BS 40104: A New British Standard on Retrofit Assessment

Updated on 13/05/25

BS 40104, the British Standard for the retrofit assessment of dwellings, is due to be published on August 5th 2025. The public consultation ended in September 2024, where we encouraged everyone to have their say.

The Environment Study Centre consider that these proposals provide a major step forward in the improvement to retrofit assessment, especially given the UK’s goal of retrofitting 29 million homes by 2050. BS 40104 is a standalone document, but there is an intention for it to replace sections 7.3 and 7.4 of the 2023 version of PAS 2035, the UK standard for retrofitting dwellings.

Improving the Retrofit Assessment of Domestic Buildings

Our retrofit expert, Professor John Edwards, a co-author of this document and leading expert on all older building issues, shared: “This has been such a worthwhile exercise, as we can see a real improvement in the way we undertake the retrofit assessment of dwellings.” Professor Edwards added: “Like all standards, it is produced through consensus of the people producing it, but it’s vitally important that everyone has the opportunity to comment on it.”

Developed in collaboration with retrofit professionals, BS 40104 highlights the growing awareness and urgency surrounding housing and the UK’s net zero goals. Retrofit assessments are becoming increasingly important for improving energy efficiency and reducing the carbon footprint of our existing housing stock. BS 40104 integrates expertise on climate resilience, reflecting an understanding of the potential long-term impacts of climate change on domestic buildings.

Changes Outlined in BS 40104

Since November 2024, an estimated 225,000 retrofit assessments have been carried out in the UK. The changes to the retrofit assessment of dwellings includes a more robust approach to the equipment being used, an increased focus on health and safety issues, a need to focus on the prediction of future climate change hazards, a more detailed analysis of ground conditions and topography together with exposure and shelter along with a much better ventilation assessment with occupancy analysis.

It’s worth noting how the new standard recognises the role of advanced equipment and technology in accurately evaluating a building’s performance. Furthermore, by placing an emphasis on health and safety, BS 40104 aims to ensure that retrofits consider the wellbeing of a building’s occupants alongside sustainability. With occupancy analysis as a requirement within the standard, domestic retrofits will need to be tailored to actual household usage and behavioural patterns.

To learn more about BS 40104 as a retrofit professional, you might consider taking our Level 3 Award in Energy Efficiency for Older and Traditional Buildings. At the Environment Study Centre, we developed the original course over a decade ago, led by Professor Edwards.

BS 40104 and BS 7913

According to Professor Edwards: “On the assessment of significance for buildings which do not have any special protection, it’s just as robust as that described in the 2023 version of PAS 2035, but the assessment has been made easier to undertake”. For protected buildings, there is reference to BS 7913, the British Standard for working on traditional and protected buildings, produced by a panel chaired by Professor Edwards.

This link to BS 7913 is important as traditional and historic buildings make up 1 in 5 British homes. By aligning the two standards, BSI aims to offer a comprehensive approach to retrofits that takes into account the historical character and heritage considerations of older buildings, while working to improve their sustainability.

Why Retrofit Assessment Requires a More Robust Approach

In reality, BS 40104 emphasises the approach that has always been taken by Edwards Hart Consultants, whose work has often involved retrofit projects by others, which have not been entirely successful, and therefore require a more robust approach. To put this into perspective, only 43% of providers measure building performance after envelope changes, highlighting a skills gap among retrofit professionals.

With the UK housing sector under pressure to decarbonise by 2050, there’s a critical need for training and professional development within the retrofit industry. By mandating a standard for retrofit assessments, BS 4104 aims to address past retrofit issues that have occurred due to errors in implementation and inadequate planning. At the Environment Study Centre, we welcome the standard as it will provide greater accountability and better outcomes for building owners and occupants.

Access Retrofit Training

You can follow the BS 40104 development process here. To learn more about retrofit, read our sustainable construction blog, including articles on what retrofitting is, the challenges of retrofit inspection and PAS 2038 compliance.

Many aspects of BS 40104, are already covered in Environment Study Centre courses, such as the 2-Day Level 3 Award in Traditional Building Retrofit Course. Similar content is also included in the eBook, ‘Retrofit of Traditional Buildings,’ written by Professor John Edwards, to which our students gain a free year’s access.

To learn more about our courses and enrol, please contact us here.