Energy Performance of Buildings (Scotland) Regulations 2025: stock model research

1 Executive Summary

1.1 Overview

Domestic Energy Performance Certificates (EPC) are based on energy calculations from home survey data. The current energy calculation model for this is the ‘Standard Assessment Procedure’ (SAP), but this is due to be superseded in 2026 by a new ‘Home Energy Model’ (HEM). This change will coincide with several changes to the EPC, with the key change being the introduction to the EPC of new scores including the ‘Heat Retention Rating’ (HRR).

At present the key score on an EPC is an energy cost rating called the SAP rating^[1], which is a 1-100+ score presented on a distinctive graphic which shows how the score maps onto seven rating bands represented by the letters A-G. This SAP rating is related to the estimated annual fuel bill per m² floor area of a dwelling.

The new HRR will be based on the amount of energy a dwelling requires to maintain comfortable temperature (energy demand per m² of floor area)^[2]. As well as appearing on new EPCs as a primary rating, it will be used widely in policy and Government consultations on the energy performance of dwellings, for example for private rented and social housing standards where the HRR could be used to support implementation.

The primary objective of this research was to inform the thresholds of an A-G band scale for the Heat Retention Rating. An important consideration is the Scottish Government’s commitment to maintain equivalence between the SAP band C and a ‘Good’ Heat Retention Rating performance, i.e. an HRR band of C or better.

This report presents the results of modelling households in the achieved sample of the 2019, 2022 and 2023 Scottish House Condition Survey with both HEM and SAP. The HRR and other HEM ratings and metrics have been calculated using the latest Scottish EPC HEM wrapper, which produces input data that is then calculated using HEM. SAP was also used to provide results for the same dataset. These results allow some understanding of the effects of the change in methodology, and band thresholds for the HRR are proposed that will result in maintaining broad equivalence between the current SAP rating system and the HRR when this change happens.

1.2 Key conclusions

A primary aim of the project was to find the threshold value for the HRR where 80% of homes at SAP 9.94 band C achieved a ‘Good’ HRR rating, which is defined as achieving a band C or better performance. We found that 80% of homes which are currently at SAP band C are using less than 159 kWh/m2/year of energy to maintain a comfortable temperature. We therefore recommend a value of 159 kWh/m²/year as the maximum value for HRR band C, assuming that the HRR is rounded to an integer value, to achieve this goal when HEM is used to calculate the primary rating.

We also considered a value of 91 kWh/m²/year as a minimum integer value for a HRR band C. In combination with the maximum value of 159 kWh/m²/year set as the upper threshold, we would estimate that 38% of homes would fall into HRR band C as Table 4 in Section 6.3. This compares with 50% of homes falling into SAP 9.94 band C (see Figure 2). This variation is reflected in an increased proportion of homes falling the proposed HRR bands A and B, compared with SAP 9.94 bands A and B. The proportion of homes achieving band C or better in the HRR (53.68%) and SAP (53.87%) are approximately the same.

We expect some homes to change band because of the new calculation methodology. For example:

• a well-insulated home with expensive heating might show a positive shift
• a home which is poorly insulated but has solar PV installed, which reduces the fuel bill but does not change fabric efficiency, might show a negative shift

A discussion of other HEM derived ratings and metrics provided by the Scottish wrapper is provided in the appendix 7 Appendix: preliminary additional Scottish HEM wrapper ratings and indicators.

1.3 Production of calculation dataset

The Scottish House Condition Survey^[3] (SHCS) was the basis for our analysis. The SHCS is published annually and the statistics reported in the SHCS are based on a national survey of the housing stock.

Each home in the dataset includes the SAP rating, SAP band, and the data from an RdSAP-compliant energy survey – noting that there were some areas where assumptions were required to convert the data into a compliant input data file for the HEM wrapper.

Cotality used SHCS physical inspection data from 2019, 2022 and 2023 to produce a full RdSAP dataset (the most appropriate survey data format for existing homes), using assumptions where necessary. The datasets and data preparation methodology are described in more detail in Section 4.1 Preparation and ingestion of Scottish Housing Data.

The SAP ratings in the SHCS did not fully align to the SAP ratings calculated from the dataset. This is partly because the SAP ratings in the SHCS were calculated using RdSAP 9.93^[4] (rather than RdSAP 9.94 as the project brief), and partly to ensure that case input data for SAP ratings aligned with that created for the HEM wrapper. As a result the Cotality-calculated SAP ratings were used as the main point of comparison with the HEM HRR results to maintain referential integrity within the research design.

1.4 Energy calculation methodology

‘HEM’, as used in this report, may refer to HEM itself in combination with a ‘wrapper’ (for example when referring to the HEM HRR). A wrapper is a separate component that defines the inputs, assumptions and outputs needed for a specific use case, such as producing a Scottish EPC certificate. Wrappers allow the same HEM core calculation engine to be used for many different purposes, without having to reimplement the underlying physics model.

The version of HEM (0.34) used for this analysis is still under development by DESNZ, and the latest version of the Future Homes Standard wrapper (0.25) is designed only for calculations on new homes. At present there is no approved HEM methodology for existing homes.

RdSAP is the SAP methodology for existing homes, and there have been various versions with RdSAP 10 having replaced RdSAP 9.94 in June 2025. It works by expanding a smaller “reduced data” dataset into a complete set of SAP data.

This project made use of Cotality’s BRE-accredited RdSAP engine, built with bulk processing in mind. As the HEM input data format has strong commonalities with the SAP input data format it was possible, with assumptions, to populate a HEM dataset using an amended version of the Cotality RdSAP engine. The project then made use of the Scottish EPC wrapper (commit 2248e2d), which itself uses much of the Future Homes Standard wrapper (0.25), and HEM (0.34) to run the HEM calculations.

When this development work was completed, Cotality were able to run calculations on the dataset to produce both SAP results and HEM outputs, forming the basis of the analysis presented above and in this report.

This is explained in more detail in Section 4: Methodology.