Paris is not a place where walls are torn down casually. There’s too much history at play—and too much good taste. But when renovating an 18th-century apartment on a side street in the Marais, architect Clément Lesnoff-Rocard boldly undid a number of alterations to the home over the years. What was left when he was done? Parisian limestone, plenty of light, rounded walls, and the freedom to sudden...