San Francisco Museum of Modern Art is reopening after three years of renovation, including an extension by architecture firm Snøhetta that piggybacks on the original Mario Botta-designed building (+ slideshow).

Snøhetta's striated addition sits behind the Postmodern building completed by Botta in 1995, giving nearly three times more gallery space to the museum.

The 235,000-square-foot (21,832 square metres) expansion includes seven floors of gallery space and three levels for administration departments, rising above the Swiss-Italian architect's boxy red-brick design.

Snøhetta aimed to respect the original structure, which was frostily received by critics when it first opened.
The extension's east facade is covered in over 700 fibre-reinforced polymer panels that form rippled patterns, influenced by the water and fog of San Francisco Bay.
From the west, the extension appears to crouch over Botta's building as if receiving a piggyback.

The museum is located in the city's SoMa neighbourhood, and the extension is designed to better connect the institution with its surrounding streets.

To access a new public entrance from Howard Street, visitors walk past a glazed gallery containing sculptor Richard Serra's 2006 weathering-steel Sequence piece.

Related story: Snøhetta team to design river walk next to America's second largest waterfall

An exterior staircase leads up to the main entrance hall, while wooden bleacher seating facing the sculpture also provide...