The arithmetic behind a regulated carbon reduction figure

A percentage reduction in regulated carbon emissions is a comparison between two calculated figures: the emissions of a notional building defined by the current Part L methodology, and the emissions of your building modelled with its proposed specification. Both numbers come out of the same compliance software. The percentage is the difference expressed against the baseline, and almost every dispute about one comes down to which emissions were counted and which edition of the methodology produced the baseline.

Get the definition right and the arithmetic is straightforward. Get it wrong and a sustainability officer recalculates it, finds a different number, and the application stalls.

Defining the baseline correctly

The baseline is the target emission rate produced by the compliance calculation for a notional building of the same size, shape and use as yours, with the fabric and services specification the methodology prescribes. It is not a figure you choose, and it is not the emissions of a building built to the previous standard.

Three points decide whether your baseline is defensible.

The edition of the methodology matters, because the notional building specification changed with the current Part L and the carbon factors changed with it. A baseline produced under a superseded version gives a percentage that cannot be compared with a policy written against the current one. State the edition and the software version on the front of the calculation.

The geometry has to match. The notional building takes its form from your building, so a change in glazed proportion or floor area changes the baseline as well as the proposal. That is why a percentage improvement cannot be carried across from a similar scheme.

And for mixed developments the baseline is assembled rather than single. Dwellings are calculated under the domestic methodology and non-domestic parts under the non-domestic one, then combined by weighting, and the weighting basis must be stated.

Which emissions sit inside the percentage

Regulated emissions are those from fixed building services: space heating, hot water, fixed cooling, ventilation and fixed internal lighting. Unregulated emissions come from cooking, appliances, small power, and in most cases external lighting and lifts.

Policy percentages are written against regulated emissions. Including unregulated emissions in either side of the comparison produces a different and unacceptable figure, and mixing the two between the baseline and the proposal is the single most common error in these submissions.

Where a policy asks for a separate unregulated or whole-building figure, report it in its own row rather than folding it into the headline percentage.

Stage by stage reporting through the hierarchy

Most local policies require the saving to be reported in stages, following the energy hierarchy of reducing demand first, then supplying heat efficiently, then adding renewables, then monitoring in use.

1. Report the baseline emissions from the notional building, in tonnes of carbon dioxide per year.
2. Report emissions after demand reduction measures, with the fabric and ventilation specification that produced them listed.
3. Report emissions after the heat supply strategy, naming the generator, its efficiency and any heat network connection.
4. Report emissions after on-site renewables, with the installed capacity and the area it occupies.
5. Report the cumulative saving in tonnes and as a percentage of the baseline at each stage.
6. State any shortfall against the policy target and the offset payment calculated at the authority's current published rate.

The arithmetic rule that catches people is that savings at each stage are sequential rather than additive. A measure that reduces demand lowers the quantity of heat the supply stage then has to provide, so adding percentage savings from separate studies overstates the total.

Showing the numbers in a table

Officers check three things on the table: that the rows reconcile, that the cumulative percentages follow from the tonnage figures, and that the final figure matches the one asserted in the text of the Energy Statement.

Give every row the same units and the same number of decimal places. Carry the tonnage to two decimal places and the percentage to one. Add a column showing the cumulative saving in tonnes alongside the percentage, because that is what an offset payment is calculated from where a shortfall exists.

Include the software output as an appendix, not a retyped summary. A reviewer who cannot trace the table back to the calculation files will ask for them, and that is a three week round trip.

Rounding and presentation conventions

Round once, at the end. Rounding each stage and then summing produces a total that does not reconcile with the tonnage figures, and it is the first thing a careful reviewer notices.

Report the percentage against the baseline rather than against the previous stage, unless the policy asks for the latter, and say which convention you have used. A 35 per cent cumulative saving and a 35 per cent saving at the renewables stage are different claims.

Where the policy sets a target expressed as a minimum, do not present a figure that only meets it after rounding. A calculated 34.6 per cent presented as 35 per cent against a 35 per cent policy target invites a recalculation that nobody wants.

Common arithmetic errors

The recurring faults are specific and easy to check before submission.

A baseline from a superseded Part L edition. Unregulated emissions included on one side of the comparison only. Photovoltaic output counted at both the demand reduction and renewables stages. Savings from separate stages added rather than applied sequentially. Carbon factors from an older version of the methodology, which matters most for electrically heated schemes because the electricity factor has changed substantially. A photovoltaic capacity in the calculation that exceeds the roof area available once plant and access are allowed for. And a design-stage calculation presented in support of a claim about as-built performance.

Where the roof is the constraint, the competition for roof space between plant, panels and green roof has to be resolved on a drawing before the renewables figure is credible.

What an assessor recalculates

Expect an officer or their consultant to re-run the files rather than read the summary. They will check the notional building parameters, the methodology and software version, the carbon factors applied, the fabric inputs against the drawn construction, the system efficiencies against the proposed plant, the renewable capacity against the roof layout, and whether the stage by stage figures reconcile.

They will also cross-check the overheating assessment, because a low solar transmittance specification that delivers the carbon figure can fail the summer criteria, and the two documents have to describe the same building. The same applies to a Whole Life Carbon Assessment and a Sustainability Statement, which frequently quote the regulated figure and then diverge from it after a late design change.

So fix one number in one document and reference it everywhere else. That discipline prevents most of the queries these submissions attract.

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This is general guidance, not legal advice. Check the position 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.