From the World Heritage inscription:
The four major town houses – Hôtel Tassel, Hôtel Solvay, Hôtel van Eetvelde, and Maison & Atelier Horta – located in Brussels and designed by the architect Victor Horta, one of the earliest initiators of Art Nouveau, are some of the most remarkable pioneering works of architecture of the end of the 19th century. The stylistic revolution represented by these works is characterised by their open plan, the diffusion of light, and the brilliant joining of the curved lines of decoration with the structure of the building.
I have no idea why this is a world heritage site. This is in the running for the lamest world heritage site I have ever visited. There didn’t seem to be anything special about the houses in question. The only thing I could think of was “why isn’t there a world heritage site for Frank Lloyd Wright?” The house in the photograph is Hotel Tassel.