As populations expand around the world, the pressure to build upwards is growing. In cities such as New York, adding floors to a building is commonplace, while in Hong Kong thousands of rooftop extensions are added illegally. In Britain, the practice is less usual but it may become more so in future, as a less disruptive alternative to basement excavations. John Parsons, associate director of built environment at the Royal Institution of Chartered Surveyors, says: “If you want to extend in a sensitive area, then this is the way to do it.”