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

3 Research Background

3.1 Changing assessment methodology

The Standard Assessment Procedure (SAP) has been the basis of Energy Performance Certificates (EPCs) since their introduction into law in Scotland in 2008, with SAP used for ‘off plan’ assessment of new homes and RdSAP (Reduced Data SAP) used for assessing existing homes based on a property survey carried out by a member of an approved scheme. These methodologies are key to EPCs as they exist today, which provide prospective building owners, homeowners and tenants with a measure of their property’s energy performance. Since the first Scottish EPCs were produced in January 2009, they have been increasingly important in a wide range of policies related to improving energy performance of buildings.

SAP is based on a monthly time-step calculation, and while it is a robust and tested methodology, it does not provide the resolution to specifically model technologies and tariffs where the timing of operation and energy use within each 24-hour period is an important factor. SAP is therefore due to be replaced by the Home Energy Model (HEM) across the UK. This change is not just an opportunity to improve the flexibility of the domestic energy model but also an ideal time to consider reform of the EPC’s purpose and usefulness for its intended audience.

A key part of the new system is the Scottish EPC wrapper, which is a software package that works in conjunction with the core HEM calculation engine. It is designed to take existing home survey data (comparable but different to SAP input data) and translate it into the format required by HEM, while also generating the specific metrics required for new Scottish EPCs. Whilst the core of HEM will be the same for all users of the methodology, the wrapper also allows parameters to be set that will align with national policy conditions, for example using nominal weather data for national ratings.

This reform is vital for potential future housing energy efficiency policies which have recently been the subject of consultation, including the proposed Social Housing Net Zero Standard (SHNZS), and a Minimum Energy Efficiency Standard (MEES) for privately rented homes. The new Heat Retention Rating (HRR) could serve as the primary benchmark for these standards, helping to inform related policy decisions and drive the necessary improvements to Scotland's housing stock. Other new ratings derived from the Scottish EPC wrapper may also provide the means to benchmark and compare performance on cost and emissions scales.