If the extension and the internal works are over 50%, then the whole building will need to achieve 6 stars. This is because regulation 233 of the building regulations 2018 stipulates that any extension work over the last 3 years that is more than 50% of the original volume, the whole code applies.
So, the whole building must achieve a 6-star energy rating report.
We will treat the existing and alteration works together as a “new” building and provide a 6-star energy rating report.
The client will need to then apply for partial compliance requesting exemption for the existing walls, windows and flooring due to the cost to bring these up to the current 6 star code, the argument being it will cost far more in construction costs to perform these upgrades then the potential in energy savings the owners may receive throughout the life of the home.