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Welsh & English Part L Compliance – The Key Differences Explained

Posted : June 17, 2015 by , Category: Building Regulations

welsh-and-english-compliance-blogOn the 31st July 2014 the Welsh government launched an update to their Building Regulations. This latest revision is the first version of the standard in which Wales has its own Building Regulations that were separate from England.

Prior to this transition date Buildings in Wales had to demonstrate that compliance with Part L 2010 was achieved which was applicable in both England and Wales. The Building Regulations in both England and Wales have now both been updated recently and there are some divergences in the requirements.

The updated regulations for both England and Wales build on the previous Part L 2010 requirements where a Notional building was used to determine the minimum Carbon Emissions rate for the Design. Neither have taken a dramatic departure from the previous methodology though the two governments have sought to reduce targets by different amounts and achieved this by modifying the Notional building in different ways.

In England, early consultation papers proposed changes that would result in a 20% reduction in Non-Domestic buildings when applied across the overall building mix. By the time the updated Part L had launched this level of reduction was reduced to just 9%. Under the Welsh regulations however, a 20% reduction was targeted in the final version.

In both England and Wales the Notional Building directly informs the Minimum requirements meaning that if you matched or exceed all of the Notional Building Attributes in the Actual Design, you should have a design that meets the minimum standards. As different levels of reduction were targeted by the two government’s this is reflected in the different properties associated with the Notional Buildings. While the two Notional Buildings remain largely the same there are a few key differences between them.

Photovoltaics (PVs)
The most notable and significant difference is the inclusion of PV’s within the Welsh Notional Building. PV’s with an area of 5.3% of the floor area (up to a maximum area of 50% of the roof) will be applied producing 120kWh/m² each year. Where the roof area limit does not play a factor this results in a reduction to the TER of 3.3 kg/m²/annum.

This reduction remains the same irrespective of the building type meaning that in buildings that have an inherently low energy consumption (i.e. schools) this can constitute a significant proportion of the overall annual CO2 emissions. It is worth noting that it is not a requirement to include PV’s or any on site renewable technologies within the Actual design however the reduction associated with the PV panels would need to be offset through improvements made elsewhere in the design.

Lighting Efficacy
Other differences between the English and Welsh Notional Buildings include a higher lighting efficacy in the Welsh Notional Building. The Welsh notional building is based on a lighting efficacy of 65 luminaire lm/W compared to 60 luminaire lm/W as used in England.

This results in a lighting power density that is ≈7% lower in the Welsh Notional Building. As well as reducing the lighting load this has the consequence of lowering internal gains effecting both heating and cooling loads as well.

Notional Constructions and Air Permeability
Another difference of note is that under the Welsh regulations the Notional Building applies different Fabric properties based on the HVAC system assigned to a space. Where a zone has mechanical cooling available the assigned glazing will have a U-value of 1.8 W/m².K and where there is no mechanical cooling a U-Value of 1.6 W/m².K is applied.

There is a similar scenario with regards to air permeability where in buildings over 250m² buildings with cooling have an air permeability of 5 m3/m² @ 50Pa assigned whereas spaces without mechanical cooling have an air permeability of 3 m3/m² @ 50Pa.

This could have the unintended consequence of making the Target Emission Rates easier to achieve in air conditioned buildings which may lead to designs unnecessarily adding cooling in order to make obtaining compliance easier.

Fuel Selection
One final difference of note between the English Part L 2013 and the Welsh Part L 2014 is the heating fuel selection for the Notional Building. Under the English regulations the Notional Building heating fuel largely uses the same fuel selection as the Actual Design.

Under the Welsh Regulations the Notional building uses Gas provided it is available on site. This becomes particularly significant if the design include a Low Carbon heat source such as Biomass as this allows designers to demonstrate a much more significant CO2 reduction.

The following shows some compliance analysis for a range of sample test models using both the English Part L 2013 and the Welsh Part L 2014 (with the location set to Cardiff in all cases)to compare the impact the different Notional Building characteristics have on the Target Emission Rate.

The chart below shows a comparison of the Notional Building energy consumption for both the English Part L 2013 and the Welsh Part L 2014. This example shows the performance in an Air Conditioned Office. From this we can identify the impact on energy consumption resulting from the different Notional Building properties. We can see that in the Welsh Notional Building there is a greater heating demand as a consequence of the poorer U-Values and Air Permeability applied when a space is air conditioned. The higher peak heating load also results in a greater Auxiliary Energy demand. This difference also results in a reduced cooling demand.








The chart below illustrates the Target Emission Rates for the English and Welsh Regulations with the contribution associated with the PV indicated on the charts. It can be seen that if the PV was excluded from the analysis the two TER’s would be very similar however with the PV contribution from in the Welsh Notional Building the TER is 13% lower than the English counterpart.







A similar comparison on a Naturally Ventilated Office shows a Welsh TER that is 20% lower than the English TER adding weight to the idea that it may be easier to demonstrate regulation compliance in Air Conditioned buildings.

As the contribution from PV remains constant regardless of the building type, the proportional contribution can vary significantly.

In the two charts shown below we see a comparison of a Hotel and a School. In the Hotel building there is a large overall energy demand largely driven by the hot water demand. As a result the PV panel only reduces the TER by 4%. If the designers decided not to install PV in the Actual Building this difference could likely be offset quite easily through other efficiency measures.

In the school building however the overall energy use is significantly lower and we see the PV system reducing the TER by 22%. This proportion of saving would be much more challenging to offset should the designers decide against a form of onsite renewable energy.

















The final example shown below compares the performance of a hotel building equipped with a Biomass boiler. DHW loads are the most significant load in this buildings type making the heating fuel source and performance very influential in the resulting Target Emissions Rate.

Under the English Regulations the Notional Building will adopt the same fuel type as the Actual design, in this case a biomass boiler, whereas under the Welsh regulations the Notional Building will use a Natural Gas Boiler.

The result of this is a Welsh TER that is more than double its English Counterpart making it significantly easier to demonstrate compliance.








The Virtual Environment has allowed users to perform Compliance assessments under the Welsh Part L 2013 for some time now using the VE-SBEM method. In VE 2015 you can also perform analysis using the DSM approach. This allows you to perform assessments on larger more complex buildings than before and enables you to assess the performance of both the Actual and Notional buildings in great detail using Vista.

VE 2015 is available now from the IES Download Centre.

If you wish to purchase VE-DSM for Part L2 2014 (Wales) please email sales@iesve.com for information.




  1. Peter Rankin says:

    Really good article, but you’ve missed out an important difference. Primary energy is targeted and measured in Part L Wales, but not in Part L England.

  2. Phil says:

    Why has the tpec and bpec not been mentioned.. This is a big deal in our experience? See our webpage form last year when the welsh regs came in.

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