There's nothing like a new "green" roof to bring out goat jokes. How else could the American Swedish Institute (ASI) keep its new roof mowed except by tethering a goat up there?

After the umpteenth goat query, ASI director Bruce Karstadt knuckled under and got one. The fiberglass critter is not expected to be of much use among the sedum and native grasses that will eventually sprout from the sloping roof of the Nelson Cultural Center, the new wing on the south side of the institute's headquarters in Minneapolis.

But even a fiberglass goat can inject a playful welcome to a marketing plan. Since the critter arrived a few weeks ago, it's been photographed exploring its new home and munching roses in the new courtyard linking the Nelson Center to the castle-like mansion that has housed the ASI for the past 83 years. There's even an online campaign to name the critter.

"People who have seen goats on grass roofs really expected we'd have one," said Karstadt, who appreciates the concept's "kid" appeal, though not its barnyard reality.

The ASI completed its expansion in just one year's time with a modest $13.5 million construction budget in a $20.5 million project. The result is a handsome, modernist addition to the museum's original Beaux Arts home. The center opens Saturday with a free daylong party featuring Abba impersonators, Scandinavian craft demonstrations, kids' activities and an evening concert by jazz-folk musicians from the Faroe Islands.

A cluster of interlocking rectangular boxes, the Nelson Center will serve as the ASI's new entrance. It features a reception hall, event space, catering kitchen, offices, cafe, craft workshop and classroom, exhibit gallery and shop. Sited on what was once a parking lot, the 34,000-square-feet addition wraps around a new courtyard inspired by traditional Swedish farmsteads and homes. Glass walls, skylights and a rooftop terrace frame vistas of the mansion, which remains the focal point of the complex. An additional 10,000 square feet of the mansion's galleries were renovated, too.

Designed by Minneapolis-based HGA, the building is up-to-the-minute but rich in traditional details. The lobby staircase is ivory-painted steel with porcelain-tile treads, very minimalist and contemporary. But the leather-wrapped handrail taps Swedish heritage. The event space -- a box that will seat 325 for concerts and lectures or 250 for dinner -- is the essence of bare-bones functionality. But its ceiling, loosely inspired by Stockholm's City Hall, is supported by slender oak beams that recall the long oars of Viking boats.

"We had to be very strategic about our use of materials" because of the tight budget, said Tim Carl, HGA's lead designer for the project. In addition to his 22-member team of architects, engineers, lighting and landscape specialists in the United States, Carl consulted with two Swedish designers, Åke Axelsson and Karin Ahlgren, about the building's placement and massing. The bio-roofs, geothermal heating and cooling system, and other "green" elements are both budget-driven and Swedish.

"Swedes don't think of 'green' as an afterthought -- it's embedded into their whole design process," said Karstadt. "Their government policy is that if something is beautiful, people will care for it and it will be more sustainable."

The no-frills tab dictated steel rather than glass panels for the lobby staircase, and prefabricated rather than custom steel frames for the courtyard window-wall. The exterior is clad in slate shingles that echo the mansion's slate roof, but their edges are cleaved rather than cut. Such shingles are not only less expensive but look more rustic and handmade -- cherished qualities in Sweden.

There were no compromises in the new elevator tower that now provides access to all three floors of the mansion. Carefully inserted between the mansion and its former carriage house (now offices), the elevator was positioned to avoid damaging the historic interiors. On the top floor, a glass hallway connects it to the building, threading between romantic turrets, gargoyles and copper-clad downspouts that offer an enchanting contrast with the handsome new courtyard and Nelson Center far below.

Mansion to museum

Built in 1903, the mansion was originally home to the family of Swan Turnblad, Minneapolis publisher of a Swedish-language newspaper. Garnished with towers and a glass conservatory atop the porte cochère, the limestone building trumpets Turnblad's success. Inside there's a two-story entrance hall with grand staircase, carved woodwork, exquisite porcelain stoves and a charming, skylit ballroom on the third floor. It has been home to the American Swedish Institute since 1929 and on the National Register of Historic Places since 1971.

The once prestigious neighborhood deteriorated in the mid 20th century, but has stabilized in recent decades. ASI officials hope their expansion will spark further rejuvenation. There are already 24 weddings booked into the Nelson Center in the next year, plus academic conferences, business meetings and other civic events. Somali, Mexican and American Indian kids from neighborhood schools are involved in ASI programs, and the organization is adapting its mission to changing times. Where it once used an immigrant's dream castle to transmit Swedish heritage and American success stories, it's now embracing a bigger vision.

"At a time of global migration, we're leaping from navel gazing about 19th-century immigrants to a multicultural mission," Karstadt said.

Mary Abbe • 612-673-4431