Primary energy and the three Part L 2021 compliance targets

A new dwelling has to meet three separate targets under the 2021 edition of Approved Document L Volume 1, not one. The Target Primary Energy Rate is the headline metric, the Target Emission Rate carries over from earlier editions, and the Target Fabric Energy Efficiency Rate constrains the building envelope independently of the services. All three are generated from a notional dwelling of the same geometry, and the design has to beat all three. A scheme that passes on carbon and fails on primary energy does not comply.

The Part L 2021 primary energy target is the one that changes design decisions, because it penalises conversion and delivery losses that the carbon metric alone no longer captures.

The three targets a new dwelling must meet

Each target is calculated by applying a defined notional specification to the actual dwelling geometry, then comparing the designed dwelling against it.

Alongside the three rates sit limiting values for individual elements and minimum efficiencies for building services. Those backstops matter in a trade-off exercise, because they cap how far a designer can push one element to compensate for another.

The calculation route for dwellings is the Standard Assessment Procedure, in the version current when the work is notified. The outputs are the dwelling's own primary energy rate, emission rate and fabric energy efficiency rate, each compared with its target.

What primary energy measures

Primary energy counts the energy used at the point of extraction, generation and delivery, not simply the energy delivered to the meter. Electricity carries losses in generation and transmission, so each delivered kilowatt hour represents more than one kilowatt hour of primary energy. Gas carries smaller losses in processing and distribution.

Including the metric does two things. It rewards reducing demand rather than switching fuels, since a fuel switch can cut carbon while leaving total energy use untouched. And it makes the efficiency of the heating system visible in a way the carbon metric does not, because a heat pump delivering several units of heat per unit of electricity performs well on primary energy while direct electric resistance heating performs badly.

For a designer, the practical consequence is that oversized or poorly controlled systems now show up twice. A heat pump sized for a peak that the fabric should have removed will pass on carbon and strain the primary energy target.

How the fabric target constrains design

The fabric target is assessed on space heating and cooling demand with the services stripped out, which is why it cannot be bought off with renewables. Photovoltaic output reduces the primary energy and emission rates. It does nothing for fabric energy efficiency.

Three design variables dominate the result. Form factor, meaning the ratio of heat loss area to floor area, which is why a detached house with projections is harder than a mid-terrace of the same floor area. Glazing area and orientation, where a large north elevation of glass adds heat loss without useful gain. And thermal bridging, which is often treated as a late-stage detail when it belongs in the early calculation, a point we set out in thermal bridging detail at planning stage.

Airtightness is the fourth, and it is the one with a gap between design and test. A design air permeability that the contractor cannot achieve produces a dwelling that fails on test and then needs retrospective measures. Set the design figure at something the build system has delivered before.

Where a design passes one target and fails another

Four patterns recur in assessments.

1. Good fabric, poor services. The fabric target passes, primary energy fails because the chosen system has low seasonal efficiency or high distribution losses.
2. Electric heating with large photovoltaic arrays. Carbon passes on the strength of the array, primary energy fails because resistance heating consumes delivered electricity at a one-to-one ratio.
3. Heat network connection with high network losses. Carbon may pass, primary energy depends entirely on the network's declared efficiency, and a notional figure will not survive scrutiny.
4. Compact apartment blocks with heavy glazing. Primary energy passes because the form factor is favourable, fabric energy efficiency fails because the glazing drives demand.

The order of fixing matters. Work on form, glazing and thermal bridging first, because those move all three results. Adding generation last moves two of the three and masks a fabric problem that reappears at overheating assessment. Where overheating is in play, the parallel assessment belongs in an Overheating Risk Assessment rather than as a paragraph in the energy chapter.

Heat network connection deserves its own attention early, since safeguarding a future connection changes the plant room layout and the risers, as covered in safeguarding a future heat network connection.

What the planning energy statement has to show

Building Regulations compliance is not a planning matter, yet planning authorities routinely require an energy statement that demonstrates the position, because development plan policies set targets expressed against the Building Regulations baseline. In London, the London Plan's zero-carbon policy requires on-site reductions beyond the Building Regulations baseline, with a minimum share delivered through energy efficiency measures and a cash in lieu contribution for the shortfall. Check the Greater London Authority's current energy assessment guidance for the figures and the reporting format, since both are updated.

An Energy Statement that answers the policy question properly contains the following:

• The calculation version used, with the date, and the notional specification assumptions.
• All three rates against all three targets, for a representative sample of dwelling types with the sample justified.
• The energy hierarchy set out stage by stage, showing the reduction attributable to fabric before any system or generation measures.
• The services specification with the seasonal efficiencies assumed and their source.
• Photovoltaic area and output with the roof plan it depends on, because an array that does not fit is the commonest error in a submission.
• The overheating position, or a statement of where it is dealt with.

State what is assumed and what is fixed. An energy statement that reads as a set of commitments is easier to condition, and an authority will often condition the submitted figures, which is a reason to avoid promising a specification the contractor has not priced.

Transitional arrangements and start on site

The 2021 edition applied to work notified from 15 June 2022. Transitional provisions allowed the earlier standards to apply where a building notice, initial notice or full plans were given before that date and the work on each individual building started within 12 months.

Two points still matter on sites running under those arrangements. The transitional entitlement attaches to the individual building rather than to the site as a whole, so a phased development can find later plots falling under the current edition. And a planning permission granted before the uplift does not carry any entitlement, because the two regimes are separate. Applicants sometimes assume an older permission locks in older standards, and it does not.

Check the position building by building when programming a long site, and record the notification dates somewhere findable. Reconstructing them three years later from email is a poor use of a weekend.

What changes under the Future Homes Standard

The Future Homes Standard has been announced as the next step, with consultation already undertaken and the detail to follow in a further edition of Approved Document L. As at October 2026 the position on the final standard, its transitional arrangements and the calculation methodology should be verified on gov.uk rather than taken from secondary summaries, because the drafting has moved more than once.

What is reasonably clear is the direction. A tighter primary energy expectation, no new dwellings heated by fossil fuel, and fabric standards that continue to be assessed separately from services. Designing to the current fabric target with margin is the hedge that costs least, since fabric is the element that cannot be improved at low cost once the shell is up.

And the three-target structure itself is worth assuming will persist. It has proved effective at stopping a single metric being gamed.

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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.