Today, New York’s Museum of Modern Art (MoMA) revealed the completed renovation of the east end of its campus, anchored by its original 1939 Philip Goodwin/Edward Durell Stone structure, now called the Lauder Building.

Barely a decade after completing a $425-million expansion by Japanese architect Yoshio Taniguchi in 2004, MoMA announced plans to expand again—this time enlisting New York-based Diller Scofidio + Renfro (DS+R) in collaboration with Gensler. While the museum has seen annual attendance double to almost 3 million since the Taniguchi addition, according to MoMA, attracting more guests is not the goal of this latest project. “We want the best possible public spaces for our visitors,” says MoMA director Glenn Lowry, adding that “pace and pause” are the guiding principles for the new scheme.

That scheme includes the $50 million Phase I renovation that opens this month—in time to mount the Frank Lloyd Wright at 150: Unpacking the Archive exhibition in the newly reconfigured third-floor galleries, on view from June 12 to October 1. More significantly however, $400 million of renovation and new construction—partially paid for by a $100 million gift from David Geffen—comprise a Phase II westward expansion that is anticipated to open in 2019.

Phase I improvements are subtle but vital for improving the circulation in, and logic of the new museum, which Lowry describes as fostering a “synthetic narrative focused on the collection.” They include exchanging a solid wall for glass to expose more of the rear sculpture garden from lobby spaces; extending the historic Bauhaus stair in the 1939 building to the ground floor; inserting a new lounge/event space; and consolidating temporary galleries while renovating the café and adding an espresso bar with charging stations. “We turned up the volume on the materials,” said DS+R partner Charles Renfro during a recent tour of the new spaces. Floors in the new dining areas are covered in black-stained oak and the lounge features a heavily-veined black marble. DS+R used 2-inch-thick structural steel plate to create elegant profiles for the revamped staircase and areas of built-in seating. According to Renfro, “It’s been an exploration of thinness and lightness.”

Already well under construction, the Phase II portion of the project has stirred controversy and provoked outcries from the very announcement for its plans in 2013—which included razing the American Folk Art Museum (2001), MoMA’s diminutive and discrete neighbor to the west designed by Tod Williams Billie Tsien Architects | Partners. With that building reduced to rubble the following year, new contemporary galleries, a performance space, a daylit gallery, and lounge are currently being erected in its place. New and existing galleries will be seamlessly connected and serviced by multiple circulation cores. Galleries on the second, fourth, and fifth floors will link to spaces within 53W53, a 1,050-foot-tall luxury condo tower designed by Jean Nouvel, and stretching MoMA’s footprint almost to the Avenue of the Americas.

Controversy continued early last year when aspects of Phase II, including moving floors and retractable walls that open to the sidewalk, were cut. Flashy elements eliminated, subtlety wins out here as well. “This project is really about archaeology—mining what’s already there,” said DS+R founding partner Liz Diller. (Aside from the original Goodwin Stone building, the MoMA complex includes later additions by Philip Johnson, Cesar Pelli, and that of Taniguchi.) To allow the reconfigured main lobby to connect to the west, the museum store will be lowered one level but will remain visible from the street and lobby. There, a double-height space at the entry—part of Taniguchi’s original design that was later compressed to accommodate a video room—will be restored. Street-level galleries will also be added.

At its core, the addition is meant to increase gallery space—which will go from 135,000 to 175,000 square feet. All collection galleries will be “aggregated into a coherent whole,” according to Diller. Achieving that cohesion, both physically and curatorially, is the challenge. Says Diller, “It requires the analysis of a detective and the skill of a surgeon.” Lowry adds, “Our curators and the architectural team spent more than two years in conversations about the nature of our collection, the history of our installations, the continually changing nature of art, and our opportunities and responsibilities for engaging our audiences.”

The main entrance will soon temporarily move to the Lauder Building. But unlike with the Taniguchi expansion, MoMA will remain open throughout construction until just a couple of months before the new complex is finished, likely in May 2019, though that date also hinges on completion of the Nouvel building. It’s only then—80 years after MoMA’s first building opened its doors—that we’ll see if the understated design matches the museum’s ambitious goals.