting worth attention:
- The trigger. Payment before first occupation is common. Payment in phases tied to occupation of each block is better for cash flow and is often achievable on a phased scheme.
- Indexation. Most obligations index the sum from the date of the agreement to the date of payment. Check which index and which base date.
- Recalculation. Some agreements fix the sum at the date of the agreement, others recalculate it against the as-built performance. The second is riskier and needs to be understood before signing.
- Monitoring. The be seen stage requires reporting of actual performance after completion, and some authorities link that reporting to a review of the payment.
Viability and the appraisal
The offset payment is a scheme cost and it enters the viability appraisal alongside affordable housing and other obligations.
Where a scheme cannot bear the full policy package, the route is a Viability Assessment prepared in accordance with the national approach to viability and the relevant professional guidance, setting out the benchmark land value, the assumed costs and the residual. Councils generally treat carbon offset payments as one of the last items to be reduced, behind affordable housing, so do not assume it is the easy concession.
Where viability is under real pressure, a better argument is usually to spend the money on site instead. Fabric improvements, better heat pump specification or additional photovoltaics deliver a reduction in the residual emissions figure, which reduces the payment at 30 times the annual saving. In most appraisals the on-site measure costs less than the offset it avoids.
Timing matters too. An obligation agreed on a scheme that is then sold before build-out passes to the successor in title, so a purchaser needs to price it. Anyone acquiring a consented site should read the section 106 schedule for the offset figure, the indexation base date and the trigger, because those three lines can move a land value materially.
Reducing the bill
Four practical moves, in order of return.
Model the energy strategy at massing stage. Form factor and orientation set the ceiling on everything that follows, and they are fixed by the planning drawings.
Push the on-site reduction past the policy minimum where the marginal cost is lower than the offset price per tonne saved. Work out that crossover figure early and use it to make specification decisions.
Check whether the authority allows in-kind delivery. Some policies permit a developer to fund carbon reduction works elsewhere, such as retrofit of existing buildings in the borough, instead of a cash payment. Where available it can be better value and better publicity.
Make sure the numbers in the energy strategy, the sustainability case and the section 106 schedule all match. A Sustainability Statement quoting a different residual figure from the energy statement will be queried, and the query costs weeks.
Illustrative example: a scheme achieving the policy minimum on-site reduction with a residual of 12 tonnes of regulated carbon a year. At 30 years, that is 360 tonnes to be offset, and the payment follows directly from the borough's adopted price per tonne. Cutting the residual to 8 tonnes through improved fabric removes 120 tonnes from the calculation.
This is general guidance, not legal advice; check the adopted policy and the current price per tonne with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
How is a carbon offset payment calculated?
Residual annual regulated carbon emissions after on-site measures, multiplied by an assumed period, multiplied by a price per tonne set in the adopted policy. The period is commonly 30 years, representing an assumed building life for the purposes of the calculation. The price per tonne varies considerably between authorities and has been reviewed upwards in many.
Which emissions does the payment cover?
Regulated emissions in most policies, meaning space heating, hot water, ventilation, fixed cooling and lighting. Unregulated emissions from appliances and cooking usually sit outside the calculation, though some authorities have begun to address them. Embodied carbon is dealt with separately through whole life carbon assessment where a policy requires one.
Is the payment secured through CIL or section 106?
Section 106. The Community Infrastructure Levy operates under its own regulations to fund infrastructure and cannot be used for this purpose, so the two obligations are cumulative rather than alternatives. Check the trigger for payment, the indexation provisions and whether the sum is fixed or recalculated against as-built performance.
Can the payment be reduced on viability grounds?
It enters the viability appraisal alongside other obligations, but councils generally treat carbon offset contributions as one of the last items to reduce, behind affordable housing. A stronger route is usually to spend the money on site instead, since a saving in annual residual emissions is multiplied by the assumed period.
Can carbon reduction be delivered in kind instead?
Some policies allow it, for example by funding retrofit of existing buildings in the borough rather than paying into the fund. Where the option exists it can represent better value and better public presentation than a cash contribution. Check the adopted policy wording, because the mechanism is not universal.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.