The New Museum was founded in the 1970s to showcase work by innovative, early-career artists. Three decades later, it followed pretty much the same philosophy when it chose SANAA , the not-yet-famous Japanese architecture firm, to design its new Lower Manhattan building. Though experts in the use of glass, the firm’s partners, Kazuo Sejima and Ryue Nishizawa, reacted to the museum’s site, on the not-yet-gentrified Bowery; its relatively modest budget; and its limited need for natural light by producing a series of stacked stucco boxes covered in a dull gray metal mesh. But when it opened in 2007, the building already seemed too small. “I loved it, but it had its limitations,” says Lisa Phillips, the museum’s director since 1999.…