For the purposes of Display Energy Certificates (DECs), a building is a construction with a roof, walls and it uses energy to condition the indoor climate.  The indoor climate is conditioned where the building has, or would be expected to have, any of the following fixed services:
Heating;
Cooling; and/or
Mechanical ventilation (this means the supply of fresh air and extraction of stale air – not just an extraction fan).
It is not uncommon for a site to have multiple buildings or a large building built in multiple phases.  If two or more structures on the same site are linked by a fully enclosed structure consisting of a roof and walls, even if the link is not itself conditioned, then the combined structure can be considered a single building for Display Energy Certificate assessment purposes.  This is on the proviso that the structures have not been designed or altered to be used separately (usually by separate occupiers). This approach is particularly sensible when the different parts are serviced by the same common systems and where sufficient sub-metering is not in place to separate energy consumption.  However, each part may still be treated separately which can be useful in identifying energy efficiency improvements and monitoring energy usage in buildings with significantly different constructions.