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.”

Building upwards is tough. In most cities developers must persuade planners that their designs can improve the aesthetic aspect of a building or, at least, not detract from it. In London, architect Richard Hywel Evans gained the support of planners for a five-bedroom penthouse at Lansdowne Court, a pair of neighbouring mansion blocks in London’s Notting Hill. The two halves of the 400 sq m home are connected by a glass bridge. The penthouse will return to the market later this year through Knight Frank and Domus Nova for ￡7.85m.

Planners liked the architect’s design, because the simple, contemporary look of the steel-and-glass mansard roof is in keeping with the modernist 1930s buildings below it. Also, it crowns two buildings that previously had boring, flat roofs.