The conceptual design for San Francisco’s most prominent new architectural project, the expansion of SFMOMA by Snøhetta, was revealed by the architects today. “We wanted to provide something to the city that isn’t limited to the contents inside the building…the museum shouldn’t be purely seen as an educational experience, but a social experience as well,” said Snøhetta principal Craig Dykers.

The 225,000-square-foot, $250 million addition is a simple rectangular form that will peer out like a mountainous landscape behind the boxy Mario Botta-designed museum. The 1995 Botta building’s façade on Third Street, the primary entrance, will remain untouched, with the Snøhetta addition layered between its iconic striped oculus tower and the Gothic grandeur of the Timothy Pfleuger Pacific Telephone Building to the east.

A concave scoop-out along the top edge will give the structure a curved appearance, and a ramping terrace will allow visitors to literally climb the upper levels of the building—an idea explored to great effect by Snøhetta in its Oslo Opera House, where visitors can walk from the ground level to the roof of the building.

Because of the complicated infill site allotted for the extension, there’s been particular curiosity about not only how the structure would look, but how it would open up—or close off—the museum to its surroundings. Hemmed in as it is by buildings, the plan tries to make the most out of its long back façade, adding a secondary entrance on the opposite side of the museum. Instead of maxing the extension out to the lot line, the design draws back a bit, eking out a new pedestrian throughway that parallels Third Street.

“The welcoming and luminous character of Snøhetta’s design and its embrace of the surrounding neighborhood further SFMOMA’s role as a center for learning, interaction, and inspiration for the people of San Francisco and the region,” said SFMOMA Director Neal Benezra in a statement. The firm won the commission in July last year, but has spent most of the intervening time developing the building’s program and financial model, beginning work on the conceptual design only four months ago. “Many people would have thought that we would have designed the building many, many months ago,” said Dykers. “With the client, we moved forward together to understand the needs of the institution before we let the computer models roll, as they say.”

The spare, 350-foot-long volume gets to the heart of the puzzle at the back of SFMOMA, where the administrative wing juts out from the center of the main building, bifurcating the buildable square footage. This wing will be remodeled into gallery space and incorporated into the Snøhetta addition. The new administrative offices will be located on the top two levels of the building, with five levels of galleries beneath.

The addition’s primary street presence will be at its south flank on Howard, where a double-height glass-walled gallery will give SFMOMA a truly public-facing space to display art. However, rather than put an entrance on the street itself, the architects located it along the back, accessible from Howard down a new passageway dignified with a raised promenade that will arch over a car tunnel. Along the promenade, a raised garden and an immense living wall will be lush embellishments to the narrow space.

The new entrance was located strategically so it will be visible from Second Street to the east–albeit down the long narrow lens of Natoma Street–and on axis with the new Transbay Terminal, just a couple of blocks away. By bringing attention to the mid-block, the architects hope to energize the alleyways and side streets in this area. “It’s an invisible site that’s being brought to life,” said Dykers.

At this stage of the design, the façade material has not yet been determined. But the architects are weighing various masonry-type options, including a concrete from Finland etched in a pattern of dots, revealing the stone aggregate beneath the surface. However, with so many vantage points to see and be seen—including the terraces along the building and perhaps even on the roof, as well as the elevated entrance—if all goes to plan, the visitors themselves will do much to enliven the exterior of the museum.

At this stage, the design of the interior gallery spaces has yet to be worked out. The schematic design is expected to be completed by November, and construction is anticipated to start at the end of 2013 and be completed by 2015. The capital campaign for the project has raised $150 million to date, a little more than half the funds required.