Colby College’s Cotter Union, a postmodern interloper on a Georgian-revival campus, has been renovated and expanded.

Waterville, Me. — This afternoon Colby College celebrates the reopening of a 1985 campus center designed by Centerbrook Architects, one of several firms founded by architecture’s most playful postmodernist, Charles Moore. The building, the Cotter Union, has undergone a $10-million renovation and expansion overseen by Bohlin Cywinski Jackson.

The good news for fans of Moore, who died in 1993, is that the overhaul preserves most of the original building’s best features while adding a big new “living room” for Colby’s students and adhering to the college’s pledge to follow LEED guidelines.

Cotter, planned in the wake of the college’s 1984 decision to abolish its fraternities, was an unlikely addition to Colby’s sober Georgian-revival campus. Designed to straddle a major campus walkway, the structure was essentially two buildings connected by a bridge that was the composition’s visual highlight.

The smaller side of the union held a pub that opened onto a terrace, while the larger held the campus post office, meeting rooms, and a multipurpose room, Page Commons, whose interior resembles the face of an Italian hill town. The building was — and still is — full of the kinds of entertaining details Moore loved: here, giant Ionic scrolls that look like column capitals drawn by Daffy Duck; there, a stairway twisting as it descended, for no apparent reason; over there, a long, severe roof plane sweeping down as if attempting to contain the mirth below.

But the building had several problems, according to Peter Bohlin, a principal at Bohlin Cywinski Jackson. Its entrances were oddly located, and many students ended up coming in through a back door that led to a cramped fire stair. That took them into a warren of corridors between the post office and a 1996 addition to the back of the building. Accessibility was a serious challenge — the pub, for instance, had tables on several levels — and the college wanted a big space where students from residence halls all over the campus could come to see and be seen.

When a potential donor’s interest prompted the college to ask Mr. Bohlin’s firm for a feasibility study — the firm was in the middle of another project at Colby — Mr. Bohlin had “some trepidation,” he says, both because there was a lot to like about the building and because several Centerbrook architects are friends of his. The renovation and addition, he says, “tries very hard to maintain the spirit of the original” while dealing with its circulation issues and adding 7,000 square feet of space.

Mr. Bohlin’s firm basically enclosed the area that separated the two halves of the original union, creating an open, high-ceilinged “heart” with a snack bar and tables on one side and easy chairs and a giant television screen on the other. A curving glass wall at the far end of the space is punctured by a big, copper-sheathed doorway that clarifies the main route through the building. The bright yellow ceiling over the middle part of the space has a gentle curve to it and rises to meet a new balcony carved into Centerbrook’s bridge.

Monica Barton, a senior associate at Bohlin Cywinski Jackson who oversaw the project, says that challenges included bringing air conditioning to areas that had not had it previously and finding space for larger, accessible bathrooms. Several other improvements were made as well, including redesign of the area around the post office and construction of a new pub off the game room; both overlook a space where bands can perform. Working with the quarter-century-old postmodern vocabulary of the Centerbrook building, Ms. Barton says, was not difficult. “It’s not the BCJ style, but I enjoy working out of context.”

The biggest challenges arose from a decision, not long before renovation began, to seek LEED certification for the building. Luckily, she says, the contractor was eager to participate, and Maine’s cold winters meant that using heavy insulation had been part of the plan all along. After some initial scrambling, Ms. Barton says, she is “comfortable that we’ll at least get certified.”

Midway through the project, the college made another change, asking Bohlin Cywinski Jackson to design another addition, this one of 9,000 square feet, for the college’s bookstore. Construction began on that project during the summer, and is expected to be completed next year. —Lawrence Biemiller

The renovation preserved Centerbrook’s main facade, although it added doors beneath the building’s bridge.

A copper-sheathed entrance on the other side of the building leads to the new gathering space.

The “heart” of the building is a vast open area with a snack bar on one side and easy chairs on the other.

The back side of Centerbrook’s bridge was opened to become a balcony overlooking the new space.

Page Commons, a multipurpose space with an Italian-hill-town motif, was not altered.

The overall appearance of the building is little changed (Chronicle photographs by Lawrence Biemiller).