Building regulations - proposed changes to energy and environmental standards: stage 2 consultation and BRIA

5 New dwellings

5.1 Level of challenge for new dwellings

The February 2023 energy standards introduced a level of challenge for new dwellings with an aggregate reduction in carbon emissions of 32% over the 2015 standards. This level of challenge is implemented through the Target Delivered Energy Rate (TDER): set based on a specification linked to the choice of main heating fuel type.

Through 2024 and 2025 the Scottish Government carried out detailed modelling to determine whether an uplift in performance standards for new dwellings is reasonable and the options available at which the level of challenge could be set at the time of implementation.

The modelling undertaken by the Scottish Government’s technical consultant, defined a set of archetype characteristics that are reflective of the current build mix in Scotland:

• Detached house;
• End terrace house;
• Mid terrace house; and
• Block of flats of 16 dwellings over 4 storeys

The modelling considered four levels of building specification:

• As defined by the 2023 notional dwelling;
• Option 1 - Good Practice Level of Challenge;
• Option 2 - Best Practice Level of Challenge; and,
• Option 3 - Business as Usual (BAU) model to reflect what is being built in Scotland at the point of undertaking the modelling.

Three modelling packages were utilised in the research to allow comparison of results:

• Standard Assessment Procedure (SAP) version 10.2;
• Home Energy Model (HEM) version 0.33 with the Future Homes Standard Wrapper (FHS) version 0.24; and
• Passivhaus Planning Package (PHPP) version 10.6.

5.1.1. Proposals

Following discussion with the Technical Working Group Option 1 and Option 2 are applied and indicated for consultation alongside an option to retain the current 2023 level of challenge for new dwellings.

To best reflect the proposed calculation methodology (see section 8 of this consultation) the figures from the HEM/FHS modelling are utilised in the summary presented in Table 1 for Option 1 and Option 2.

Table 1: New dwellings – annual carbon emissions abatement compared to the 2023 energy standards – proposed reduction as modelled in HEM v0.33 / FHS v0.24.

New Dwellings	Option 1 – Good Practice	Option 2 – Best Practice
Annual abatement (%)	6%	37%
Annual abatement (kT)	44 kt CO2e	252 kt CO2e

Please refer to the associated published research for full details of the target setting specifications used to generate the potential reductions in carbon emissions presented above: ‘Identification and Assessment of Improvements to Energy Standard for New Domestic Buildings within Scottish Building Regulations’.

The main elements are presented in Table 2:

Table 2: Options for uplift in the target setting level of challenge for new dwellings.

Fabric	Option 1	Option 2	2028 Option 3
External Wall U-value (W/m2K)	0.15	0.12	0.15
Floor U-value (W/m2K)	0.11	0.10	0.12
Roof U-value (W/m2K)	0.11	0.10	0.09
Window and glazed door U-value (W/m2K)	1.2	0.8	1.2
Window and glazed door g-value	0.52	0.52	0.63
Air permeability (m3/(h.m2) @50Pa)	3	1.5	5
Ventilation	Option 1	Option 2	Option 3
System	dMEV	MVHR	dMEV
SFP (W/(l/s))	0.15	0.80	-
Heat recovery (%)	N/A	90%	N/A
Lighting	Option 1	Option 2	Option 3
Efficacy of all fixed lighting (lm/W)	100	120	80
Heating and hot water – heat pump	Option 1	Option 2	Option 3
System	Air source heat pump	Air source heat pump	Air source heat pump
Heating efficiency (%)	285%	285%	250%
Hot water efficiency (%)	282%	282%	250%
Standing loss (kWh/day)	ErP class A	ErP class A	If cylinder present: declared loss factor = 0.85 x (0.2 + 0.051 x V2/3).
Wastewater heat recovery efficiency (%)	N/A	N/A	N/A
Heating and hot water – heat network	Option 1	Option 2	Option 3
System	Heat Network	Heat Network	Heat Network
Heating efficiency (%)	As actual dwelling	As actual dwelling	As actual dwelling
Hot water efficiency (%)	As actual dwelling	As actual dwelling	As actual dwelling
Standing loss (kWh/day)	1.00 (CIBSE CP1)	1.00 (CIBSE CP1)	-
Wastewater heat recovery efficiency (%)	58%	58%	N/A
Heating and hot water – any other solution	Option 1	Option 2	Option 3
System	Direct Electric	Direct electric	Gas boiler
Heating efficiency (%)	100%	100%	93%
Hot water efficiency (%)	100%	100%	87%
Standing loss (kWh/day)	ErP class A	ErP class A	If cylinder present: declared loss factor = 0.85 x (0.2 + 0.051 x V2/3).
Wastewater heat recovery efficiency (%)	58%	58%	55%
Onsite generation of power – heat pump	Option 1	Option 2	Option 3
System	N/A	Photovoltaic Panels	N/A
kWp	N/A	Houses: 0.2 x ground floor area / 4.5 Flats: 0.2 x dwelling floor area / (4.5 x number of storeys in block)	N/A
Onsite generation of power – heat network	Option 1	Option 2	Option 3
System	Photovoltaic panels	Photovoltaic panels	Photovoltaic panels
kWp	Houses: 0.3 x ground floor area / 4.5 Flats: 0.3 x dwelling floor area / (4.5 x number of storeys in block)	Houses: 0.3 x ground floor area / 4.5 Flats: 0.3 x dwelling floor area / (4.5 x number of storeys in block)	Houses: 0.4 x ground floor area / 6.5 Flats: 0.4 x dwelling floor area / (6.5 x number of storeys in block)
Onsite generation of power – any other solution	Option 1	Option 2	Option 3
System	Photovoltaic panels	Photovoltaic panels	Photovoltaic panels
kWp	Houses: 0.4 x ground floor area / 4.5 Flats: 0.4 x dwelling floor area / (4.5 x number of storeys in block)	Houses: 0.4 x ground floor area / 4.5 Flats: 0.4 x dwelling floor area / (4.5 x number of storeys in block)	Houses: 0.4 x ground floor area / 6.5 Flats: 0.4 x dwelling floor area / (6.5 x number of storeys in block)

The February 2023 standards introduced a step change in the performance of new dwellings with the research associated with the current review indicating that improvements in most measures that result in lower energy buildings would not be cost effective to introduce at this time.

However, the slight increases in services efficiencies do reflect improvement in technology over the past few years. One area that has been improved is a step reduction in the proposed target setting notional dwelling air permeability. This is to complement the proposals in section 9 of this consultation and to reflect the importance that is drawn to this element within the Passivhaus system.

Consultation Question 6

Do you have any comments on the roof U-value proposals across the Good and Best Practice options noting that these represent a relaxation in the level of challenge for this element?

Yes

No

Please provide a summary of the reason(s) for your view.

Consultation Question 7

What uplift to the current energy standards for new dwellings do you consider appropriate as an outcome of this review?

Option 1: ‘Good Practice’ standard (6% emissions reduction)

Option 2: ‘Best Practice’ standard (37% emissions reduction)

Option 3: Retain current standard (0% emissions reduction)

Other (please specify)

Please provide a summary of the reason(s) for your view.

For new dwellings connecting to a communal heating system

Consultation proposals maintain the current option of three dwelling specifications based on the heat source: heat pumps, heat networks and any other solution, the latter being assessed against the direct electric heating specification.

It is proposed, where dwellings on a development are supplied from a communal heating system (as defined in the Heat Networks (Scotland) Act 2021), also located within the development, that the use and benefit of high efficiency generation on site should be recognised.

This would be achieved by enabling dwellings supplied from such a communal heating system to be modelled against the heat pump notional dwelling, with the net efficiency of communal heat supply determined by a suitably qualified party and input into the calculation.

Such an option will support the use of high-efficiency communal heating systems which give an overall delivered energy outcome for the development, similar to the use of individual heat pumps.

Consultation Question 8

Do you support an approach to enable recognition of higher efficiency generation within a development regardless of whether it takes place within individual buildings or a communal heating system?

Yes

No

Please provide a summary of the reason(s) for your view.

Cost neutrality

The development and confirmation of the proposed levels of challenge across the available heat sources has been completed with a focus on capital cost neutrality. As indicated within the associated published research the capital cost fluctuation to build a heat pump, heat network or direct electric dwelling is minor. This principle allows the current and proposed energy metrics to fluctuate to compensate for the differences in efficiencies and costs for each of the available heat solutions.

Consultation Question 9

Are you currently designing or building dwellings to the February 2023 standards and able to share cost information in confidence for any of the available heat solutions?

Yes

No

If you answered ‘Yes’, please contact the review team on bsdenergystandardsreview@gov.scot.

5.2 Method of target setting for new dwellings

Standard 6.1 aims to limit the delivered energy needed at a new dwelling to meet energy demand through the setting of an overall performance target calculated by an approved methodology.

The calculated delivered energy demand for the proposed dwelling: the Dwelling Delivered Energy Rate (DDER) measured in kilowatt hours per square metre per year (kWh/m²/yr), should be less than or equal to the Target Delivered Energy Rate (TDER). The TDER is calculated by a ‘notional dwelling’. The notional dwelling includes a range of specification items that are either defined or those that replicate what is specified in the actual dwelling.

However, it must be recognised, that when a given dwelling specification is fully defined and is modelled in the approved methodology a single DDER and single TDER will be reported.

Given the proposal in section 4, the following section should be read noting the intent to introduce a space heating demand target alongside retaining the delivered energy demand target.

Target fixed elements

When a dwelling specification item differs from those that are defined within table 6.1 of the current domestic building standards technical handbook the TDER does not change. For example, the TDER for a given dwelling specification does not consider the actual building fabric U-values as these are ‘fixed’ by the notional dwelling i.e. at 0.15 W/m²K for walls. When everything else within a given dwelling specification remains the same the infinite choice of fabric U-values should always result in a single TDER as the DDER fluctuates. This principle is the same for all elements of the notional dwelling with a stated value in Table 6.1 of the current domestic building standards technical handbook.

Target fluctuating elements

For dwelling specification items that are defined as being the same as the actual dwelling, the TDER will fluctuate depending on the final dwelling specification. For example, the TDER for a given dwelling specification will be modified based on the heat source defined in the actual dwelling. This principle is the same for all elements of the notional dwelling where the specification item is defined as the ‘same as the actual dwelling’ or a similar phrase in Table 6.1 of the current domestic building standards technical handbook.

Passivhaus

The Passivhaus system does not use a notional dwelling approach, instead a single ‘absolute’ target must be met for total energy use and space heating demand. Where this single target is set, it is in effect ‘fixing’ all elements of the notional dwelling on a single dwelling specification that results in those targets being met. There are no notional dwelling aspects that are defined as the same as the actual dwelling. This means that any modification to the actual dwelling specification does not modify the ‘absolute’ targets that need to be met.

5.2.1. Proposals

The Scottish Government proposes to reduce the number of ‘target fluctuating elements’ within a target setting dwelling to a number of clearly defined principles. We propose that the specification items that allow the TDER to fluctuate are:

• Dwelling location;
• Dwelling archetype (flat, mid-terrace, end-terrace and detached);
• Dwelling orientation; and
• Dwelling heat source

Dwelling location

It is proposed that the target setting dwelling location is defined as the actual dwelling location. For a given dwelling specification this will produce a range of TDERs, the range being determined by the number of climate data files within the compliance methodology.

Scottish building regulations currently apply a single climate data set within the domestic calculation methodologies. Responses within the stage one consultation were in favour of a move to adopt regional climate data to enable a more informed approach to dwelling design and reporting on performance which better reflects the impact of location on the building. Given this response we have implemented the use of available climate data within the consultation version of the domestic tool.

The final version of the tools will implement the 14 weather files available from CIBSE 2025 (or a more appropriate updated set). Given the proposal to allow the TDER to fluctuate based on the dwelling location this will result in a range of TDERs determined by these 14 locations.

Dwelling archetype

It is proposed that the target setting dwelling archetype is defined as the actual dwelling archetype. For a given dwelling specification this will produce a range of TDERs, the range being determined by the four dwelling archetypes currently identified in research: flat, mid-terrace, end-terrace and detached.

Dwelling orientation

It is proposed that the target setting dwelling orientation is defined as the actual dwelling orientation. For a given dwelling specification this will produce a range of TDERs, the range being determined by the 360 degrees that are available for dwelling placement.

Dwelling heat source

It is proposed that the target setting dwelling heat source is defined as the actual dwelling heat source. For a given dwelling specification this will produce a range of TDERs, the range being determined by the three available heat options: heat pumps, heat networks and any other solution (direct electric).

It is proposed that all other dwelling specification items will be ‘fixed’ within a target setting dwelling and will not allow the TDER to fluctuate.

Consultation Question 10

Do you support the proposal to allow the target rates for dwellings to fluctuate based only on the location, orientation, archetype and heat source of the dwelling?

Yes

No

Please provide a summary of the reason(s) for your view.

When the majority of dwelling specification items are ‘fixed’ within a target setting dwelling a specific range of Target Delivered Energy Ratings (TDERs) and Target Space Heating Rates (TSHRs) are more easily defined around the four items indicated above.

The Scottish Government proposes to replace the presentation of a notional dwelling within the Technical Handbooks with a set of target range tables. These tables will present a range in which the actual DDER value should sit. It will be through the use of the approved methodologies that the dwelling specification single target and actual figures will be defined as is the case in the current method.

It is proposed that the Technical Handbooks will include a similar table to that shown in Table 3 noting that the figures are for illustration purposes only and will be defined by the level of challenge chosen (see section 5.1 of this consultation).

Table 3: Illustration of target range table to be presented in the domestic building standards technical handbook

Heat solution	Heat pump TDER (kWh/m2/yr)	Heat network TDER (kWh/m2/yr)	All other solutions TDER (kWh/m2/yr)	Heat pump TSHR (kWh/m2/yr)	Heat network TSHR (kWh/m2/yr)	All other solutions TSHR (kWh/m2/yr)
Archetype
End Terrace	X-X	X-X	X-X	X-X	X-X	X-X
Mid Terrace	X-X	X-X	X-X	X-X	X-X	X-X
Detached	X-X	X-X	X-X	X-X	X-X	X-X
Flats	X-X	X-X	X-X	X-X	X-X	X-X

To add further clarity using two examples:

• For an end-terrace dwelling that has a heat pump as its heat source the TDER for that dwelling will sit within a range. The target range is defined by the fluctuation in TDER dependent on where the actual dwelling is located and the orientation in which it is fixed.
• For an end-terrace dwelling the target space heating rate for that dwelling will sit within a range. The target range is defined by the fluctuation in space heating rates dependent on where the actual dwelling is located and the orientation in which it is fixed. Note that the space heating rate should not be affected by the choice of heat solution in the actual dwelling and so the range is the same across all heat options

Consultation Question 11

Do you agree with the proposal to present the target rates for dwellings within a target range table, replacing the notional dwelling specification currently shown in Table 6.1 in the Technical Handbooks?

Yes

No

Please provide a summary of the reason(s) for your view.

5.3 Fabric performance for new dwellings

The guidance to Standard 6.2 sets out robust maximum recommended thermal (‘backstop’) values for elements of the building insulation envelope. This defines the general level of expectation in limiting heat loss. That level is set, alongside the requirements of standard 6.1, to enable a degree of flexibility in the delivery of cost-effective levels of fabric performance that are viable for projects across Scotland.

The guidance also emphasises the importance of addressing air infiltration in new dwellings to reduce heat loss and energy demand. Whilst it remains difficult to achieve a specified air infiltration rate to any degree of accuracy, the February 2023 energy standards introduced the requirement for air pressure tests to be performed on all new dwellings as set out in CIBSE TM23: ‘Testing buildings for air leakage’, to verify that the infiltration rate declared at the design stage is achieved on completion.

The Passivhaus system sets a single total energy use and a single space heating demand target without being explicit on the fabric U-values that must be met. Additionally, one of the key principles of Passivhaus is to achieve a particular airtightness level set at equal to or less than 0.6 ACH @ 50 Pa.

5.3.1. Proposals

Given the extent of change within building standards with the implementation of the February 2023 standards and the effect of other measures within Section 6 and proposals within this consultation has on the need to improve fabric U-values, we propose to retain the maximum U-values as indicated in table 6.2 of the current domestic building standards technical handbook and as copied in table 4 below. This position reflects the reporting on the cost/benefit of further improvements to fabric performance within the associated research paper: ‘Identification and Assessment of Improvements to Energy Standard for New Domestic Buildings within Scottish Building Regulations’, and also the need to maintain a reasonable degree of flexibility between the consultation proposal for the domestic level of challenge set in section 5.1 of this consultation and the maximum performance of individual elements.

Table 4: Proposed maximum area weighted U-values for fabric elements for new dwellings

Type of element	Area weighted average U-value (W/m2K) for all elements of the same type
Wall	0.17
Floor	0.15
Roof	0.12
Windows	1.4
Roof Windows	1.4
Doors	1.4
Rooflights	2.1
Cavity separating walls	0.0

Given the theme of this review, responses to the stage one consultation and the associated research papers, we propose to set a provision in guidance for an ‘upper limit’ for fabric infiltration. For any new building that has a declared design infiltration of 5 or poorer, we propose that a design statement should be provided alongside the application for building warrant. This should set out the approach taken in the fabric specification and the specific considerations that have led to less of an emphasis on limiting fabric infiltration. We would anticipate that this would be integrated with the reporting on compliance for energy and environmental standards indicated in section 9 of this consultation.

Consultation Question 12

Do you agree with the proposal to set a provision in guidance for a maximum ‘upper limit’ to the declared design air infiltration rate?

Yes

No

Please provide a summary of the reason(s) for your view.

5.4 Standard 6.1 approved compliance methodologies for new dwellings

Standard 6.1 ‘Energy demand’ requires that “the energy performance is calculated in accordance with a methodology of calculation approved under regulation 7(a) of the Energy Performance of Buildings (Scotland) Regulations 2008”. The current approved methodology for dwellings is identified in guidance published in support of standard 6.1: Standard Assessment Procedure (SAP) with version 10 the methodology to be used for compliance with the current standards.

A significant review of the UK calculation methodology for determining the energy performance of dwellings is underway with further information provided in the July 2024 stage one consultation. Information on the current development of the Home Energy Model (HEM) as a replacement to the Standard Assessment Procedure (SAP) can be found at: Home Energy Model: replacement for the Standard Assessment Procedure (SAP) - GOV.UK.

The Scottish Government has procured the development of an updated compliance tool for dwellings with its core engine based on the Home Energy Model. This work has comprised of several iterative steps.

Assessment of available potential compliance tools

The key outcome sought from a calculation methodology should be that it is robust and representative of the performance of the building, presenting an accurate illustration of the effect that any combination of building elements will deliver. That representation should approximate the outcome expected in the real world.

Through 2024 the Scottish Government procured research to review the outputs of various building energy models against a number of standardised dwelling specifications: ‘Identification and Assessment of Improvements to Energy Standard for New Domestic Buildings within Scottish Building Regulations’.

The building energy models reviewed were:

• Standard Assessment Procedure (SAP) version 10.2
• Home Energy Model (HEM) version 0.33 with the Future Homes Standard (FHS) wrapper version 0.24
• Passivhaus Planning Package (PHPP) version 10.6.

Table 5 indicates a summary of the delivered energy outputs from the building energy models across the four building archetypes. The specification applied across the archetypes are defined as the Business as Usual models which represent what is currently being developed in Scotland.

Table 5: Summary of delivered energy outputs from a selection of methodologies

Heat Pump Delivered Energy (kWh/m2/yr)	SAP 10.2	HEM v0.33 FHS v0.24	PHPP v10.6
End Terrace	22.7	31.3	32.0
Mid Terrace	20.1	28.3	27.6
Detached House	24.5	35.6	31.1
Block of Flats	22.7	30.3	26.5
Direct Electric Delivered Energy (kWh/m2/yr)	SAP 10.2	HEM v0.33 FHS v0.24	PHPP v10.6
End Terrace	37.9	61.9	79.3
Mid Terrace	33.3	51.5	64.9
Detached House	44.9	79.5	78.2
Block of Flats	42.9	60.4	59.1
Heat Network Delivered Energy (kWh/m2/yr)	SAP 10.2	HEM v0.33 FHS v0.24	PHPP v10.6
End Terrace	43.0	70.2	85.8
Mid Terrace	38.2	60.6	71.3
Detached House	50.7	86.7	84.8
Block of Flats	47.0	62.6	64.1

The modelling indicates that there is a step change increase in reported outcome of Delivered Energy between the current SAP 10.2 and the HEM/FHS version developed at the time of the modelling. The development of HEM/FHS has brought outputs closer to the current PHPP modelling tool with the average difference between SAP and PHPP output being in the range of 20% and the average difference between the HEM/FHS and PHPP in the range of 10%, the remaining difference being due to the standardisation of assumptions and climate locations in the HEM/FHS methodology.

Development of Scottish consultation compliance tool

Building on the 2024 research the Scottish Government procured the development of a Scottish ‘wrapper’ linked to the UK Government base HEM tool, taking the place of the FHS ‘wrapper’.

Given the development timelines the current development of this HEM/Scottish wrapper is based on HEM v0.34; a later iteration of the HEM methodology than that modelled in the above research.

The Scottish Government ‘wrapper’ applies updates to the UKG FHS ‘wrapper’ to capture changes that were implemented within the February 2023 standards that differed from approaches applied in the rest of the UK, including:

• The benefit of electricity generated onsite that is applied within the standard 6.1 compliance calculation will only be that proportion that is used onsite
• Focus on Delivered Energy as the main compliance metric with changes to the methodology for dwellings connected to heat networks

The development work also applies aspects that are proposed elsewhere within this stage two consultation; however, this work is still live and the version of the tool provided alongside the consultation does not capture all the proposed changes. The current consultation version of the HEM/Scottish wrapper has been tested for usability prior to launch.

Consultation Question 13

Do you have any views or comments on the development and usability of the consultation version of the HEM/Scottish wrapper for new dwellings?

Yes

No

Please provide a summary of the reason(s) for your view.

Future development of the Scottish compliance tool

A final version of the Scottish compliance tool will build on the consultation version and an agreed base UK Government HEM and will apply confirmed actions once the responses to this consultation have been analysed. The Scottish government continues to be involved in the development of HEM and will decide on the base version to be applied at an appropriate point.

Consideration of additional compliance methodologies

The Passivhaus standard is supported by the Passivhaus Planning Package (PHPP) as a design and calculation tool for use within certified Passivhaus projects from inception. PHPP is based on the same building physics principles as SAP and HEM/FHS and with slight modifications to the reporting could produce the equivalent proposed metrics.

5.4.1. Proposals

As part of confirming the Scottish equivalent to the Passivhaus standard we propose to recognise a modified version of PHPP as an approved methodology indicated in guidance to Standard 6.1 ‘Energy demand’ for new dwellings. We will work with the Passivhaus Trust and Passive House Institute to modify the existing PHPP tool to align it with the requirements of a Building Regulations compliance tool and capture the intent of additional changes as part of this review as indicated in proposals to other questions. This work will also compare outputs from PHPP and the proposed Scottish version of HEM and wrapper to capture potential updates to both methodologies, so outputs are aligned where possible.

The final modified PHPP regulatory compliance tool will be presented for testing once complete.

Consultation Question 14

Do you agree with the proposal to recognise a modified version of PHPP as an additional tool that implements the approved methodology for standard 6.1 ‘Energy demand’ for new dwellings?

Yes

No

Please provide a summary of the reason(s) for your view.