The Park Avenue office of Roy Stillman looks more like a room at the Metropolitan Museum of Art than one inside an office building. On one wall, eight vases and globes by Louis Comfort Tiffany rest on teak shelves, a fraction of an expansive personal collection. Mr. Stillman, a developer, sits behind a monolithic desk made by Jacques Adnet, where two nickel blocks support a slab of leather, across the room from a beloved, one-of-a-kind Jean Royere couch. Behind that is a tapestry by Henri Rousseau of a lively jungle scene.

Yet, at the moment, the most precious antique in Mr. Stillman’s collection might just be the one he is restoring himself: 36 Bleecker Street in NoHo. And the most important piece is one that has been long gone, a refrain common to many of the city’s old buildings — the decorative pediment crowning the sixth floor.

Older than most of his heirlooms, the property dates to 1885, when it was built as the Schumacher and Ettlinger lithography studio, helping establish the neighborhood as one of the city’s foremost printing districts. But when Mr. Stillman came across it in 2012, it had been reduced to a document-storage warehouse covered in grimy white paint and aluminum siding. Now he has stripped all that away, revealing red brick and cast iron, barrel-vaulted ceilings and a light-filled courtyard.

Nothing has made him prouder, though, than that pediment, unearthed in archives and rebuilt by hand.