Target emission rate and target primary energy rate

Approved Document L 2021 gives a new dwelling two separate carbon and energy targets, and the target emission rate target primary energy rate pair can disagree about the same heating system. The emission rate is measured in kilograms of carbon dioxide per square metre per year. The primary energy rate is measured in kilowatt hours of primary energy per square metre per year, counting the energy consumed at the point of generation and the losses in delivering it. A design can clear one comfortably and fail the other, and which one it fails tells you exactly what to change.

Both are calculated targets rather than fixed numbers, derived from a notional building of your own geometry. That is the part most often misunderstood.

What each target measures

The dwelling emission rate is the modelled regulated carbon dioxide emissions of your design, and it has to be no greater than the target emission rate. Regulated emissions cover space heating, hot water, ventilation, pumps, fans and fixed lighting. They exclude cooking, appliances and anything a tenant plugs in, which is why the figure bears only a loose relationship to a real energy bill.

The dwelling primary energy rate is the modelled primary energy demand of the same design, and it has to be no greater than the target primary energy rate. Primary energy applies a factor to each fuel that reflects extraction, generation, transmission and distribution losses. For grid electricity the factor is well above one because of generation and network losses. For mains gas it is a little above one.

Alongside both sits the target fabric energy efficiency, which the 2021 edition retained. That one is about the building envelope and ventilation heat loss alone, with no services in it, so it cannot be traded against a better boiler.

Why a primary energy target was added

Carbon factors for grid electricity have fallen sharply as generation has decarbonised, and they are expected to keep falling. That creates a problem for a regime that judges buildings only on carbon: an inefficient electric system can pass on emissions simply because the grid it draws on has improved, while wasting a great deal of energy.

The primary energy target closes that gap. It is insensitive to how clean the grid is and sensitive to how much energy the building actually uses, so it penalises waste directly. The 2021 edition treats primary energy as the leading metric, with the emission rate alongside it and the fabric target underneath.

For a designer that means the questions have changed order. The first question is no longer which fuel produces less carbon. It is how much delivered energy the building needs, which is a fabric and system efficiency question.

The notional building sets both

Neither target is a number you can look up. Both are produced by running a notional dwelling, matching your design in size, shape and orientation, through the approved calculation methodology with the fabric and services specification set out in the Approved Document. Multiply the notional result by the stated factor and you have your target.

Three consequences follow from that, and they catch people out on every scheme.

A compact building gets a harder target than a badly shaped one, because the notional building shares the design's form factor. Improving the massing tightens the target at the same time as it improves the design, so the margin does not grow as much as the intuition suggests.

Flats and houses get different notional specifications, so a mixed scheme has different targets on different plots and a single fabric specification will not sit in the same place relative to each.

Orientation matters, because the notional building takes the design's glazing areas and orientations. Rebalancing glazing between elevations changes both the design result and the target.

Where a design passes one and fails the other

The classic case is direct electric heating. Panel heaters and electric boilers convert one unit of delivered electricity into one unit of heat, so delivered energy is high. With a low carbon factor for electricity the emission rate can look survivable. The primary energy rate will not, because every kilowatt hour is multiplied by the electricity primary energy factor. That design fails on primary energy and no amount of photovoltaic capacity on a flat roof will rescue it.

The opposite case is a gas boiler scheme with a good fabric specification. Delivered energy is moderate and the gas primary energy factor is low, so the primary energy target is reachable. The emission rate is where it struggles, because the carbon content of gas is now higher per delivered kilowatt hour than that of electricity.

A heat pump improves both at once, which is why it has become the default answer. Delivering three or so units of heat per unit of electricity cuts delivered energy by around two thirds, and that reduction flows straight through both calculations. The design risk moves elsewhere: flow temperatures, emitter sizing, hot water cylinder losses and the overheating assessment under Part O, which is a separate compliance exercise and is dealt with in an overheating risk assessment.

Communal systems deserve the caution in that last row. Distribution losses in a poorly designed network can be large enough to reverse the result, and the same issue sits behind the arguments about combustion plant discussed in combined heat and power and the air quality objection.

Fuel choice pulls the targets apart

Because the two metrics apply different factor sets to the same delivered energy, fuel switching moves them in different directions and by different amounts. Shifting from gas to electricity reduces carbon per delivered unit and increases primary energy per delivered unit. Whether the design improves overall depends entirely on the system efficiency you achieve, not on the fuel label.

That is why a planning policy expressed as a percentage improvement over the Part L target is not a stable measure. The baseline moves with each edition of the Approved Document and with each revision of the calculation methodology, so a 35 per cent improvement against a 2013 baseline and the same percentage against a 2021 baseline describe different buildings. State which edition and which methodology version your percentage is calculated against, every time.

Reading the compliance output

The compliance output sheet from the approved methodology is the document a building control body and a planning officer will both look at, and reading it properly beats scanning the pass flags.

1. Confirm which version of the calculation methodology produced it, because factor sets differ between versions.
2. Check all three comparisons: design against target for emissions, primary energy and fabric energy efficiency.
3. Look at the margin on each as well as the pass. A one per cent margin on primary energy will not survive a value engineering exercise.
4. Check whether photovoltaic output is included and how much of the result depends on it, since panels are the first thing removed on site.
5. Confirm the stage. As-designed and as-built calculations are different documents, and planning conditions are often discharged against the first while the second is what gets built.
6. Read the notional building specification assumed, particularly for flats within a mixed scheme.

Reporting both figures in an energy statement

An energy statement that reports only carbon savings is incomplete under the 2021 regime. Report the design and target figures for emissions and for primary energy, with the margin on each, and state the methodology version and the Approved Document edition.

Where a local policy sets a carbon reduction percentage, show the arithmetic: baseline, each stage of the energy hierarchy, and the residual. Where the policy is silent on primary energy, report it anyway, because a scheme that meets the carbon policy while failing primary energy at building control will have to be redesigned after permission.

Two further points for the statement. Explain any dependence on photovoltaics and state the area and orientation assumed, since a roof that later carries plant will not deliver it. And where the scheme is non-domestic, the same two-target structure applies through the separate volume of the Approved Document, with the commercial specifics that matter for large shed schemes covered in energy statements for warehouse and logistics schemes.

The Future Homes Standard has been announced and consulted on but its commencement and the metric structure it will use should be checked against the current government position rather than assumed, as at September 2026.

Enter your postcode on PlanScope to see which reports your site is likely to need. 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.