To ensure the building looked like an old brownstone from the outside, Mr. Peterson took a thin layer of real brownstone and attached it to light aluminum honeycomb.

WSJ: Up and Down Brownstone

The building at 224 E. 14th St. looks like any other on the block, but a section of the brownstone-finished facade folds into Mr. Peterson's apartment to create an open-air living room.

WSJ: Up and Down Brownstone

"Originally in an old brownstone there would have been a parlor-floor balcony, " Mr. Peterson said of the retractable living-room wall.

WSJ: Up and Down Brownstone