A deep dive into: Compliance Metrics

A deep dive into: Compliance Metrics

Achieving compliance with the current Part L- Conservation of Fuel and Power (2013) of the Building Regulations is achieved by comparing the actual building with a ‘notional building’, with two metrics currently used to illustrate this in England.

Compliance is achieved by showing that the Dwelling Emission Rate (DER) is equal to or lower the Target Emission Rate (TER), and also that the Dwelling Fabric Energy Efficiency (DFEE) rate is equal to or lower the Target Fabric Energy Efficiency (TFEE) rate as well as meeting the minimum standards set out in Part L. These metrics are calculated using the UK Government’s Standard Assessment Procedure (SAP), which is the methodology used to assess and compare the energy and environmental performance of dwellings.

The publishing of the outcome of the Future Homes Standard consultation has shown that the UK Government intends to introduce a third compliance metric that will need to be met for Part L, the Primary Energy Rate.

In order to comply with Part L, all three compliance metrics will have to be met when the interim update to Part L of the Building Regulations comes into force. The interim updated is expected to be finalised and announced in December of this year, with a view to coming into force in June 2022.  As a result, it is important to understand what these three compliance metrics mean, how they are calculated and ultimately how you can comply with them.

Target Emission Rate

The Target Emission Rate is defined in kgCO2e/m2/year, and is based on the emissions produced by the Notional Building, as set out in the SAP methodology. It is essentially the maximum emissions allowed for a particular dwelling. Included in the calculation are the carbon emissions from space and hot water heating, internal lighting and from any pumps and fans. The notional building is heated by mains gas; meaning that the Target Emission Rate is calculated using the carbon factor for mains gas.

As different fuel types have different carbon contents, the Dwelling Emission Rate will differ depending on the fuel used to heat it. These “Emission Factors” change over time, with the carbon content of electricity decreasing notably since the last time Part L of the Building Regulations was updated thanks to the ongoing decarbonisation of our electricity production. Whilst these Emission Factors change over time, they are locked into the SAP methodology when that version is finalised. The fact that SAP was last updated in 2012 is why the Emission Factor for electricity will change so much in the interim update to Part L, as 10 years will have passed by the time it comes into force.

Previously, it was possible for “Fuel Factors” to be applied in the SAP calculations to provide relief to new homes that are built using more carbon intensive fuels than gas for heating, either because there is no mains gas available or because the Government wanted to encourage electrical heating. However, grid electricity now has a lower carbon emission factor than gas, and thus it no longer needs a fuel factor to support its use. As a result, these Fuel Factors have been removed.

In practice this means that developments using higher carbon fuels, such as oil and LPG, would need to install considerable mitigating measures to comply with the TER, as the notional building it is compared to is gas-heated.

Target Fabric Energy Efficiency Rate

The Target Fabric Energy Efficiency Rate is defined as the space heating and cooling requirements per square metre of floor area, presented in kWh/m2/year, and is based on how well the fabric of the dwelling performs. The TFEE is calculated according to the specification of U-values set out in Appendix R of SAP for different types of building envelope, such as heat-loss floors, roofs, walls and glazing. For those unfamiliar, U-values are expressions of the thermal transmittance of a material or structure, shown in units of W/m2.K.

The higher the U-value, the faster heat is transmitted through the material or structure; as a result, when constructing a building lower U-values for the elements are desirable resulting in a better insulated building. The U-values used to produce the notional building have been proposed to be improved in the outcome of the Future Homes Standard consultation, as well as the ‘backstop’ U-values (minimum U-values building elements must achieve). In practice this means that the Target Fabric Energy Efficiency rate will become more stringent, and thus construction specifications that currently comply with Part L may need updating to ensure they comply with these more stringent conditions.

Target Primary Energy Rate

The Target Primary Energy Rate (TPER) is defined in kWhPE/m2/year, and is the maximum allowable primary energy usage by the dwelling. Primary energy is defined by the BRE as “energy from renewable and non-renewable sources which has not undergone any conversion of transformation process”. To illustrate what this means, the energy contained in fossil fuels is a source of primary energy, however, a unit of electricity produced by burning that fossil fuel would not be considered primary energy, as it had to go through a conversion process.

The calculation accounts for factors such as:

  • The efficiency of the property’s heating system;
  • Power station efficiency for electricity; and
  • The energy used to produce the fuel and deliver it to the property.

The Primary Energy metric therefore takes into account upstream energy uses, i.e. the energy used to produce the fuel before it is used in the dwelling. These are accounted for when producing the Primary Energy Rate by converting the energy consumption using a Primary Energy Factor. The Primary Energy Factor differs depending on the fuel type and indicates how many kWh of energy were used to produce 1 kWh of energy used in the dwelling. For example, the Primary Energy Factor for natural gas is 1.130 kWh/kWh, meaning 1.13 kWh of energy is used in the extraction and transportation of the natural gas for each 1 kWh supplied to the dwelling.

An example calculation, provided by BRE[1] can be seen below:

Dwelling A with a 90% efficient gas boiler has a heating demand of 10,000 kWh. The Primary Energy factor for mains gas is 1.130 kWh/kWh. The associated primary energy use would therefore be (10,000 kWh / 0.90) x 1.130 = 12,556 kWh.

Electricity is a bit different, given that it is produced from many different sources, both renewable and non-renewable. As such, it has been calculated from the primary energy of the input fuels, using a weighted average from the 2020-2025 projected electricity generation mix[2].

As with the other two existing compliance metrics, the Target value will be set according to the notional building specification.

Backstop values

In addition to these three compliance metrics, there are backstop values that must also be achieved in order to meet compliance with Part L of the Building Regulations. The limiting U-values for fabric elements in new dwellings in the upcoming Building Regulations uplift can be seen below:


Element Type Maximum U-value (W/(m2.K)
All roof types 0.16
Wall 0.26
Floor 0.18
Party wall 0.20
Swimming pool basin 0.25
Window 1.6
Rooflight 2.2
Doors (including glazed doors) 1.6

In order to meet the TFEE, the performance of some elements will need to be significantly better than the limiting standards in the table above.

In addition to this, the backstop for air permeability has decreased from 10m3 per hour per m2 @ 50Pa to 8.0m3 per hour per m2 @ 50Pa. This is in addition to the new regulations requiring that an air pressure test is carried out on every dwelling, which was not previously required.

Minimum efficiency standards for building services must also still be met. Guidance on specific minimum efficiencies for different system types can be found in the Domestic Building Services Compliance Guide for current Building Regulations, but have been put into the Approved Document L (Consultation Version) for the proposed uplift to Building Regulations, and are likely to be incorporated into one single Approved Document in the future.

Want to find out how your products and buildings perform against the new Building Regulations? Take a look at our Product Modelling service. 

[1] BRE (2019). Briefing Note- Derivation and use of Primary Energy factors in SAP. Source.

[2] BRE (2020). SAP 10 Technical Paper- CO2 and Primary Energy Factors for SAP. Source.

Article published 13/07/21

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