This is the "Stoclet Palace", a private mansion built by architect Josef Hoffmann between 1905 and 1911 for banker and art lover Adolphe Stoclet. Considered Hoffman's masterpiece, the Stoclet's house is one of the most refined and luxurious private houses of the twentieth century.
The mansion is still occupied by the Stoclet family and is not open to visitors. It was designated as a world heritage site by UNESCO in June 2009.

When I was a child I had a card game with this palace. Every time I passing by I thinking of it!