The Städel in Frankfurt is one of Germany's most important museums. Its collection of some 2,900 paintings, 600 sculptures, 500 photographs, and more than 100,000 drawings and prints represents 700-plus years of Occidental art history from the early 14th century to the Renaissance, Baroque, early modern, and contemporary periods. Highlights include pieces by Dürer, Rembrandt, Vermeer, Monet, Picasso, Beckmann, and Richter.

Founded in 1815 by banker and merchant Johann Friedrich Städel, the institution has acquired so much by continually expanding its holdings through an active acquisition policy. Recently, the museum's contemporary collection received a boost with the transfer of 600 works from the Deutsche Bank collection and more than 200 photographs from DZ Bank, adding such luminaries as Olafur Eliasson, Nan Goldin, Cindy Sherman, and Andreas Gursky.

To accommodate and display this new wealth of contemporary art, the Städel decided to undertake the largest expansion in its 200-year history. In the autumn of 2007, the museum invited eight architectural firms to enter a competition to design a new wing and renovate the existing facilities: Diller Scofidio + Renfro of New York; Gigon/Guyer Architekten of Zurich; Jabornegg & Pálffy of Vienna; Kuehn Malvezzi Architekten of Berlin; Sanaa/Kazuyo Sejima, Ryue Nishizawa & Associates of Tokyo; UNStudio of Amsterdam; and local Frankfurt firms Wandel Hoefer Lorch + Hirsch Müller and Schneider+Schumacher. In February 2008, an international jury chose the proposal by Schneider+Schumacher.

The firm's winning design put the 9,800-square-foot exhibition area in a volume beneath the museum's existing garden. This allowed the architects to maintain the cherished green space, regularly used by the museum and its visitors, with an intensive green roof. To indicate that more exists beneath the lawn than just earth and nature's critters, Schneider+Schumacher gave the extension's poured-in-place concrete roof a gently sloping convex shape, creating a domed hill that animates the landscape.

The firm studded the surface with a grid of 195 porthole skylights that range in diameter from 5 feet at the perimeter to 8 feet in the middle. Underground, a mere 12 columns support the roof, which reaches as high as 28 feet, providing an open, flexible space that can be reconfigured to fit the museum's evolving needs. Kuehn Malvezzi Architekten of Berlin designed the first exhibition presentation, laying out a system of interlocking galleries that allows visitors to choose their own path through the contemporary art holdings.

The skylights also provide illumination for the exhibition spaces below. The architects worked with Berlin- and Bonn-based lighting design firm Licht Kunst Licht to develop a natural and electric lighting scheme that would be as flexible as the space itself, and thus capable of meeting the needs of individual artworks—whether they be sunlight-loving sculptures or light-sensitive drawings. The arrangement of the different-diameter skylights, with smaller ones on the fringes and larger ones in the center at the raised dome, provides the first method of controlling daylight, organizing the interior into smaller, easier-to-control compartments. Each skylight is outfitted with an automated shading system that provides four levels of allowable light penetration and a fifth level for total blackout, so that, throughout the exhibition area, the skylights can be adjusted to the requirements of the artworks below.

The design team integrated the electric lighting scheme into the skylights using custom-designed LED ring fixtures that sit inside the skylight housing. Each fixture features both warm-white (2700K) and cold-white (4500K) LEDs, which can be tuned to match daylight conditions, while daylight sensors and a control system adjust the levels of the LEDs, raising light levels as clouds pass overhead or as evening approaches—thus ensuring a constant level of light. Each skylight also features a diffuser foil system that makes it indistinguishable to decipher whether the sun or the LEDs are providing the illumination. As needed, specially fabricated LED accent spotlights with a variety of optics can be inserted into sockets in the skylights to add to the ambient illumination or to highlight individual artworks.

At the Städel, light and architecture come together so completely that it is difficult to tell where one begins and the other ends. On the interior, the porthole skylights and profile of the domed ceiling create a lively environment in what would otherwise be a hole in the ground outfitted with white walls. In the garden above, the skylights let the world know that this is a piece of architecture, an impression that only grows as the sun goes down and the field of polka dots begins to illuminate the outside from within.