Cyril Sweett reviews the recommendations from the Zero Carbon Hub’s Carbon Compliance Task Group and considers the implications for the definition of a zero-carbon home from 2016

In December 2006, the communities minister unveiled plans for all new homes to have zero net carbon emissions from 2016. The challenge this objective presented was unmistakable. However, few thought that the definition of exactly what constitutes a zero-carbon home would remain unresolved more than four years later.

The time taken to formulate a detailed definition partly reflects the technical and commercial difficulties of meeting the zero carbon standard for all homes, whether built as part of a large block of flats in central London, a housing estate in the Midlands or a single home in the Lake District.

To help make the zero carbon standard more achievable for all development types, a new definition was introduced in 2008 (see Figure 1)This was more flexible, introducing three criteria that new homes would be required to meet:

  • Minimum standards of energy efficiency
  • Minimum carbon reductions on site, either through further energy efficiency measures or the use of low or zero-carbon technologies (together these two components are known as carbon compliance)
  • Purchase/delivery of additional carbon savings required to reduce the home’s carbon emissions to zero through “allowable solutions”.

Graph 1

A key element of this definition is the level at which carbon compliance is set, determining the maximum amount of carbon that can be directly emitted from a new home over the course of a year.

The Zero Carbon Hub was set up in 2008 to co-ordinate delivery of the low/zero carbon homes strategy. In 2009 the Hub made recommendations on the minimum Fabric Energy Efficient Standard, FEES for new homes. These were accepted by the government in 2010. At the time, it was indicated that the overall level of carbon compliance would be in a range equivalent
to a 44-100% improvement on the standard set by Part L 2006, with an initial proposal of 70%.

Following its work on the minimum energy efficiency standard, the Hub was asked to advise on the appropriate level(s) for carbon compliance. In December, it gave initial recommendations to the government. This article indicates the process by which these were developed, and their implications.

Defining carbon compliance

To help develop the carbon compliance standard the Hub convened a task group of housebuilders, environmental and consumer groups, engineers and architects. The core Task Group, together with the members of the technical, commercial and policy working groups, totalled more than 80 people.

One of the Hub’s first actions was to redefine the standards against which carbon compliance would be measured in terms of kg of CO2e per m2 per year. This new approach provides a more consistent benchmark than continuing to compare performance with the requirements of Part L in 2006. Using this new performance measure, a 44% improvement becomes 14kg CO2/m2/year, 70% is 6kg CO2/m2/year, with a 100% improvement being 0kg CO2/m2/year.

Three apartment block types (low, mid and high rise) were used to assess the technical and commercial implications of different levels.
Some of the key considerations when determining the standard are outlined below.

Technical feasibility

As a national minimum standard that must be achievable by all new developments:

  • different standards might be required for different building types to ensure that the standards are technically achievable
  • technologies such as wind energy, or biomass combined heat and power (CHP) that will only be appropriate in certain locations/project types were not used as a basis for setting the standard
  • technologies that are not yet commercially available were not considered.


It was felt that meeting the standard should not be at the expense of good place-making nor by imposing house designs that look very different. Therefore:

  • the carbon compliance standard should be achievable using more than one core heat source (for example, gas/electricity-based) as well as biomass where appropriate.
  • a maximum roof area equivalent to 40% of the ground floor area was considered available for solar technologies such as photovoltaics.

Cost and viability

This includes capital and lifecycle costs and also the potential impact of the zero carbon standard on project viability in the context of other regulatory costs such as affordable housing, community infrastructure levy and S106 payments.

Impact on householders

This includes the potential for savings in running costs in comparison with a new home built to 2010 regulations.

Based on the technical, commercial and policy work undertaken, the Task Group made the following recommendations:

  • the carbon compliance standard should relate to “built performance” not simply designed performance
  • the standard should be achieved “in aggregate” across a development site (so with some homes being below the standard provided others are above)
  • the minimum standards for different building types should be: 10kg CO2/m2/year for detached houses, 11kg CO2/m2/year for other houses and 14kg CO2/m2/year for low-rise apartments. Standards for medium (eight storeys) and high-rise (20 storeys) apartments are subject to further work early this year.

Figure 2 summarises the recommended standards in relation to past, current and future Building Regulation requirements.

Graph 2

Technical implications

Comprehensive modelling was undertaken to assess the technical implications of achieving different levels of carbon compliance. Using a modified version of the SAP 2009 tool, a wide range of low and zero-carbon technologies were assessed for each house type (modifications included the use of revised emission factors to reflect future changes in the energy supply mix of 0.527 kg per kWh for electricity and 0.227 kg per kWh for gas). Two options for fabric energy efficiency were used, the minimum FEES (an improvement on Part L 2010), and a higher standard that is approximately equivalent to the Passivhaus specification.

From the huge range of modelled options, the technical Work Group refined the technologies under consideration into systems believed to be widely applicable for developments in any context. For houses, this comprised gas boiler and air source heat pump (ASHP)-powered heating systems, both with and without solar hot water systems, together with photovoltaic (PV) panels (photovoltaics were used as a proxy for all electricity-generating technologies on the basis that they are the most widely applicable technology).

For low-rise apartments some additional “shared” options were included, namely ground source heat pumps (GSHPs), gas CHP and biomass boilers. These additional options were modelled on the basis that the system would be shared between all the apartments within a single block.

The technical viability of different carbon compliance standards was assessed for each home type by calculating the amount of PV that would be required to achieve each level. The performance of the PV panel was calculated on the basis that the panels were placed at an angle of 45º, with little shading and on an orientation of SW or SE. Where the PV array (or combined PV and solar hot water array) would be less than 40% of the building’s ground floor area, this was considered generally possible without necessitating a “solar design”.

Figure 3 shows the technical viability of meeting varying potential carbon compliance levels with different combinations of energy efficiency and renewable energy technologies. Where the cells are green or yellow the option does not require solar design; where the cells are orange or red, then orientation and roof design options are likely to be constrained.

The technical analysis showed that the potential to achieve different carbon compliance levels varies considerably between dwelling types. Achieving the previously proposed carbon compliance level of 70% (or 6kg CO2/m2/year) would be particularly challenging for apartment blocks without breaching the Task Group’s approach to design and technical flexibility.

Graph 3

Carbon compliance costs

Cyril Sweett supported the Hub’s work by analysing the capital and lifecycle costs of meeting different carbon compliance levels. The approach to compiling base cost data is described in the box.

The data gathered reflects costs in 2010. The future price of these technologies in 2016 was estimated using UK and international market projections together with learning rates (the extent to which the costs of a product or service reduce as its market grows). The impact of learning in the renewable energy sector is expected to deliver significant reductions in technology costs between now and 2016 with the installed cost of PV, for example, projected to fall by around 45%.

Figures 4a and 4b show the estimated additional capital costs of targeting different levels of carbon compliance in an end terrace house, with and without the effects of industry learning. These costs include for:

  • meeting the minimum FEES standard
  • installation of sufficient LZC technologies to meet the specified option for carbon compliance
  • purchase/delivery of allowable solutions at a price of £75 per tonne of residual carbon (after the carbon compliance level has been achieved) for 30 years.

Figures 4a and 4b show that, with an allowable solutions price of £75 per tonne (the central price used previously in government analysis in a range between £50 and £200), the costs of achieving zero carbon status are higher when the carbon compliance level is lower. Within each core heat technology group (for example, gas boiler or ASHP) the incremental cost of moving to lower levels of carbon compliance is linked to the cost of the additional PV required. Excluding the effects of learning, this is between £300 and £600 per kg CO2/m2/year reduction in the level. However, if industry learning is taken into account this increase is reduced significantly, to between £80 and £175 per kg CO2/m2/year. Once a technology is no longer technically feasible, then it is necessary to “jump” to the next technology (the addition of a solar hot water system in this instance), resulting in a greater increase in cost. In practice, it is unlikely that dramatic step changes in cost would occur because further carbon savings will be achievable through targeted improvements in energy efficiency beyond the FEES.

Graph 4

Graph 5

Analysis of detached houses and low-rise apartments are shown in Figures 5 and 6.

Graph 6

Graph 7

These cost estimates represent a significant reduction on the costs originally projected for zero-carbon homes (up to £40,000 per home) but will still be of concern to an industry where many sites are already not economically viable for development.

More work needed

If accepted by government, the Hub’s work to define carbon compliance in new homes represents a major step towards a clear, workable definition of a zero-carbon home, but there is still more to do.

The issue of higher-rise apartments (above four storeys) is still to be resolved, as is the approach to demonstrating that the actual performance of new homes meets their design criteria. Other thorny issues include the extent to which local factors such as climatic conditions or local community views should be incorporated into the zero carbon framework. These issues will be picked up in the Hub’s final report, due in February.

However, the largest uncertainty remains the approach to, and cost of, allowable solutions. It should be remembered that with the current recommendations, about two-thirds of the total carbon savings required to achieve zero carbon status are delivered through allowable solutions. Funds generated for delivering allowable solutions are likely to be considerable, at about £5,000-6,000 per new house

Fabric Energy Efficiency Standard

The FEES is defined in kWh per m2 per year and relates only to space heating and cooling. It excludes other factors such as the efficiency of the heating system, hot water demand and lighting.

Two minimum standards were defined:

  • 39kWh/m2/yr for apartment blocks and mid-terrace houses
  • 46 kWh/m2/yr for semi-detached, end-of-terrace and detached houses.

This standard is higher than that required by current Building Regulations 2010 but is not as stringent as the Passivhaus standard.

Compiling cost data

A survey of housebuilders and technology suppliers was used to update cost benchmarks for a range of defined system sizes. Analysis revealed significant variations in costs across the housing industry, even after they were adjusted for consistency by standardising.

  • Quality thresholds - to mid-range commercially available levels likely to be installed by a housebuilder and obtained in a competitive procurement process
  • Scope - to include all materials and work necessary to produce a functioning system including:

core technology

power supplies, connections,

fuel delivery mechanisms

additional preliminaries, such as extra scaffolding hire for installation of solar technologies

additional space requirements (for example, biomass boilers and fuel storage)

on-site upgrades to electrical infrastructure to support the use of heat pumps

general builders’ work in connection

  • Scale - price data assumes that the technology is being procured to match a development of broadly 200 units or fewer
  • Sites were assumed to be free of location-specific abnormalities such as challenging ground conditions, sequencing, and logistics.

Average, high and low costs were identified for each technology type and for some these costs were separated into fixed and variable price elements to allow the costs to be scaled across a limited range of sizes.


To see the carbon compliance Task Group’s interim report,