When a young couple purchased the parlor and second levels in a five-story row house condo, planning to start and raise a family there, the two floors were connected by a less-than-ideal spiral stair.

“Sweet and homey” were the watchwords when interior designer Keren Richter tackled the garden floor of a 19th century single-family townhouse. The downstairs rooms, part and parcel of a top-to-bottom ...