SHELTER ISLAND, N.Y. — As the founding design partner of Kohn Pedersen Fox Associates, William Pedersen is best known for soaring skyscrapers like the World Financial Center in Shanghai or sprawling developments like the recently completed Hudson Yards in New York.

But in Mr. Pedersen’s more than 60 years of architectural practice, the project that means the most is the modest one he just completed here, where he and his wife, Elizabeth Pedersen, have summered since 1975.

“I probably had the most pleasurable professional experience of my life,” Mr. Pedersen, still lithe at 81, said during a recent walk through the Shelter Island History Center.

The center is the new name for the reconfigured complex run by the Shelter Island Historical Society, custodian of the island’s archives and artifacts. Until recently, those treasures were housed in the attic of the historical society’s decaying Havens House Museum, built in 1743. Despite a slight update in 1966, the museum was desperately in need of rescue.