Target emission rate under Part L, explained

The target emission rate under Part L of the Building Regulations is the maximum carbon dioxide emission rate your building is allowed, expressed in kilogrammes of carbon dioxide per square metre per year. It is not a national figure. It is calculated for your building by modelling a notional building with the same size, shape and use, fitted with the specification set out in Approved Document L, and taking that notional building's emission rate as the target.

So the target moves with the design. Change the form, the glazing area or the mix of uses, and the target changes with it, which is why quoting a target emission rate without the building it came from tells a reader nothing.

What the target rate measures

The rate covers regulated energy only: space heating and cooling, hot water, ventilation, fixed lighting and the associated pumps and fans. Appliances, cooking, IT equipment and other occupier loads sit outside it, which is why a compliant building can still have high metered consumption.

Emissions are calculated by multiplying delivered energy for each fuel by the carbon factor for that fuel, then dividing by the total floor area. Those carbon factors are published within the calculation methodology and they change between editions, which is one of the reasons a design modelled under an older version cannot simply be carried forward.

The comparison is a ratio, not an absolute standard. Compliance means the calculated rate for your design is no higher than the target, and how much lower it is becomes the compliance margin that planning policy then works from.

The notional building and how it is defined

The notional building shares your building's geometry: the same floor area, the same shape, the same orientation, and the same broad pattern of use. What it does not share is the specification.

Approved Document L sets that specification, covering fabric U-values, thermal bridging, air permeability, the efficiency of heating and hot water plant, ventilation, lighting efficacy and, for some dwelling types in the current edition, on-site renewable generation. The software builds this shadow building automatically from your geometry and runs it through the same calculation.

One consequence is often misunderstood. Because the notional building has the same form as yours, a poor form factor does not make the target easier to hit in relative terms. A sprawling, highly articulated house raises the notional emission rate and the design emission rate together. It fails on absolute energy use and on cost, and on a policy target expressed as a percentage it can still look fine, which is one of the honest limitations of percentage-based carbon policies.

The other consequence is that the notional specification is a rising benchmark. Each uplift to the Approved Document tightens the notional building, so the same design that passed comfortably under one edition may not pass under the next.

Design emission rate against target

For dwellings the calculation runs through the Standard Assessment Procedure and produces a dwelling emission rate, which must not exceed the target emission rate. For buildings other than dwellings, the National Calculation Methodology runs through an approved simplified or dynamic model and produces a building emission rate, again capped by the target.

The comparison is done at design stage and again as built, and the two can differ. As-built values reflect the actual air test result, the plant actually installed and any substitutions made during construction. Where a planning condition ties a carbon commitment to the design-stage figure, an as-built shortfall becomes a discharge problem rather than a building control one.

Three things change the design rate most on a typical scheme: the heating system and its fuel, the air permeability achieved, and the extent of on-site generation. Fabric improvements move the number too, though with diminishing returns once the notional specification is met.

Primary energy and fabric metrics

Carbon is not the only test. The current edition of Part L sets additional targets that have to be satisfied alongside the emission rate.

The fabric target is the one that stops a poorly insulated building being rescued by a low-carbon heat source. A design can hit the carbon target with a heat pump and a leaky envelope and still fail the fabric test, which is the intended behaviour.

The primary energy target catches electricity-heavy designs, because primary energy accounts for the energy used to deliver a unit at the meter. As the grid decarbonises, carbon and primary energy pull in different directions, so a design passing one can fail the other. Check all three before committing to a services strategy.

What the 2021 uplift changed

The 2021 edition of Part L was published as an interim step ahead of the Future Homes and Future Buildings Standard, with the government stating an intended reduction in carbon emissions of around 31 per cent for new dwellings and around 27 per cent for other new buildings compared with the previous standard.

Two changes did most of the work. The notional specification was tightened, and the carbon factors used in the calculation were revised to reflect a much less carbon-intensive electricity grid. That second change altered the economics of the decision between a gas boiler and a heat pump more than any fabric measure did.

The practical point for anyone writing an Energy Statement is to state which edition the baseline is drawn from. A percentage improvement over the 2013 standard and the same percentage over the 2021 standard are entirely different commitments, and local plan policies written before 2021 sometimes still name the older baseline.

The Future Homes Standard has been announced and its commencement arrangements should be checked on gov.uk at the point of writing, along with any transitional provisions that apply to your permission.

Where planning policy goes further

Building Regulations set a floor. Planning policy in many areas sets something higher, and the two are consented separately.

London Plan 2021 Policy SI 2 asks major development to be net zero-carbon and requires a minimum on-site reduction of 35 per cent beyond Building Regulations, with any shortfall addressed through a carbon offset payment. Local plans elsewhere adopt similar structures with different percentages, and some set their own absolute energy targets instead.

Where a policy expresses its requirement as a percentage beyond Part L, three questions have to be answered in the submission. Which edition of Part L is the baseline. Whether the reduction is required on-site or may be met in part off-site. And whether the target applies to regulated emissions, which it almost always does.

Getting those three wrong is the most common reason an energy statement is returned, and it is easily avoided by quoting the policy wording in the document. Carry the same figures into the Sustainability Statement, because two documents disagreeing on the compliance margin will be spotted.

Reporting the compliance margin

A reviewer needs to be able to reproduce your number, or at least see exactly where it came from.

1. Name the edition of Approved Document L applied, and the transitional position if relevant.
2. Name the calculation software and its version, and the assessor.
3. Give the target and design values for emission rate, and for dwellings the primary energy and fabric rates as well.
4. State the percentage improvement, and say which pair of figures it was calculated from.
5. State whether the improvement is achieved on-site, and identify any offset contribution.
6. Attach the output reports as an appendix rather than summarising them.

Where the scheme relies on a services strategy that also affects summer conditions, keep the assumptions consistent with the Overheating Risk Assessment, since a ventilation strategy that works for one model and not the other is a familiar cause of late redesign.

Browse all 66 PlanScope report types to see how the energy, sustainability and overheating documents fit together on a scheme.

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