With a series of outdoor spaces oriented around a new building, the firm plans to create connections to San Francisco's Yerba Buena neighborhood. When Mario Botta’s building for SFMOMA opened in 1995, the heavy street presence of the museum’s stacked blocks of red-orange brick—protecting a striped, periscope-like turret—created a formidable cultural outpost among the scruffy warehouses that then dominated the area south of Market Street in San Francisco. Today, the Oslo- and New York-based firm Snøhetta unveiled details of its plan to expand the museum by 235,000-square-feet with a new 10-story concrete building. It will connect SFMOMA via a system of alleys, walkways, and plazas to the surrounding neighborhood, which has been transformed over the last decade by development spurred in part by the pioneering institution. “The next stage for the museum is about reaching out and opening to surroundings,” says Snøhetta principal Craig Dykers, “It’s about becoming more extroverted.”

Working with local firm EHDD, Snøhetta conceived a scheme that tucks the new tower, a narrow form housing 130,000 square feet of additional exhibition space, behind the existing building. The plan invites visitors up from surrounding streets though 40,000 square feet of multi-level public space bounded by an 18-foot-wide promenade. According to today’s announcement, the museum, which averaged 625,000 annual visitors over the last ten years, plans to raise $555 million for the expansion—revised up from $480 million—with more than $200 million reserved for the endowment and the remainder going toward capital costs. So far, it has pulled in  $437 million toward that goal. Groundbreaking is scheduled for summer 2013, with completion expected in 2016.

Snøhetta won a closed competition to design the expansion, beating out an initial field of 30 and a short list that included David Adjaye, Diller Scofidio + Renfro, and Foster + Partners. The scale of the expansion was dictated by the Doris and Donald Fisher Collection, which came to the museum in 2009. The Gap founders had just abandoned plans to build a new structure to house their holdings of 20th-century and contemporary art in San Francisco’s Presidio park following opposition from preservation, parks, and local resident groups. Instead the collectors gave 1,100 works to SFMOMA as a long-term loan.

The plan calls for the demolition of a fire station on Howard Street, around the corner from the museum’s 3rd Street entrance—SFMOMA will pay for its relocation. The cleared space will allow Snøhetta to insert its new building behind the Botta and an adjacent W Hotel. (The hotel unsuccessfully opposed zoning changes required for the project because of its effect on the W’s access to an alley and parking garage.) The firm will connect Howard Street to the interior of the block with a terraced public promenade. On the ground floor of the addition, a glass-enclosed gallery designed to show large sculptural work—Richard Serra’s 13-foot twists of steel, titled Sequence and part of the Fisher loan, will inaugurate the space—follows the new public plaza up from Howard street to a ticketing and visitor orientation area at the center of the block.

By creating a new space for ticketing and information, Snøhetta has taken those functions out of Botta’s atrium, where the firm’s plan calls for the most dramatic and potentially controversial alterations of the original building. Accross from the current entrance, Botta’s take on a grand stair, a vertical shaft striped with alternating bands of rough and polished black granite, will be removed in favor of a wide new stair designed to draw visitors through the atrium to the central plaza in back. The firm will also take out walls in some upper-floor galleries, opening them to the atrium. “The current Botta entrance doesn’t suggest you’re in an art museum,” says museum director Neal Benezra, who added that the Swiss architect was “very gracious” about the change, saying, “It’s your museum now.”

The upper stories of Snøhetta’s expansion will house galleries integrated with those in the Botta building as well as two floors of administrative offices. With rippling facades punctuated by strips of glazing, the new building rises like a wall behind the original, its presence softened by a dipping roof line and a series of terraces. A balcony on the northeast side faces the site of Pelli Clarke Pelli’s Transbay Transit Center and looks over an existing rooftop sculpture garden, designed by Mark Jensen Architects and opened in 2009, and a new sculpture garden just above the promenade.

Dykers relates the plan’s vertically staggered spaces to the city’s famous hills. “We are creating a series of terraces that act like the San Francisco landscape,” says Dykers. “Because of the topography, you always feel like you’re entering and leaving the city as you move through it, and the layering of space is a big part of that experience.”