IN A SALON WHERE subjects like universal education and health care benefits once held center stage—a room groaning with elaborate boiserie, inlaid mirrors and parquet flooring—a taxidermic monkey now looms over a massive stone-gray table. Rick Owens, the iconic expatriate American designer, has taken up residence in the former French Socialist Party headquarters on the Place du Palais Bourbon, a few blocks south of the Seine.

"The building was empty for 20 years before we got it in 2004. It was hideous: office cubicles,...