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© The British Standards Institution 2025

Published by BSI Standards Limited 2025

ISBN 978 0 539 21602 8

ICS 91.120.10; 91.140.10

The following BSI references relate to the work on this document:
Committee reference CB/401
Draft for comment 24/30453505 DC

Foreword

Publishing information

This British Standard is published by BSI Standards Limited, under licence from The British Standards Institution, and came into effect on 30 September 2025. It was prepared by Technical Committee CB/401, Retrofitting energy efficiency measures. A list of organizations represented on this committee can be obtained on request to the committee manager.

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Introduction

This British Standard provides the processes and recommendations for retrofit assessments based on a whole-dwelling approach, which forms the foundation for all the subsequent design and installation stages of any domestic retrofit.

A retrofit assessment is the essential first step towards a successful retrofit project. An assessment aims to provide the evidence, and the intellectual and practical foundations, for decision making about the most appropriate solutions for each dwelling in terms of energy efficiency, health and the public interest.

The retrofit assessment process is based on the whole-dwelling approach which emphasises reviewing the building's construction, significance, condition, ventilation, location, energy performance and occupancy issues. A quality assessment is determined not only by the assessor’s ability to gather data on the dwelling, but also by their capacity to understand where further specialist input might be appropriate and can be engaged. The clear presentation of gathered data facilitates the next step in the retrofit process.

A whole-dwelling approach is used to prevent retrofit works from being implemented in isolation which can unintentionally damage the building's overall performance; minimal assumptions are allowed in the assessment process. This risk-based approach is embedded in the principles outlined in Each home counts [1] and PAS 2035; however, this British Standard outlines good practice for all retrofit assessments for domestic dwellings.

1 Scope

This British Standard provides recommendations and guidance for the retrofit assessment of domestic dwellings in the UK, based on a whole-dwelling approach.

This British Standard covers:

context assessment;
condition assessment;
ventilation assessment;
occupancy assessment;
energy performance assessment;
site-specific context assessment;
significance assessment;
reporting and lodgement; and
competencies.

This British Standard is intended for use by competent persons assessing dwellings prior to retrofit.

This British Standard might be of interest to small and medium-sized enterprises in the construction field and to retrofit project stakeholders.

This British Standard does not cover fire, electrical and gas safety, and the structural elements of dwellings.

NOTE While fire, electrical and gas safety is out of scope in terms of inspection and installation, the assessor is expected to be aware of issues to flag as part of basic identification and information gathering.

2 Normative references

3 Terms and definitions

4 Assessment process

4.1 Stage 1: Context assessment

4.2 Stage 2: On-site assessment

4.3 Stage 3: Reporting and lodgement

5 Context assessment

5.1 Process

5.2 Output

6 On-site assessment

6.1 Condition assessment

6.2 Ventilation assessment

6.3 Occupancy assessment

6.4 Energy performance assessment

6.5 Site-specific context assessment

6.6 Significance assessment

7 Reporting and lodgement

8 Competencies

Bibliography

BS 40104:2025

Retrofit assessment for domestic dwellings. Code of practice

Current

Published:

24 Sep 2025