SAN FRANCISCO — Inside the newly expanded San Francisco Museum of Modern Art, one of the most startling sights is the absence of works on the towering walls flanking the main zigzagging staircase.

The blank walls are awaiting the arrival of a pair of paintings next year by Julie Mehretu, the Ethiopian-born, New York-based artist. “She’s working on the commission,” said Gary Garrels, a senior curator, climbing stairs with the ease of someone who has given many museum tours. “The paintings are so large that she has to use an old church in Harlem as her studio.”

When the museum officially reopens on May 14, after a three-year closing, a $305 million addition by the architecture firm Snohetta and a campaign that elicited some 3,000 works of art from donors, it will have bragging rights on many fronts.

Spanning a full city block at its widest, with a dynamic white structure that resembles a cruise ship, the museum will be the largest in the Bay Area. It will have more exhibition space dedicated to photography than the Getty in Los Angeles, and more gallery space than the current Museum of Modern Art in Manhattan.