They moved quickly and struck a creative deal to buy it: They would pay $1.4 million, but only if they were allowed to complete a targeted, temporary renovation to convert the house into a two-family home before the closing.

“We filed with the city to get a Letter of No Objection for it to be a two-family,” Mr. Johnston said. “And we did a quick $4,000 renovation to add a second kitchen and a dividing wall between two units, so we were able to leverage a mortgage for a two-family.”

They closed in July 2016 after selling their Bushwick apartment, which had roughly doubled in value over the previous five years, and split the profit between the new purchase and anticipated renovation costs. Then they got to work.

As proponents of sustainable building, they wanted to design their home according to passive house standards that would radically reduce the building’s energy consumption. And as practitioners of modern architecture, they wanted to make it lighter and brighter, and introduce simple, clean-lined details.

Finding themselves with a bonanza of late 19th-century architectural details, however, they decided to split the difference and blend the new with the old. When demolition began in March 2017, Mr. Johnston and Ms. Mandl instructed their contractor, LB General Contracting, to gingerly pry out and preserve every significant piece of woodwork while stripping the interior down to the studs.

They erected a tent in the backyard to store the material they planned to reinstall and donated the rest to the architectural salvage store Big Reuse. Hoping to keep a few of the plaster elements, but unable to move them for construction, they left a decorative archway and an ornate ceiling medallion in place on the parlor level and crossed their fingers. The archway survived, but vibrations from the construction work eventually sent the medallion crashing to the floor.