Laura Balser and her husband, Matt Smith, bought a penthouse condo for $2.4 million at the W Residences in downtown Atlanta in 2009. For the next three years, they remained the lone occupants of the 74-unit tower.

Because the concierge desk was fully staffed, "we got treated like rock stars," says Lt. Col. Smith, a member of the Georgia Army National Guard. Occasionally the couple would use the hallway outside their home as overflow space. They let their cats roam, and once Ms. Balser, a principal at human-resources...