With space at a premium, extending into the basement has become cost effective in recent years. However it is critical that natural light is considered part of the scheme if you want to avoid a subterranean existence.
Glass floors are ideally suited to this sort of design, as are wide sections of vertical glazing uninterrupted by framework. In order to achieve the large span of the vertical glass without framework, a small glass beam stands down from the ceiling.
On this project the garden level was half way between the ground and basement floor levels and, by treating the facade as one plane of glass, it becomes difficult to differentiate between floors and the extension becomes one building of theatrical proportions.