Hôtel Solvay Brussels

Horta designed this in 1894 at the age of 33, and it's considered one of his masterpieces. It was commissioned by the Solvay family (soft-drink manufacturers), who gave him free rein in matters of design and budget. It's open only to ARAU tour groups; if you can't join one, the hints of art nouveau visible on the outside are worth a look.