A chosen case study house forms the basis of this paper, which is a pilot energy-efficient social housing project, completed by one of the largest housing developers in the UK. The main aim of this study is to inform the redesign of flexible energy-efficient housing units. The housing, designed for social tenants, was built by the Accent Group in 2005, using modern construction methods and sustainable materials, based on extensive research from the adaptable and "Grow Home" principles of Avi Friedman as well as open building implementation. The first pilot scheme was designed in collaboration with the Building Energy Analysis Unit at the University of Sheffield, together with the Goddard Wybor Practise, and was a successful housing development with respect to being environmentally friendly and a low-energy design scheme for the UK climate. This paper presents redesigning of flexible terraced housing units, and their performance evaluation, using a building simulation method as well as the passive-house planning package. The aim was to plan a row of terraced houses that can not only utilize a flexible design concept in floor planning layout, but also to reduce energy consumption with a passive design with particular attention paid to material selection. In addition, building simulation work has been OPEN ACCESS Sustainability 2015, 7 5489 carried out with the use of DesignBuilder software for both thermal and energy performance evaluation. The study examines the annual energy performance and comfort conditions in the designed house to be situated in the Northeast of England, UK. A terraced house unit design is considered a flexible home that can adjust to the needs of different tenants for the purpose of achieving a sustainable building under different aspects, such as low energy, low carbon, use of renewables, and low impact materials, with flexibility by design.