Part L 2021 fuel factors and the energy statement baseline
The Part L baseline is the notional dwelling, not a percentage. Current SAP factors favour electric heating on carbon and penalise it on primary energy, which changes what an energy statement has to prove.
The Part L 2021 energy statement baseline is not a percentage improvement on an abstract standard. It is the notional dwelling: a same-shape, same-size version of your building with a fixed specification set out in the Approved Document, modelled with the current SAP carbon and primary energy factors. Your design has to beat that notional building on three separate measures, and a planning energy statement that asserts a carbon saving without showing which measure it is improving on, and against what baseline, will come back from the officer.
The change that caught most practices out was not the fabric standard. It was the move to the current carbon factors, which cut the carbon intensity attributed to grid electricity and raised the relative penalty on gas. A design that scraped a pass under the previous edition can sit comfortably ahead under the current one if it is electrically heated, and can fail if it is not.
What the notional dwelling fixes
The notional dwelling has the same size, shape, orientation and number of storeys as the actual dwelling. Everything else is specified: fabric U-values, thermal bridging, air permeability, glazing area as a proportion of floor area, ventilation type, heating system and efficiency, hot water generation, lighting and the presence of photovoltaic generation on houses.
Two consequences follow, and both matter at planning stage.
First, the baseline moves with your design. Change the glazing proportion and the notional building changes with it, within the limits in the Approved Document. You cannot bank a saving simply by designing a more compact form, because the notional building is equally compact.
Second, the notional specification already includes measures a planning officer may assume are your enhancement. Photovoltaic generation is in the notional specification for houses, so fitting the panels the notional building assumes is not an improvement on the baseline. Energy statements that count those panels as a planning betterment are the single most common error I see.
How the current carbon factors changed the electric case
The carbon factor applied to grid-supplied electricity in the current SAP methodology is far lower than the figure used under the 2013 edition. Generation has decarbonised and the methodology caught up with it. Gas carries a factor that has barely moved.
The arithmetic consequence is direct. A heat pump with a seasonal efficiency well above one, drawing electricity with a low carbon factor, now produces a much better dioxide emission result than a condensing boiler, and the gap is wider than it was. For a planning policy expressed as a percentage carbon improvement over the Building Regulations baseline, that favours all-electric designs without any change to the fabric.
Primary energy pulls the other way. The primary energy factor for electricity accounts for generation and distribution losses and is higher per unit delivered than for gas, so an all-electric scheme that looks comfortable on carbon can be tight on primary energy. This is why a scheme can pass the emissions target and fail the primary energy target, and why quoting only the carbon figure in a planning submission is incomplete.
Three targets, not one
| Target | What it measures | Where it usually bites |
|---|---|---|
| Target emission rate | Dioxide emissions per square metre per year | The headline figure planning policies reference |
| Target primary energy rate | Primary energy demand per square metre per year | All-electric schemes and schemes with high ventilation loads |
| Target fabric energy efficiency rate | Space heating and cooling demand from fabric alone | Poor form factor, large glazed areas, weak junction detailing |
The fabric energy efficiency target is the one that cannot be bought back with plant. It is calculated from the building fabric and form, so a thin, highly glazed flat with poor junctions fails it regardless of how efficient the heat pump is. Where a planning policy asks for a fabric-first approach, this is the number that demonstrates it.
Alongside these sit the limiting standards on individual elements, the minimum standards for building services efficiency, and Part O overheating, which is assessed separately and often pulls the design the opposite way from the energy case. Our note on overheating in roof level and mansard flats covers that conflict for the dwelling types where it is sharpest.
Where planning policy asks for more than the Regulations
Many adopted local plans and the London Plan require a carbon improvement beyond the Regulations baseline, and some require net zero carbon with an offset payment for the shortfall. Three drafting points keep those submissions clean.
State the baseline edition and the calculation tool version. An energy statement that says "35 per cent better than Part L" without naming the edition and the software version is unverifiable, and a reviewer cannot reproduce the result.
Show the stages separately. Where a policy follows the energy hierarchy, report the demand reduction from fabric and form, then the efficient supply stage, then the renewable contribution, with the emission rate after each. A single end figure hides which stage did the work and invites a request for a revised statement.
And distinguish regulated from unregulated energy. Policies that ask for a percentage against regulated emissions only, or against total emissions including cooking and appliances, produce different numbers from the same design. Say which you have used.
An Energy Statement prepared for planning and a Part L compliance submission are different documents with the same underlying model. The planning one explains and justifies. The compliance one proves.
Showing the calculation in a planning submission
What a reviewing officer or a borough energy consultant wants to see, in this order:
- The baseline notional dwelling outputs: target emission rate, target primary energy rate and target fabric energy efficiency rate, per dwelling type or per block.
- The as-designed outputs against each target, with the percentage margin on each.
- The specification assumptions in a table: U-values by element, linear thermal transmittance values with their source, air permeability, ventilation strategy and system efficiencies.
- The source of each thermal bridging value, distinguishing default values from accredited construction details or calculated psi values.
- The compliance software output appended in full, not summarised.
- Where offset payment applies, the residual emissions over the policy period and the arithmetic of the payment.
Point four is where most queries land. Default thermal bridging values are conservative, and designs that rely on calculated psi values have to show the calculation and the detail it came from. Our note on thermal bridging detail at planning stage covers how much detail is reasonable to commit to before tender.
Errors that cause a resubmission
Counting the notional photovoltaic array as an enhancement, as above.
Modelling a heat pump at manufacturer's coefficient of performance rather than the seasonal figure the methodology requires, which overstates the carbon saving and will not survive the Building Regulations submission.
Using a different floor area convention between the planning statement and the compliance output, so the per-square-metre figures do not reconcile.
Carrying over a carbon factor from an older version of the calculation methodology, which produces a result nobody can reproduce.
Reporting a block average where the policy requires every dwelling to comply, hiding a failing single-aspect unit behind a good one.
And describing a heat network connection as the strategy when no network exists and no agreement is in place. If the proposal is a safeguarded connection, say so and specify the plant space and pipe route, as our note on safeguarding a future heat network connection sets out.
Keeping the planning and regulations cases consistent
The discharge of an energy condition is where inconsistency shows up. If the planning statement promised a 40 per cent improvement with triple glazing and an exhaust air heat pump, and the tendered build has double glazing and a different unit, the as-built figures will not match and the condition will not discharge without a revised submission.
Two habits prevent this. Keep one model and one specification schedule for both submissions, versioned, with a note of what changed and when. And resist the temptation to promise the best achievable number at planning stage. A scheme that commits to a margin it can hold through value engineering discharges its condition quietly. One that commits to the theoretical maximum spends the next year negotiating.
The honest uncertainty worth naming: the Future Homes Standard will change the notional specification and the policy expectations around it, and the transitional arrangements determine which edition applies to your scheme. Check the commencement and transitional position on gov.uk for the specific application date rather than assuming.
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Frequently asked questions
What is the notional dwelling in Part L 2021?
It is a modelled building with the same size, shape, orientation and storey count as your design, but with a specification fixed by the Approved Document: set U-values, thermal bridging, air permeability, glazing proportion, ventilation, heating and hot water efficiencies, lighting and, for houses, photovoltaic generation. Its outputs become the three targets your design must beat.
Why does an electric scheme pass on carbon but fail on primary energy?
The current carbon factor for grid electricity is low because generation has decarbonised, so a heat pump produces a strong emissions result. The primary energy factor for electricity accounts for generation and distribution losses and is higher per unit delivered than gas. A design can therefore clear the emission target comfortably and sit tight against the primary energy target.
Can the notional photovoltaic array count as a planning enhancement?
No. Where photovoltaic generation is part of the notional specification, fitting the same array only matches the baseline rather than improving on it. Counting it as a betterment against a local policy percentage is the most common error in submitted energy statements and usually triggers a request for a revised document.
What should a planning energy statement include that a Part L output does not?
The baseline edition and software version, the three target figures with the as-designed margin on each, the specification assumptions in a table, the source of each thermal bridging value, and the results reported stage by stage through the energy hierarchy. The compliance output should be appended in full rather than summarised.
Which target cannot be met by better plant?
The target fabric energy efficiency rate. It is calculated from fabric and form alone, so a highly glazed dwelling with poor junction detailing fails it regardless of heating system efficiency. Where a policy asks for a fabric-first approach, this figure is the one that demonstrates compliance with it.
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.