The notional building and why baselines confuse officers

The notional building carbon baseline is the invisible object every energy statement is measured against, and almost every argument between an architect and a planning officer about percentage improvements comes down to not saying which version of it was used. The notional building is not a real design. It is a building of exactly the same size, shape and orientation as yours, modelled with a specification written into the Building Regulations, and the targets your scheme has to beat are generated from it.

Change the edition of the regulations and the notional building changes. The scheme does not, but the percentage does.

What the notional building is

When a dwelling is assessed, the calculation runs twice. Once for the actual design, and once for a notional equivalent with the same geometry and a prescribed specification covering fabric values, air permeability, services efficiency and any low carbon technology assumed.

The notional run produces the targets. The actual run produces the rates your design achieves. Compliance under the 2021 edition of Part L for dwellings means beating three of them: a carbon dioxide emission rate, a primary energy rate and a fabric energy efficiency rate. For buildings other than dwellings the same logic applies through a different calculation engine.

The point of the method is fairness between designs. A north-facing flat and a detached house are measured against notional versions of themselves rather than against a single national number, so an inefficient shape is not penalised for being that shape. The side effect is that the baseline moves from building to building as well as from edition to edition.

Why a percentage improvement needs a stated baseline

A local plan policy requiring a 35 per cent improvement over Part L is incomplete without three more pieces of information, and officers rarely ask for all three.

Which edition. The 2021 notional building is considerably better than its 2013 predecessor, so a design that cleared 35 per cent against the older baseline may sit close to parity against the newer one. The scheme has not got worse.

Which metric. An improvement measured against the carbon dioxide target and the same improvement measured against the primary energy target give different numbers, sometimes by several percentage points, because the two targets weight electricity differently.

Which scope. Part L regulates heating, hot water, fixed lighting, ventilation and cooling. It does not regulate appliances, cooking or small power. A percentage quoted on regulated emissions is not comparable with one quoted on total site energy.

State all three in a single sentence near the front of the Energy Statement and most of the confusion disappears.

Regulated and unregulated energy

The split matters more now than it did, because as fabric and services improve, unregulated energy becomes a larger share of what the building actually uses.

An officer reading an energy statement that reports a large regulated carbon saving may reasonably ask what the building will consume in total. That question is fair, and the answer is a different calculation: a predicted energy use figure rather than a compliance figure.

Where a policy asks for an energy use intensity figure in kilowatt hours per square metre per year, say plainly that it is a different metric from the compliance calculation and report it separately. Mixing the two in one table is the single most common cause of an officer misreading an energy statement.

Comparing schemes assessed against different editions

This is where it gets awkward on a site with a planning history.

A consented scheme from several years ago will have been assessed against an older notional building. A revised scheme assessed against the current one, achieving a lower headline percentage, looks like a reduction in ambition to anyone reading the two documents side by side. In absolute terms it may emit considerably less.

The fix is arithmetical rather than rhetorical. Report the absolute figures, in kilograms of carbon dioxide per square metre per year and in kilowatt hours per square metre per year, alongside the percentages. Absolute numbers are comparable across editions. Percentages are not.

Where a section 73 application varies a scheme consented under an earlier edition, say explicitly which edition the original was assessed against and which applies now, because the officer will otherwise assume continuity.

Presenting the figures without ambiguity

A table at the front of the energy statement removes nearly all of the risk.

Put the absolute figures before the percentage. It reads as less promotional and it is the number that survives a change of edition.

Where the scheme is in London, the energy hierarchy of lean, clean, green and seen structures the statement, and the Greater London Authority's guidance sets out what it expects at each stage along with the baseline it applies. Check the current version of that guidance rather than working from a scheme you did two years ago, since the baseline it uses has been updated.

Misreadings that turn up in officer reports

Four recur often enough to design around.

An officer treating a regulated carbon saving as a saving on the occupier's bills, which it partly is and partly is not, since bills follow energy use and tariffs rather than carbon.

An officer comparing a percentage from one scheme with a percentage from another without checking the editions, and concluding one applicant is less ambitious.

An officer reading the fabric energy efficiency result as the headline figure when the policy refers to carbon, or the reverse.

And an officer assuming that because the notional building includes a heat network connection or photovoltaics in its specification, the actual scheme must include the same technology. It does not have to. It has to meet the targets.

Pre-empt each of these in the text rather than in an appendix. A paragraph headed by what the number means is worth more than ten pages of output.

Where this meets the rest of the submission

Overheating is assessed separately under the regulations and under policy, and the measures that improve the carbon result, more insulation and better airtightness, can worsen the overheating result. An Overheating Risk Assessment using the dynamic modelling route should be run alongside the energy work rather than after it, because a late glazing change undoes both calculations.

The wider policy narrative sits in the Sustainability Statement, which is where water use, materials, circular economy measures and biodiversity belong. Keeping the energy statement to energy makes both documents shorter.

Standards in this area move. The Future Homes Standard will change the notional specification again, and any statement written now should say which rules it was prepared under and when. Our note on energy statements for warehouse and logistics schemes covers how the same baseline problem plays out in large single-storey buildings.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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