This project is an evidence based whole house retrofit of a 1948-built cavity wall semi-detached house. The foundation of the project was an in depth energy report produced at RIBA Stage 2. This collated existing energy use data from bills, which were used to define baseline energy use and carbon emissions for the house, and in turn validate a corresponding “as existing” thermal model. Iterative modeling then defined an optimal approach to building fabric improvement to improve energy efficiency by 65%, with a predicted consequential leap in thermal comfort. This then informed the subsequent design and specification, which conceptually is based on a fabric first strategy – significantly improved insulation and air-tightness – with the installation of an MVHR system to provide energy conserving cold season ventilation. The retrofit utlises low embodied energy materials, for example woodfibre and lime render external wall insulation to upgrade the existing walls, and timber framed construction to the new build elements. An interesting feature – which is more for horticultural and pleasure use than an integral part of the building energy strategy – is the attached greenhouse / winter garden element to the rear of the house.