In the financial district of Manhattan, the symbolic heart of commerce, conversions of grand bank buildings are old hat. Metro Loft, a development company that has completed 14 similar conversions, many of them in the area, is wrapping up a new rental project at 20 Broad Street. The 29-story tower, formerly offices for the New York Stock Exchange next door, opened in September, with 533 luxury apartments. Built in 1956 and once connected directly to the Stock Exchange building, the tower has a mix of apartments, from studios to two-bedrooms, starting at $2,685 a month.

The connection to banking is still tangible. Below street level, two vaults, once filled with stocks and bearer bonds, are being repurposed: one to store mechanical equipment, the other as a lounge for residents, said Nathan Berman, the founder of Metro Loft.

The developer’s gain is clear in these projects, but what do the historic buildings get out of it? In the case of 9 DeKalb, where the Dime Savings Bank will merge with the borough’s first super-tall skyscraper, the bank will be restored, inside and out.

The developer, JDS, bought the bank and its air rights for $95 million in 2016. That gave the tower an additional 385,000 square feet of development rights, adding about 30 more floors. In exchange, the developer will repair damage to broken capitals and weathered friezes and preserve much of the landmark interior, restoring the hand-painted wallpaper in the upstairs ladies’ lounge, among other things.

The restoration has also uncovered some bank lore. In the subbasement, insulated by thick concrete, security guards used to shoot their weapons against a far wall for target practice, said Marci M. Clark, an architectural historian who is a director with JDS.

On a recent morning, light poured into a subterranean vault that had not seen the sun since the early 20th century. The developer had torn down an accessory building on the lot to make way for the tower. One of the bank’s walls had been temporarily pinned, held up like a theater scrim, so the new building can be integrated behind it.

The new tower, which is expected to be completed in 2022, will be deferential to the landmark, but not derivative, said Gregg Pasquarelli, a principal at SHoP Architects, the designer. The hexagonal geometry of the building, seen in its ornate tile and coffered ceilings, will be echoed in the shape of the new structure.