The result is a real curiousity. The structure seems to offer complete transparency, and yet almost everything is in fact hidden away on the first floor or underground. The entrance via the glass facade (and a miniature indoor garden where the tree has pride of place) is visible to anybody passing by, but at the same time, the house has also been removed from Google Street View.
All is at once visible and invisible, including the owners of the property. They live in a glass house, but remain very discreet. The principal space remains the 19th century classical townhouse at the end of the courtyard, but much of their living space today is in an underground zone between the two. However, even the architects have apparently lost contact with their creation, and are unaware of how it has evolved. A message on their website simply explains that "as far as we know, the ground floor has recently been transformed into a cellar. The two courtyards have been demolished, maximising square meters". Truly a house of mystery.
For a full overview of this house, its various spaces and how it was designed to fit together, see the description on the architect's website: http://www.pottgiesser.fr/christian_pottgiesser_architecturespossibles/galvani.html