In most industrialized countries, the buildings sector is the largest contributor to energy consumption and associated carbon emissions. These emissions can be reduced by a combination of energy efficiency and the use of building integrated renewables. Additionally, either singularly or as a group, buildings can provide energy network services by timing their use and production of energy. Such grid-aware or grid-responsive buildings have been termed Active Buildings. The recent UK Government investment of £36m in the Active Building Centre is a demonstration that such buildings are of considerable interest. One problem with the concept, however, is that there is no clear definition of Active Buildings, nor a building code to design or research against. Here we develop and test an initial novel code, called ABCode1. It is based on the need to encourage: (i) the minimisation of energy consumption; (ii) building-integrated generation; (iii) the provision of grid services; and (iv) the minimisation of embodied carbon. For grid services, we find that a lack of a precise, quantifiable measure, or definition, of such services means that for the time being, theoretical hours of autonomy of the building is the most reasonable proxy for these services within such a code. Practical application Buildings have a special role in the transition to a sustainable energy infrastructure and a decarbonised society. They can become an active part of energy networks by leveraging strategies and technologies that are already available, but are not yet articulated in an integrated scheme that facilitates their uptake at scale. This work provides a review of the issues and opportunities, and introduces a practical framework aimed at helping designers and researchers study and deliver such buildings, and in particular the buildings that will form the exemplars in the first wave of Active Buildings.