With changes to Part L of the Building Regulations in England imminent, building control professionals need to start preparing now. Otherwise, the wide range of new criteria could become overwhelming.
Every time a change is made to Part L, this allows for the standard assessment procedure (SAP) to be updated with new carbon factors for each of the main fuel sources. It also means further details can be added for existing items such as Tesla batteries and PV diverters for hot water cylinders, and the inclusion of new energy-efficiency technology.
Carbon associated with electricity to be reduced
The headline change being made in SAP 10.3 is that the emissions associated with electricity will be reduced from 0.519kg of carbon dioxide per kilowatt–hour to 0.136 kg, with the effect that electricity will be considered less carbon-intensive than mains gas.
While this could mean that direct electric heating, electric underfloor mats and electric panel heaters see a comeback, the introduction of a new compliance metric known as primary energy makes this unlikely.
Primary energy will measure the kilowatt–hours used by heating, hot water, lighting, ventilation and cooling. The primary energy factor for electricity is 1.501 – the highest that could be assigned to a fuel. In comparison, mains gas is 1.13, liquid petroleum gas (LPG) is 1.14 and oil heating is 1.18. Without measures to help reduce primary energy, the only viable electric heating in principle will be heat pumps.
Assessing thermal bridging under revised Part L
The former Ministry for Housing, Communities and Local Government used to accredit construction details that assigned psi values to each junction to account for heat loss. Under the revised Part L, however, it will no longer do so, leaving assessors with two options when it comes to assigning psi values for any details they deal with: they can adopt either the default in the SAP manual, or the values defined by the user or declared by the manufacturer.
This should not concern the majority of designers/architects; however, anyone considering non-traditional construction methods or bespoke approaches may find it difficult to comply with the Building Regulations unless they consider calculating the psi values themselves.
Shower flow rate factored into SAP
SAP will also now take shower flow rate into account. The rate required depends on the configuration of the heating system. Table 1 shows these rates.
| Hot water supply type | Default flow rate (litres/minute) | Default power (kW) |
| Vented hot water system | 7 (existing dwelling) 8 (new dwelling) |
– |
| Vented hot water system and pump | 12 | – |
| Combi-boiler or unvented hot water system | 11 | – |
| Instantaneous electric shower | – | 9.3 |
| Unable to determine | 9 | – |
Table 1: Shower flow rates required under revised Part L. Source: SAP 10.3, Table J4
| Hot water supply type | Default flow rate (litres/minute) | Default power (kW) |
| Vented hot water system | 7 (existing dwelling) 8 (new dwelling) |
– |
| Vented hot water system and pump | 12 | – |
| Combi-boiler or unvented hot water system | 11 | – |
| Instantaneous electric shower | – | 9.3 |
| Unable to determine | 9 | – |
Newly built dwellings
The revised Part L proposes that newly built dwelling should have 31% lower carbon emissions than under the current standard. This can be achieved by meeting fabric standards similar to those in the present regulations, but with the addition of wastewater heat recovery (WWHR) and photovoltaic (PV) panels. The main highlights for the notional building are found in Table 2. The notional building is what every project in England is being measured against – if it always achieves the values identified in Table 2 then it will always comply.
'The revised Part L proposes that newly built dwelling should have 31% lower carbon emissions than under the current standard'
Table 2: Notional specification for new dwellings. Source: Revised Approved Document L, Volume 1, Table 1.1
* Section 6 of the revised Part L states that the mains gas boiler should achieve 92% efficiency, as required by the EU Energy-related Products Directive 2009, but SAP will continue to use the Seasonal Efficiency of Domestic Boilers in the UK (SEDBUK) rating
| Feature | Specification |
| Wall | U-value = 0.18W/m2K |
| Party wall | U-value = 0.00W/m2K |
| Floors | U-value = 0.13W/m2K |
| Roof | U-value = 0.11W/m2K |
| Doors | U-value = 1.00W/m2K |
| Windows/rooflight | U-value = 1.202W/m2K |
| Air test | Design air permeability = 5.0m3/(h.m2) at 50Pa |
| Heating | Mains gas boiler 89.5% SEDBUK, including interlock and compensator* |
| Thermal bridging | 0.05 psi value |
| Lighting | Fixed lighting capacity (lumens) = 185 × total floor area Efficacy of all fixed lighting = 80 lumens/W |
| WWHR | All showers should be connected to WWHR, including showers over baths Instantaneous WWHR with 36% of heat recovered and 98% of wastewater reused |
| PV | Capacity required for houses: kWp = 40% of ground floor area (including unheated spaces) ÷ 6.5 Capacity required for flats: kWp = 40% of each dwelling floor area ÷ (6.5 × number of storeys in block) System facing south east or south west |
Table 2: Notional specification for new dwellings. Source: Revised Approved Document L, Volume 1, Table 1.1
* Section 6 of the revised Part L states that the mains gas boiler should achieve 92% efficiency, as required by the EU Energy-related Products Directive 2009, but SAP will continue to use the Seasonal Efficiency of Domestic Boilers in the UK (SEDBUK) rating
| Feature | Specification |
| Wall | U-value = 0.18W/m2K |
| Party wall | U-value = 0.00W/m2K |
| Floors | U-value = 0.13W/m2K |
| Roof | U-value = 0.11W/m2K |
| Doors | U-value = 1.00W/m2K |
| Windows/rooflight | U-value = 1.202W/m2K |
| Air test | Design air permeability = 5.0m3/(h.m2) at 50Pa |
| Heating | Mains gas boiler 89.5% SEDBUK, including interlock and compensator* |
| Thermal bridging | 0.05 psi value |
| Lighting | Fixed lighting capacity (lumens) = 185 × total floor area Efficacy of all fixed lighting = 80 lumens/W |
| WWHR | All showers should be connected to WWHR, including showers over baths Instantaneous WWHR with 36% of heat recovered and 98% of wastewater reused |
| PV | Capacity required for houses: kWp = 40% of ground floor area (including unheated spaces) ÷ 6.5 Capacity required for flats: kWp = 40% of each dwelling floor area ÷ (6.5 × number of storeys in block) System facing south east or south west |
The air test heating efficiency – referred to in Table 2 above – relates to the flow of air (m3/hour) in or out of the building, per square metre of the building internal envelope at a reference pressure of 50 Pascals (Pa) between the inside and outside of the building.
SEDBUK – also referred to in Table 2 – is a measurement of boiler efficiency, whereby a compensator adjusts the flow of the boiler dependent on external temperature and a boiler interlock ensures the heating system does not continuously produce heat once a temperate has been reached.
If the specification in Table 2 is implemented, the dwelling will comply with the revised Part L. However, if greater design flexibility is required because of site constraints, we could design to the limiting standards shown in Table 3. These limiting standards should only be implemented to overcome challenges on site in combination with compensatory measures, because the specification we have compared against is the notional specification in Table 2.
| Feature | Specification |
| Wall | U-value = 0.26W/m2K |
| Party wall | U-value = 0.20W/m2K |
| Floors | U-value = 0.18W/m2K |
| Swimming pool basin | U-value = 0.25W/m2K |
| Roof | U-value = 0.16W/m2K |
| Doors | U-value = 1.60W/m2K |
| Windows | U-value = 1.60W/m2K |
| Rooflights | U-value = 2.20W/m2K |
| Air test | Design air permeability = 8.0m3/(h.m2) at 50Pa |
Table 3: Limiting standards. Source: Revised Approved Document L, Volume 1, Table 4.1
| Feature | Specification |
| Wall | U-value = 0.26W/m2K |
| Party wall | U-value = 0.20W/m2K |
| Floors | U-value = 0.18W/m2K |
| Swimming pool basin | U-value = 0.25W/m2K |
| Roof | U-value = 0.16W/m2K |
| Doors | U-value = 1.60W/m2K |
| Windows | U-value = 1.60W/m2K |
| Rooflights | U-value = 2.20W/m2K |
| Air test | Design air permeability = 8.0m3/(h.m2) at 50Pa |
Highly glazed extensions
When any extension is added to an existing dwelling, there is a limit to the amount of glazing allowed. This limit is defined as 25% of the extension's floor area plus the area of any openings that are no longer exposed, such as doors or windows on the original building that now lead into the extension. If this limit is exceeded, then a further assessment is required.
This assessment is used to demonstrate that the highly glazed extension will perform in the same way as one built to the limiting glazing standard; that is, it will not result in any more carbon dioxide or energy consumption, and not decrease the energy efficiency of the fabric. This is normally carried out using the SAP.
The new notional U-value for highly glazed extensions can be seen in Table 4.
Table 4: Notional U-value for highly glazed extensions. Source: Revised Approved Document L, Volume 1, Table 4.2
* Until 14 June 2023 ** After 14 June 2023
| Feature | U-value |
| Wall | 0.18W/m2K |
| Party wall | 0.00W/m2K |
| Floors | 0.18W/m2K |
| Swimming pool basin | 0.25W/m2K |
| Roof | 0.15W/m2K |
| Doors with >60% internal face glazed | 1.40W/m2K door energy rating (DER) band C |
| Doors: all | 1.40W/m2K DER rating band B |
| Doors: timber only | 1.80W/m2K DER rating band E* 1.40W/m2K DER rating band B** |
| Windows: all | 1.40W/m2K window energy rating (WER) rating band B |
| Windows: timber-framed only | 1.60W/m2K WER rating band C* 1.40W/m2K WER rating band B** |
| Rooflights | 2.20W/m2K |
Table 4: Notional U-value for highly glazed extensions. Source: Revised Approved Document L, Volume 1, Table 4.2
* Until 14 June 2023 ** After 14 June 2023
| Feature | U-value |
| Wall | 0.18W/m2K |
| Party wall | 0.00W/m2K |
| Floors | 0.18W/m2K |
| Swimming pool basin | 0.25W/m2K |
| Roof | 0.15W/m2K |
| Doors with >60% internal face glazed | 1.40W/m2K door energy rating (DER) band C |
| Doors: all | 1.40W/m2K DER rating band B |
| Doors: timber only | 1.80W/m2K DER rating band E* 1.40W/m2K DER rating band B** |
| Windows: all | 1.40W/m2K window energy rating (WER) rating band B |
| Windows: timber-framed only | 1.60W/m2K WER rating band C* 1.40W/m2K WER rating band B** |
| Rooflights | 2.20W/m2K |
Note, with regard to Table 4 contents: timber doors and timber-framed windows have a dedicated U-value until 14 June 2023; after this date they will need to meet the same U-value as all other kinds of door or window. This is to allow manufacturers more time to adjust, as it will be more challenging for timber products to comply with the new requirements.
Photographic evidence of thermal bridging required
The new regulations will also introduce a requirement for photographic evidence as part of the final sign-off stage for the SAP. The photographs should concentrate on the potential cold thermal bridges found in key locations in dwellings.
Foundations or substructure and ground floor must be photographed, to show thermal continuity and the quality of insulation:
-
at the ground-floor perimeter edge
-
at the external door threshold
-
below the damp-proof course on the external wall.
Each main external wall type also needs photographing, to show thermal continuity and the quality of insulation:
-
in the junction between ground floor and wall
-
for structural penetrating elements.
For each main roof type, photographs must show thermal continuity and quality of insulation:
-
at the joist or rafter level
-
at eaves and gable edges.
One image is needed for each opening type in each wall or roof to show thermal continuity and quality of insulation:
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at the window positioning in relation to cavity closer or insulation line
-
at the external door positioning in relation to the cavity closer.
Airtightness details should also be photographed, if not included or visible in images showing continuity of insulation.
All building services plant for space heating, hot water, ventilation and low- or zero-carbon technology must be photographed, including:
-
plant or equipment ID labels, including make and model
-
continuity of insulation on the primary pipework
-
continuity of insulation on the mechanical ventilation ductwork outside the thermal envelope.
The photographs are needed for each dwelling on a development as a record of all the above locations during the construction of a property. They should be provided to the energy assessor and the building control body once the process is complete.
'The new regulations will also introduce a requirement for photographic evidence as part of the final sign-off stage for the SAP'
Changes made to grounds for transitional agreements
The transitional agreement that establishes how the revised Part L will be applied to existing sites is moving away from site-wide exemption to plot-by-plot exemption. In the past, Part L has stipulated that, once work had commenced on a site, all plots were locked into the current Building Regulations; however, each plot will now be subject to the regulations in place at the time work commences on it.
To qualify for the new transitional agreement, a developer must:
-
submit a building notice, full plans application or initial notice, or deposit plans, by 15 June 2022 and
-
commence work on each individual plot by 15 June 2023.
Commencement is defined as:
-
excavation for strip of trench foundation
-
digging out and preparation of ground foundations
-
piling, boring for piles or pile-driving and
-
completing drainage work specification to the building.
Implementation and implications
While the revised regulations are designed to encourage the uptake of heat pumps, it is likely that other measures will be introduced in the next three years to push new builds towards the technology. However, mains gas boilers can still comply with the regulations if the notional specification is implemented.
With photographs now also required to prove compliance with Approved Document L, it is unlikely that dwellings will be signed off by the SAP assessor or building control body unless this evidence is provided.
Note that while these changes only apply in England at present, Part L regulations here have in the past been used as foundations for those of the other UK nations, so it is likely that similar changes will be made in Scotland, Wales, and Northern Ireland.
James Rivers is an SAP assessor at Ashby Energy Assessors
Contact James: Email
Related competencies include: Legal/regulatory compliance